Breeding Ammospiza nelsoni (Nelson’s Sparrow) exploits both saltmarsh and hayfields in northern habitats

ABSTRACT

Ammospiza nelsoni subvirgata (Acadian subspecies of Nelson’s Sparrow) breeds in saltmarsh from northern Massachusetts to New Brunswick and eastern Quebec. In the Canadian Maritimes, this subspecies also successfully breeds in diked agricultural lands (i.e., “dikeland”) that were originally created by Acadian settlers in the 1600s. Little is known about the reasons for or consequences of using dikeland for breeding. To fill this knowledge gap, we tracked male and female sparrows, and monitored nest fates in natural saltmarsh and human-made dikeland habitats. We collected fecal samples from adults and nestlings to examine which habitat type they were foraging in, and we also quantified vegetative cover. We hypothesized that flood risk in saltmarsh played an important role in the decision of A. n. subvirgata to nest in dikeland given that the saltmarsh is regularly inundated with tidal water. Based on nest monitoring, we estimated higher overall nest success in dikeland than saltmarsh. Fecal sample analysis showed distinct differences in diet between individuals using dikeland compared with saltmarsh. We also observed differences in vegetation. These results suggest that A. n. subvirgata are able to take advantage of readily available human-made habitats for breeding. With rising sea levels and increased storm events threatening coastal habitats, it is important to understand whether coastal-breeding birds can adapt to changes and what trade-offs exist for individuals who shift to alternative habitats.

RÉSUMÉ

Ammospiza nelsoni subvirgata se reproduit dans les marais salants du nord du Massachusetts au Nouveau-Brunswick et dans l’est du Québec. Dans les provinces maritimes canadiennes, cette sous-espèce se reproduit également avec succès dans des terres agricoles endiguées (dikelands), créées à l’origine par les colons acadiens dans les années 1600. On sait peu de choses sur les raisons ou les conséquences d’utilizer ces terres agricoles endiguées pour se reproduire. Pour combler cette lacune dans les connaissances, nous avons suivi des mâles et des femelles d’A. n. subvirgata, et nous avons surveillé le devenir des nids dans des marais salants naturels et des habitats de terres agricoles endiguées artificielles. Nous avons recueilli des échantillons fécaux d’adultes et d’oisillons afin de déterminer le type d’habitat dans lequel ils s’alimentaient, et nous avons aussi quantifié la couverture végétale. Nous avons émis l’hypothèse que le risque d’inondation dans les marais salants a joué un rôle important dans la décision d’A. n. subvirgata de nicher dans des terres agricoles endiguées, étant donné que le marais salant est régulièrement inondé par les marées. En se basant sur la surveillance des nids, nous avons estimé un succès de nidification plus élevé dans les terres agricoles endiguées que dans les marais salants. L’analyze des échantillons fécaux a montré des différences marquées dans le régime alimentaire entre les individus utilisant les terres agricoles endiguées et ceux utilisant les marais salants. Nous avons aussi observé des différences dans la végétation. Ces résultats suggèrent qu’A. n. subvirgata est capable de tirer profit d’un habitat artificiel facilement disponible pour se reproduire. Avec l’élévation du niveau des océans et l’augmentation du nombre d’événements de tempête qui menacent les habitats côtiers, il est important de comprendre si les oiseaux qui se reproduisent en zone côtière peuvent s’adapter aux changements et quels sont les compromis pour les individus qui se déplacent vers d’autres habitats.

Graphical Abstract

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INTRODUCTION

Anthropogenic changes to landscapes force wildlife to move, adapt to the changes, or face local population declines. Behavioral changes are often an early response by wildlife to anthropogenic change. For instance, wildlife often use novel habitats created by humans (Lowry et al. 2013). Some anthropogenic landscape changes are intentionally created as wildlife habitats, such as freshwater impoundments for waterfowl (Mitsch and Gosselink 2015) and nesting platforms for raptors (Bohm 1977), while other landscapes, such as agricultural fields, are unintentional wildlife habitats (Lieske et al. 2012). Many of these human-made habitats supplement natural habitats or features that are lacking, such as nesting boxes in place of natural cavities or agricultural fields where natural grasslands are limited; however, human-made and natural habitats may provide a different suite of benefits and consequences for wildlife that use them.

Adaptation allows future generations to exploit natural habitats with increased efficiency. For example, wildlife that use saltmarsh often contend with tides, salinity, competition with other individuals, and predation risks; therefore, species that rely on saltmarsh have adaptations that enable them to take advantage of this highly productive habitat despite the challenges (Greenberg 2006). Saltmarshes are unique, highly productive habitats that provide food resources for birds and other wildlife (Greenberg 2006). For example, few songbirds nest in saltmarsh (Greenberg and Maldonado 2006), but, through behavioral adaptations, Ammospiza caudacata (Saltmarsh Sparrow) can reduce the loss of nests to flooding. For example, the timing of nesting for A. caudacata is synchronized with tidal cycle and nesting cycle is known to influence nest survival in A. caudacata in New England because of this synchrony (Shriver et al. 2007). Additionally, eggs remain viable after submersion (Gjerdrum et al. 2008, Bayard and Elphick 2011), and chicks climb up grass to avoid drowning (Bayard and Elphick 2011). Despite these adaptations, 60% of failed nests were lost to flooding in one study in Maine (Shriver et al. 2007), and a range-wide study found that nest flooding was the main abiotic source of nest failure (Ruskin et al. 2017). Predation is often hypothesized to be the main cause of nest failures in tidal marsh sparrows (Roberts et al. 2017), but is often masked by flooding occurring at the same time (Greenberg et al. 2006). In tidal marsh sparrows, the 2 threats have been shown to interact and increase the risk of nest failure from flooding and predation simultaneously (Hunter 2017).

