Isotopic

J. Field Ornithol. 86(3):213–228, 2015

DOI: 10.1111/jofo.12112

Isotopic (!2 Hf ) evidence of “loop migration” and use of the Gulf of Maine Flyway by both western and eastern breeding populations of Blackpoll Warblers Rebecca L. Holberton,1,4 Steven L. Van Wilgenburg,2 Adrienne J. Leppold,1 and Keith A. Hobson3 Laboratory of Avian Biology, School of Biology and Ecology,University of Maine, Orono, Maine 04469, USA 2 Canadian Wildlife Service, Environment Canada, Saskatoon SK S7X OH4, Canada 3 Wildlife and Landscape Science, Science and Technology Branch,Environment Canada, Saskatoon