Unlike A. caudacata, which are restricted to tidal marshes, the closely related Ammospizanelsoni (Nelson’s Sparrow) uses both tidal and nontidal habitats for breeding, migrating, and overwintering. A. nelsoni is a small songbird with 3 named subspecies: A. n. nelsoni, A. n. alterus, and A. n. subvirgata. Two of the subspecies breed in freshwater marshes and meadows mainly in the Prairie provinces of Alberta, Saskatchewan, and Manitoba (A. n. nelsoni) and along the Hudson and James Bays (A. n. alterus) in Canada. The third subspecies, A. n. subvirgata, or the Acadian Nelson’s Sparrow, breeds mainly in saltmarshes of Atlantic Canada and northern New England (Shriver et al. 2020, Walsh et al. 2021). A. n. subvirgata is considered a saltmarsh-specialist species (Correll et al. 2017); however, they have also been reported to successfully breed in upland habitats, such as hayfields and dikeland of Atlantic Canada, a behavior not observed elsewhere in its range (Nocera et al. 2007a). Dikeland are areas that were once saltmarsh and exist below the high tide line; however, due to human-built raised dikes, these landscapes are now protected from tides. Compared with saltmarsh, dikeland have relatively higher diversity of plants and wildlife, including a variety of grasses, forbs, and shrubs, many of which are introduced species. Generally, saltmarshes of eastern North America have a low diversity of plants and wildlife due to high salinity, but high endemism (Greenberg 2006) and are dominated by cordgrass (Sporoborlus sp., formerly Spartina sp.; Mitsch and Gosselink 2015). Little is known about how or why A. n. subvirgata use human-made dikeland in the northern extent of their breeding range, or what the consequences might be for using dikeland as a breeding habitat.

Unlike most songbirds, A. n. subvirgata do not defend breeding territories (Shriver et al. 2010). Males have larger home ranges than females, and many individuals use different core areas in their home range depending on the tidal cycle (Shriver et al. 2010). Movement and home ranges of breeding A. n. subvirgata have not been investigated in landscapes where this subspecies uses both saltmarsh and dikeland, such as in Atlantic Canada. Previous research on A. n. subvirgata showed evidence of higher reproductive success in saltmarsh than dikeland, suggesting that saltmarshes are better quality breeding habitats (Nocera et al. 2007a). A previous investigation using the same study population as our study found higher densities of singing male A. n. subvirgata in saltmarsh habitats (88 males km⁻²) than in dikeland (25 males km⁻²; Rose 2013). Studies on other saltmarsh specialists suggest that several trade-offs exist between nesting in saltmarsh vs. farther inland, including food availability, competition, and risks of predation or flood (Greenlaw and Rising 1994; Shriver and Greenberg 2012).

To better understand how and why A. n. subvirgata uses both human-made dikeland and natural saltmarsh, we monitored the movements of individual sparrows through radio-tracking, nest success through nest monitoring, and diet through fecal samples. We hypothesized that home range sizes might differ between saltmarsh and dikeland due to habitat quality, with home ranges in the saltmarsh being smaller because individuals do not need to travel as much to find food resources or mates. We predicted, therefore, that individuals using mainly dikeland would have larger home ranges than individuals using mainly saltmarsh. We also hypothesized that A. n. subvirgata in this area nest in dikeland to escape the flood risk associated with nesting in saltmarshes. We therefore predicted that nest success would be higher in dikeland than saltmarsh. However, given that A. n. subvirgata densities appear to be higher in saltmarsh, we recognize that saltmarshes are still critically important to breeding A. n. subvirgata and that trade-offs likely exist for individuals that nest in dikeland. Specifically, we hypothesized that food resources are of better quality and/or more available in the saltmarsh, and that individuals using dikeland may remain close to saltmarsh so that they can take advantage of the food available in the adjacent saltmarsh. We, therefore, predicted that isotope analysis of fecal samples would show similar carbon values that would reflect individuals foraging in similar habitats (i.e., individuals from dikeland foraging in saltmarsh).

METHODS

Study Site

We conducted our study near Beaubassin Research Station (hereafter “Beaubassin”) in Aulac, New Brunswick (45°50ʹ51.08″ N, 64°17ʹ7.27″ W) from 2021 to 2023, and in Dorchester, New Brunswick (45°53ʹ32.01″ N, 64°32ʹ27.2″ W) in 2023 only. Beaubassin is surrounded by dikeland, with natural and restored saltmarshes along the shore of the bay (Figure 1). The site in Dorchester was added to the study after we found a single A. n. subvirgata nest there in 2023. The Dorchester site includes 2 large impoundments (human-made diked wetlands), directly adjacent to a large dike that separates a narrow saltmarsh along the Memramcook River (Figure 1). Beyond the impoundments are dikeland used for cattle pasture and hayfields. We did not conduct any radio-tracking or banding of adults at the Dorchester site, but monitored several nests at this site in 2023. The area we covered in Dorchester had a much narrower saltmarsh, measuring ~130 m wide, than the one near Beaubassin, where the saltmarsh extends up to 700 m from the dike. Although the dikeland or agricultural land area was over 7 km², we restricted our nest-searching efforts to the narrow area (~10% of the total) on or near the dike because these lands were publicly accessible.

Radio Telemetry and Home Range Estimation

A. n. subvirgata are difficult to monitor: they have soft songs, often sing and move within dense vegetation, and nest near the ground. We, therefore, used a combination of radio telemetry and color-banding to track female and male sparrows in 2021 and 2022. To capture sparrows, we set up multiple mist-nets in either dikeland or saltmarsh. In the saltmarsh, most nets were set inside ditches because A. n. subvirgata, when disturbed, would often fly into the ditches and could be coaxed into the net. In the dikeland, if a ditch was not present, we set multiple nets parallel to the dike or near shrubs or small trees where the net would be camouflaged. We had limited success with playback or passive netting and did not use these techniques beyond a few initial attempts.

Once captured, we tagged individuals with Lotek nanotags (Lotek NTQB2-2-M) registered with the Motus network (Taylor et al. 2017). In 2021, we used the clip-and-glue method (Raim 1978; Diemer et al. 2014), but switched to leg-loop harnesses (Rappole and Tipton 1991) in 2022 due to retention issues with the clip-and-glue method. The clip-and-glue method involved clipping some interscapular feathers, gluing a small piece of cotton fabric to the tag to improve adhesion, then gluing the tag to the clipped area on the individual’s back. The leg-loop harness method involved creating 2 adjacent loops out of elastic cord, then gluing the tag to the cord. Harnesses were sized to fit the individual; therefore, we made the harnesses in the field while holding the individual. All individuals successfully flew away after tagging and we confirmed that individuals continued their normal activities after relocating them using radio telemetry. The weight of the tags with the glue, cotton fabric, and cord was <0.4 g, which is <5% of handled individuals’ weights (range: 14.5–22 g). With the leg-loop harness method, we had one mortality due to entanglement of the tag antenna in grass, a problem that has been reported with this method in the past, particularly for species that spend a lot of time on the ground among grass (Hill and Elphick 2011; Choi et al. 2021).

We also banded each individual with a unique combination of color bands to ease identification during resighting, and one metal band with a unique federally issued number. For each individual, we collected information on weight (g), wing length (mm), fat score (0–5), age (i.e., hatch-year or after hatch-year), and sex, as determined by the presence of a cloacal protuberance or brood patch. In 2022, we also measured tarsus length (mm) and tail length (mm), and collected a fecal sample from each individual. All birds were handled and banded following animal use protocols of the University of New Brunswick (AUP #s: 21034, 22028, and 23025) and under banding permit #10801 E.

We recorded locations of individuals every 2–3 days throughout the breeding season and at different times of day. We used handheld radio telemetry equipment (Lotek SRX800 receiver and 8-element Yagi antenna) to ensure that the tag was on the individual by resighting the color bands or tag or flushing the individual from the location and confirming that the tag location moved. We also included locations of individuals that died or whose tags had dropped if we could confirm the color band combination (n = 68 locations). Once located, we marked the location using a handheld Garmin eTrex Global Positioning System (GPS) and recorded coordinates, time, and date. We did not track individuals on days with rain, heavy fog, or a high risk of lightning.

We created kernel density estimates derived from each individual’s utilization distribution using adehabitatHR, an R package for home range analysis (Calenge 2006). We only included individuals with ≥5 locations (n = 50), which is the minimum number of locations required by adehabitatHR to create home ranges. We used R package sp to convert the individuals’ locations to a SpatialPointsDataFrame object (Pebesma and Bivand 2005), one of the 2 data types that can be used with adehabitatHR. Then we used adehabitatHR to create 50% kernel density estimates to look at home ranges for each individual (with ≥5 locations; range: 5–26 locations; mean = 13 locations). The 50% kernel density estimate is usually considered representative of the core area of an individual, and the area where it spends most of its time. We also created kernel density estimates at the 95% level to represent the home range with 5% removed to deal with extreme values. We focused on the 50% core areas for analysis because the 95% home ranges included substantial overlap that made it difficult to interpret visually.

Nest Searching and Monitoring

In 2023, we searched for A. n. subvirgata nests in dikeland and saltmarsh habitats from 16 June to 23 August. Nest searching was both opportunistic (i.e., while completing other tasks in the field) and systematic (i.e., dedicated searching). Nearly all nests were found by first flushing the female off the nest. We used a variety of methods to find nests: walking through the habitat, rope-dragging, searching with the assistance of a thermal camera, and following a female carrying food. We found that a combination of multiple methods was the most useful. For example, in many cases, we would split up and walk around the habitat watching for behavioral signs of females with nests nearby, such as carrying food or flushing late (i.e., taking flight when someone was very close to the bird). When a female was encountered, we would leave a marking flag and return to the area with the rope. Then, after flushing the female close to the nest with the rope, we would use the thermal camera to try and pinpoint the exact location of the nest. We used an AGM Global Vision Thermal Monocular ASP-Micro TM160 to assist with nest-searching efforts. This follows the recommendation of Galligan et al. (2003) of pairing a handheld thermal camera with traditional nest searching methods. Once we found a nest, we marked the location with 2 flags placed 2 m on either side of the nest.

We checked nests once every 3 days, unless weather or logistics prevented a check on the third day, in which case we would check on the second or fourth day. During each nest check, we determined whether the nest was still active and gathered information on the number of eggs or nestlings, the age of nestlings, and whether eggs were warm or cold. We also noted whether the female flushed when we approached, which can suggest the nest is still active. If the nest was not active, we tried to determine the cause of failure; for example, if the nest itself was destroyed and eggs were smashed, we assumed it was depredated, or if the nest was wet and the area around the nest was flattened from a recent high tide, then we assumed the nest was flooded. We also monitored tides to determine when flooding was likely, and we monitored weather, particularly rain events, to determine when nest wetness might be due to rain rather than flooding. Two high tide events resulted in failures at many of our nests; on 6 July at ~0145 hours, the height of the tide at a local tidal station (Pecks Point, 45°45ʹ0″ N, 64°28ʹ58.8″ W) reached 13.02 m, and on 4 August at ~0130 hours, the tide reached a height of 13.28 m (Government of Canada 2024).

Nest Survival

We concluded whether each nest had failed or fledged based on observations at the nest. If we determined that at least one nestling had fledged, then we assigned a fate of “fledged” (i.e., a successful nest). For each failed nest, we assigned a nest failure type: flooded, depredated, or unknown. Flooded nests were due to high tides and could be determined by nest wetness during nest checks (especially if there was no precipitation in the past 24 hr) and by using tide tables. We considered a nest to be depredated if there were signs of disturbance at the nest or broken shells. If we could not determine the nest fate, we assigned a nest fate of “unknown.” Some nest failures occurred after an extreme rain event, and some of the “unknown” nest failures may have been due to this extreme weather event, which did not coincide with a particularly high tide. Nest failure due to heavy rains and winds has been reported in other tidal marsh nesting sparrows (Greenberg et al. 2006).

We weighed, measured, and collected fecal samples from nestlings once they were 7–8 days old. We used chick aging guides from the Saltmarsh Sparrow Research Initiative (Robinson 2021) and the Saltmarsh Habitat and Avian Research Program (SHARP 2021). The former reference guide is specific to A. caudacata, and the latter uses examples of A. caudacata, A. n. subvirgata, and A. maritimus (Seaside Sparrow). We found that our population of A. n. subvirgata was developmentally more advanced (by one day) than the examples in the 2 guides. We weighed nestlings (g), and measured wing chord (mm), tarsus (mm), and the length of the outermost primary feather (P9; mm).

We included 35 nests in our survival analyses, after removing a nest from the analysis that we suspected had already failed (and the clutch had not been completed by the female) at the time we found the nest. We created several models looking at nest success with a combination of 3 variables: exposure days, habitat, and nesting cycle. We measured exposure days from the first day an egg was laid, and therefore exposed to threats or could potentially fail, to the day the young successfully fledge. Habitat was a binary variable of saltmarsh or dikeland. The nesting cycle was also a binary variable: the first-cycle nests (nests that failed or fledged before 5 July), and second-cycle nests (nests that failed or fledged after 5 July). In our study area, the first series of very high tides since the beginning of the nesting season occurred around 5 July, and 6 saltmarsh nests failed. About 10 days after these high tides, we found 10 new nests, which we suspect were due to females re-nesting after failures. In 3 cases, the nests were <5 m from a recently failed nest. We considered any nest in the egg stage after 6 July to be a second-cycle nest. We included nesting cycle in the models because, although the nesting cycle of A. n. subvirgata is not known to be synchronized with lunar cycles (Shriver et al. 2007), it appeared that nest timing was at least loosely associated with the tide, and, therefore, we tested for an effect. For A. n. subvirgata, nesting cycle is estimated to be 23 days (4 days for egg laying, 9 days for incubating, and 10 days for fledging; Shriver et al. 2007).

We used a discrete proportional hazards approach and fit binomial regression models with a complementary log–log link and exposure days as an offset term to estimate daily nest survival probability (de Zwaan and Martin 2018). We compared models using corrected Akaike’s Information Criterion (AIC_c), which corrects for small sample sizes to avoid overfitting (Hurvich and Tsai 1993), and we considered models to have different support if ΔAIC_c was >2. We used the complementary log–log link because it fit the data better than a logistic curve; the complementary log–log link is appropriate for time series data when the timing of an event (e.g., nest failure) cannot be precisely determined but can be assigned to a time interval (e.g., a calendar day; Heisey et al. 2007). We used parameter estimates from the best-supported model to calculate overall nest success. We then back-calculated daily nest survival by taking the inverse exponential of the overall nest success based on the average nesting cycle for A. n. subvirgata.

Diet

We collected fecal samples from all adults banded in 2022 and all nestlings banded in 2023 to determine if adults were foraging in saltmarsh or dikeland habitats, both for their own dietary needs and to feed nestlings. To determine the foraging location, we analyzed δ¹³C. We also looked at δ¹⁵N values, which provide information about the trophic position of prey items. In 2022, adult sparrows were placed in a brown paper bag instead of the usual cloth bag. We placed all nestlings from the same nest into a single paper bag. Usually, individuals released a fecal sample into the bag before we removed them to band, tag, and measure. The paper bags with the samples were then labeled and frozen.

At the Stable Isotope in Nature Laboratory (SINLAB) at the University of New Brunswick, we analyzed carbon isotopic signatures of fecal samples from adult and nestling A. n. subvirgata collected in 2022 (adults: n = 27) and 2023 (nestlings: n = 16). Samples were dried in an oven at 60°C for 48–72 hr, then were ground, prepared in tin capsules (8 × 5 mm; Elemental Analysis), and weighed (±0.001 g). We submitted 49 samples, of which 4 were blind duplicates, to the SINLAB which used continuous flow-isotope ratio mass spectrometry to obtain δ¹³C and δ¹⁵N ratios. To ensure the quality and comparability of data, samples were measured alongside standard and reference materials. Carbon and nitrogen ratios were calibrated to international standards (Vienna Peedee Belemnite for carbon, atmospheric air for nitrogen), and are reported in delta notation (δ) parts per thousand (per mille, ‰). The SINLAB calculated analytical error as 0.2‰ for δ¹³C and 0.3‰ for δ¹⁵N based on repeat analyses of certified standard materials: bovine liver standard (developed by SINLAB, δ¹³C = −18.76‰, δ¹⁵N = 7.17‰), muskellunge muscle (developed by SINLAB, δ¹³C = −22.30‰, δ¹⁵N = 14.00‰), nicotinamide (batch 237264, δ¹³C = −32.53‰, δ¹⁵N = 2.10‰). These standards were calibrated against secondary reference materials: USGS61 (caffeine, certified by USGS, δ¹³C = −35.05‰, δ¹⁵N = −2.87‰), CH7 (polyethylene foil, certified by IAEA, δ¹⁵N = 20.3‰), N2 (ammonium sulfate, certified by IAEA, δ¹³C = −32.15‰).

In habitats like dikeland and saltmarsh, which are isotopically different, δ¹³C values from fecal samples can provide information on where the individual was foraging because of dominant photosynthetic pathways by freshwater or terrestrial (C₃) and saltmarsh (C₄) vegetation (Brittain et al. 2012). Lower δ¹³C values would suggest a diet with a terrestrial or freshwater origin indicative of vegetation using the C₃ photosynthetic pathway, while higher δ¹³C values suggest a diet with a saltwater origin from vegetation using the C₄ photosynthetic pathway (Kelly 2000). Nitrogen values provide information on the trophic position of prey items, which can be used as a proxy for food quality, as protein content is generally positively correlated with trophic position in arthropods (Wilder et al. 2013).

To compare carbon and nitrogen values from nestling fecal samples between habitats, we used Welch’s 2-sample t-tests with ɑ = 0.05. We did not run analyses on fecal samples from adults because sample sizes for individuals using mainly dikeland (n = 4) or using both habitats (n = 2) were too small, so we instead report the means.

Vegetation Surveys

We sampled 300 vegetation quadrats (0.5 m × 0.5 m²; Jones et al. 2021) at 100 locations between 6 June and 15 June 2023, when females were choosing nest site locations for first-cycle nests. In each quadrat, we made the same measurements at 5 points: each of the 4 corners, and the center of the quadrat using a meter-long ruler (SHARP 2019). For each point in the quadrat, we measured the average height of vegetation crown (an estimate based on the height of continuous vegetation around the ruler; cm), species and height (cm) of tallest plants, and thatch depth (cm). We defined thatch as the horizontal layers of dead vegetation from previous years. We randomly selected one location for each adult A. n. subvirgata that was tagged in 2021 and 2022 and had at least 5 locations based on radio-tracking or resighting. Therefore, half of the vegetation quadrats were completed at a location where a unique individual was observed during the 2021 or 2022 breeding season (n = 50). For the other 50 quadrat locations, we created a polygon outlining the study area, then overlayed a grid of points on the polygon with 100-m spacing. We used random sampling with no replacement to select locations and removed any points that were located in water. To assess differences in vegetation structure between saltmarsh and dikeland habitats, we used Welch’s 2-sample t-tests to compare means and used F-tests to compare variances, with ɑ = 0.05.

All analyses were completed in R (version 4.3.1, R Core Team 2023). All outliers were defined as values above Q3 + 1.5*IQR or below Q1 – 1.5*IQR (interquartile range).

RESULTS

Home Ranges

In 2021, we banded and tagged 43 individuals (11 females and 32 males), and we recaptured 6 individuals and retagged 5 of those (only 1 had retained the tag when recaptured). In 2022, we banded and tagged 33 individuals (7 females and 22 males) and recaptured 2 individuals from 2021, which we retagged, and another 2 from 2022 that still retained their tags. In 2021, we recorded locations of 40 A. n. subvirgata (9 females, 31 males). In 2022, we recorded locations of 40 A. n. subvirgata (7 females, 33 males). In total, we recorded locations of 72 individuals, with 8 banded individuals from 2021 also being tagged or resighted in 2022.

Kernel density estimates set to the 50% level showed A. n. subvirgata core areas were 0.07–29.04 ha with a mean core area size of 3.01 ha (Figure 2). Set at the 95% level, A. n. subvirgata home ranges were 0.30-215.11 ha with a mean of 22.17 ha (Table 1). Among the 50 individuals included in the home range analyses, only 9 were females. The mean core area and home range sizes for males were significantly larger than females (core area; t_46.0 = −2.36, P = 0.02 and home range; t_47.9 = 2.11, P = 0.03).

We divided observations of individuals into those observed mainly (i.e.,>50% of locations) or only in saltmarsh, and those who were observed mainly or only in dikeland. Nine individuals were observed mainly in dikeland and 41 individuals were observed mainly in saltmarsh (coincidentally matching the female–male ratio). There was no significant difference in mean core areas (t_10.6 = −1.32, P = 0.21) or home range (t_10.2 = −1.13, P = 0.29) sizes between dikeland and saltmarsh (Table 1).

There was substantial overlap in A. n. subvirgata core areas (Figure 2), even at the 50% level. The A. n. subvirgata core areas and home ranges were clustered in 2 main areas: the main saltmarsh bordered by the Bay of Fundy and a narrower saltmarsh between the Missaquash River and dikeland. Dikeland west of the Missaquash River showed no A. n. subvirgata home ranges and we observed no individuals there during tracking efforts. This absence of A. n. subvirgata farther inland in the dikeland suggests that individuals tended to stay close to the edges of the dikeland (i.e., close to saltmarsh). Conversely, the southeast section of the main saltmarsh that includes only one core area (light green in Figure 2) is an area where several A. n. subvirgata were observed. However, it was beyond the limits of our banding and tagging focal areas. This single individual was a male that was banded closer to where the other core areas were located but it moved beyond the previous extent of our study area.

Nest Monitoring and Survival

We found 36 A. n. subvirgata nests in 2023 (27 in saltmarsh, 9 in dikeland). Overall, 12 (33%) of those nests successfully fledged at least 1 young and 24 (66%) nests failed. Six (22%) of the nests in saltmarsh were successful and 21 (78%) failed. Six (67%) of the dikeland nests were successful and 3 (23%) failed.

After model selection (Table 2), we used parameters from the most-supported parsimonious model to estimate nest success in each habitat and both habitats combined. Across both habitat types, we estimated nest success to be 50.2 ± 2.6%. We estimated nest success to be 36.2 ± 1.1% in saltmarsh and 81.5 ± 1.7% in dikeland, which indicated a higher estimated success for nests in the dikeland than in saltmarsh (β = 1.3, P = 0.02). We estimated daily nest survival for both habitats combined as 0.970 (±0.002), for saltmarsh nests as 0.957 (±0.002), and for dikeland nests as 0.991 (±0.002), such that daily nest survival was higher for nests in dikeland than in saltmarshes (Figure 3).

Diet

Isotope analysis of fecal samples from adult and nestling A. n. subvirgata revealed that individuals in dikeland and saltmarsh have different diets. Fecal samples from adult sparrows using mainly saltmarsh had a δ¹³C of −16.1‰ (±2.3‰), and for those from mainly dikeland habitats was −21.4‰ (±5.3‰); however, this difference was not statistically significant (t_3.2 = 2.0, P = 0.13). We were only able to obtain fecal samples for 4 adults and one of these was an outlier; therefore, results must be interpreted cautiously (Figure 4).

Nestling diet was significantly different between saltmarsh and dikeland nests (t_2.7 = 9.7, P = 0.02; Figure 4). The mean δ¹³C value for saltmarsh nestlings was −16.1‰ (±2.1‰) and for dikeland nestlings was −19.9‰ (±3.3‰), such that nestlings in the dikeland have a diet made up of invertebrates in a terrestrial or freshwater environment (i.e., dikeland) and saltmarsh nestlings have a diet made up of invertebrates from a saltwater environment (i.e., saltmarsh). There was no difference in δ¹⁵N values between saltmarsh and dikeland nests (t_−0.2 = 10.1, P = 0.85), such that A. n. subvirgata nestling diets in both habitats were made up of prey items with similar trophic position and protein content.

Vegetation

There were differences in vegetation composition and structure between saltmarsh and dikeland habitats (Figure 5). The average height of vegetation and the tallest height of vegetation per quadrat were both higher in dikeland compared to saltmarsh (average height, t₁₇₀ = 6.2, P = 4.692e⁻⁹; tallest plant height, t₁₇₄ = 6.3, P = 4.579e⁻⁹). Thatch was deeper in dikeland than in saltmarsh (t₁₇₈ = 2.2, P = 0.03). Sporobolus pumilus (formerly Spartina patens) was the most common species observed in saltmarsh, with considerably more observations than any other saltmarsh species. Other commonly recorded species in the saltmarsh include Sporobolus alterniflorus (formerly Spartina alterniflora), Triglochin maritima, and Juncus gerardii, which are all native saltmarsh species. In dikeland, Alopecurus pratensis was the most common species observed, along with Bromus inermis and Phleum pratense. All 3 of these species are non-native Eurasian grasses. Many of our surveys were on or near dikes where the vegetation transitions between saltmarsh and dikeland communities, therefore, S. pumilus and A. pratensis were often recorded in dikeland and saltmarsh, respectively.

In dikeland, A. n. subvirgata were observed in areas with higher average and tallest vegetation height compared to randomly selected locations (average height, t₆₂ = −2.2, P = 0.03; tallest height, t₆₅ = −2.1, P = 0.04). Thatch depth in dikeland did not differ between locations where we observed sparrows and random locations (t₅₈ = 0.3, P = 0.78.). In saltmarsh, differences were more pronounced between areas with sparrows and randomly selected areas. We observed A. n. subvirgata in locations with lower average (t₈₁ = 4.3, P < 0.001) and tallest vegetation (t₈₃ = 4.3, P < 0.001), and shallower thatch depth (t₉₈ = 4.7, P < 0.001).

There was a difference in the variance of vegetation data in locations where individuals were observed compared with random locations in the saltmarsh. Average vegetation height, tallest vegetation height, and thatch depth were less variable where we observed A. n. subvirgata compared with random locations, such that, the locations where we observed sparrows in the saltmarsh were structurally similar (average height: F_{70, 119} = 2.99, P < 0.001; tallest: F_{71, 119} = 8.24, P < 0.001; thatch depth: F_{71, 119} = 7.28, P < 0.001). For dikeland, the variances were equal between with A. n. subvirgata observed and randomly selected locations, such that the habitat being used by individuals had similar structural diversity to randomly selected areas across the dikeland (average height: F_{74, 29} = 1.34, P = 0.39; tallest: F_{73, 29} = 1.45, P = 0.27; thatch depth: F_{74, 29} = 1.19, P = 0.62).

DISCUSSION

Our study showed that A. n. subvirgata partition their use of saltmarsh and dikeland habitats differently. We observed higher use of saltmarsh than dikeland, with most tagged individuals spending all or some of their time in saltmarsh. However, tracking data also showed that some individuals may be only using dikeland. Home range sizes did not differ between the 2 habitat types which could mean that individuals may fulfill all their needs during the breeding season in either of the 2 habitats. Home range sizes were larger for males than for females, which is expected since females likely remain close to their nest. Males do not defend territories and contribute no parental care (Shriver et al. 2010); therefore, they had larger home ranges than females. A movement study on A. n. subvirgata in Maine found similar differences in male and female home ranges, although mean sizes of home ranges were considerably larger in that study than in ours (Shriver et al. 2010). This difference in home range sizes between study areas could reflect differences in habitat quality, resource availability, or density of potential mates.

Success and daily nest survival rates were considerably higher in dikeland compared with saltmarsh. Including habitat in the models improved fit, and there were multiple lines of additional evidence to suggest that habitat is an important factor in nest survival rates for A. n. subvirgata breeding in southeastern New Brunswick. Higher nest survival rates in dikeland than saltmarshes fit the pattern seen in other sparrows that have tidal marsh and inland breeding populations. For example, populations of Melospiza melodia and M. georgiana that nest in tidal marshes had lower nest success than their inland counterparts (Greenberg et al. 2006).

Overall nest success for A. n. subvirgata in 2023 was 50.2%. In a study on the breeding ecology of A. n. subvirgata and A. caudacata in Maine, Shriver et al. (2007) estimated nest success for A. n. subvirgata to be 25.3%. The estimated nest success for saltmarsh in our study was 36.2%, which is slightly higher than the value reported by Shriver et al. (2007). Shriver et al. (2007) studied A. n. subvirgata and A. caudacata in the hybrid zone for these 2 species, which may explain some of the differences in nest success between the 2 populations. For tidal marsh nesting songbirds, Greenberg et al. (2006) reported average nest survival to be 31% with a range of 5–64%; nest survival rates in our study fall in the middle of this range, however, data from these 2 studies are now nearly two decades old and may not reflect current nest success rates in those areas.

Although we did not find new nests after 25 July, we observed evidence of possible third-cycle nests (i.e., new nests after the high tides around 5 August), which included females carrying food in mid-to-late August. This could mean that some females undertook a third nesting attempt after having 2 unsuccessful previous attempts or some females attempted a second brood much later in the season. A. n. subvirgata in Maine were observed renesting within 10 days after nest failure (Shriver et al. 2007), and we observed females renesting within meters of a recently failed nest. Second-brooding has been observed in Maine (Shriver et al. 2007), but we cannot confirm this happened at our study site. The deviance explained by adding nesting cycle to the models was negligible. A. n. subvirgata appear less synchronized with the lunar cycle than A. caudacata (Shriver et al. 2010); therefore, nesting cycle may have less influence on overall nest success for A. n. subvirgata. Additional research on A. n. subvirgata breeding ecology in the Maritimes could elucidate the importance of nesting cycle to nest success.

Tracking data showed that A. n. subvirgata in dikeland will also use saltmarsh or areas close to saltmarsh. For example, we observed female sparrows carrying food on 2 occasions from the saltmarsh to their nests in dikeland. We, therefore, expected that isotopic analyses of fecal samples would show little difference in carbon values between adults or nestlings from saltmarsh and dikeland habitats. We predicted that individuals nesting or occupying areas near saltmarsh would take advantage of better quality or more abundant food resources in saltmarshes. Instead, isotopes revealed clear differences in diets of nestlings in dikeland vs. saltmarsh nests. Due to small sample sizes, results from diet analyses for adults should be interpreted cautiously; however, 3 of the 4 adult fecal samples from dikeland had carbon values much lower than any of the adults observed using mostly saltmarsh, which suggests that there may be a difference in the diets of adult sparrows, similar to nestlings. More fecal sample data might elucidate if a true difference exists. Results from nestlings suggest that A. n. subvirgata using dikeland during the breeding season are mainly feeding their nestlings invertebrates from dikeland sources and not provisioning most food items from saltmarshes. Previous research on A. n. subvirgata breeding in dikeland found a strong positive relationship between A. n. subvirgata abundance and hayfield drainage ditches (Nocera et al. 2007b). These drainage ditches often occur directly adjacent to the dikes that separate dikeland and saltmarsh; therefore, A. n. subvirgata in dikeland are possibly choosing locations close to the dikes and saltmarsh to take advantage of these drainage ditches rather than the nearby saltmarsh habitat. Nitrogen values between saltmarsh and dikeland nests were similar indicating no difference in trophic position of prey items consumed from saltmarsh and dikeland. The amount of protein in arthropods is associated with trophic position (Wilder et al. 2013); therefore, similar nitrogen values could suggest that birds can find similar quality food in both habitats. We cannot say, however, if food availability is similar between habitats. Future research on this population might investigate whether food availability is similar and if the amount of time that females must spend away from the nest while foraging differs between saltmarshes and dikeland.

We found differences in vegetation structure between saltmarsh and dikeland, as expected. The average height, tallest height of vegetation, and thatch depth differed between saltmarsh and dikeland, with a much stronger relationship between vegetation height and habitat than for thatch depth and habitat. For vegetation heights, this difference becomes increasingly evident in late summer, when the vegetation reaches its maximum height, which is significantly higher in dikeland than in saltmarsh. Future research comparing saltmarsh and dikeland habitat might consider repeating vegetation surveys later in the breeding season to better represent the difference in these 2 habitats. Taller vegetation in the dikeland may provide better cover to ground-nesting sparrows and reduce predation risk. A study on A. caudacata and A. maritimus found evidence for tidal marsh sparrows selecting habitats with a deeper thatch layer (Gjerdrum et al. 2005). We observed that many nests, particularly those in the saltmarsh, were built into the thatch layer with the upper layer of thatch often acting as a partial canopy. The thatch layer was significantly deeper in dikeland than in saltmarsh but with a low effect size; therefore, the difference in thatch may be smaller than in vegetation heights, but it could still influence nest site locations if females prefer to nest in deeper thatch.

Our study can provide several recommendations for management and future research. Given the higher densities of A. n. subvirgata in saltmarsh compared to dikeland, and their higher use of saltmarsh, we recommend that saltmarsh conservation and restoration remain a high priority. Global losses of saltmarsh habitats by the end of the 21st century are estimated to be 40–95% (Valiela et al. 2018). Anthropogenic climate change is increasing the number and severity of storms, which could result in higher rates of nest loss for tidal marsh nesting species (Michener et al. 1997). Management of saltmarsh habitats should, therefore, consider climate projections and future sea-level rise. There are still many knowledge gaps with regard to A. n. subvirgata, particularly the population that we studied in Atlantic Canada. Future research should focus on resource availability and habitat quality of dikeland and saltmarsh for A. n. subvirgata to provide clear management recommendations for this species. It would also be useful to examine fledgling survival and habitat use, as very little is known about this species. Although A. nelsoni is considered globally as Least Concern (BirdLife International 2024), the species is on multiple watch lists and priority lists (Nocera et al. 2007a), and the population of A. n. subvirgata in the northeastern United States declined annually by 4.2% from 1994 to 2012 (Correll et al. 2017). Additionally, the closely related A. caudacata that occupies slightly lower elevations of saltmarsh habitats in the United States is globally listed as Endangered (BirdLife International 2024) and is estimated to be extinct by the middle of the century (Field et al. 2017, 2019). Proactive planning is required to keep A. n. subvirgata from following the same trajectory as A. caudacata. There is concern for the future of the Bay of Fundy’s dikeland and associated saltmarshes, with many restoration projects completed or underway (Boone et al. 2017). Understanding the resource requirements of A. n. subvirgata may provide useful information to be considered during land and restoration decisions, which could improve persistence of A. n. subvirgata populations.

Coastal habitat loss, due to sea-level rise and anthropogenic changes, is affecting coastal nesting species globally (Rush et al. 2009, Valiela et al. 2018, Klingbeil et al. 2021). A. n. subvirgata provides an example of how a tidal marsh breeding species has responded and adapted to historic coastal habitat loss by exploiting human-made habitats. This highlights the behavioral plasticity of the A. n. subvirgata, and the potential importance of alternative habitats close to coastal areas where habitat loss has occurred. Habitat restoration or human-made habitats may provide refuge to populations experiencing habitat loss due to anthropogenic changes or sea-level rise. In the southern part of the A. n. subvirgata breeding range, alternative habitats are very limited, therefore, with increasing habitat loss, A. n. subvirgata breeding in New England may not have as many opportunities to adapt to alternative nesting habitats as those using dikeland in Atlantic Canada. This emphasizes the importance of saltmarsh protection, restoration, and the creation or conservation of alternative habitats that meet the needs of breeding A. n. subvirgata. Currently, coastal areas along the Bay of Fundy appear less affected by sea-level rise than other coastal areas (Saintilan et al. 2022). However, as sea levels continue to rise, dikeland could become increasingly important to breeding A. n. subvirgata in Atlantic Canada. The Canadian population of A. n. subvirgata may provide insights to conservation managers and researchers regarding how breeding birds can adapt to novel habitats in the face of ongoing change.

Acknowledgments

We wish to recognize the extensive efforts put forth by S. Neima, H. Drake, E. Peacock, R. Schweighardt, F. Maika, and J. Eckerson in the field, as well as countless volunteers who contributed to field data collection. We thank D. de Zwaan for assistance with analyses, and Dr. Chris Elphick and the reviewers who provided comments on the manuscript. We also thank the SINLAB for the stable isotope analysis of fecal samples.

Funding statement

Funding for this research was provided by Mitacs and Ducks Unlimited Canada through a Mitacs Accelerate Fellowship, as well as from the Society of Canadian Ornithologists through the Taverner Award to Kiirsti Owen.

Ethics statement

All field activities were conducted within the guidelines of the University of New Brunswick animal care protocol #s. 21034, 22028, and 23025 and under banding permit #10801 E.

Author contributions

K.C.O. was responsible for concept, methodology, data collection, and analyses. K.C.O., M.L.M., N.R.M., and J.J.N. were responsible for manuscript writing and editing. N.R.M., M.L.M., and J.J.N. contributed materials, resources, and funding.

Conflicts of interest statement

We declare no conflicts of interest.

Data availability

Analyses reported in this article can be reproduced using the data provided by Owen et al. (2024).

LITERATURE CITED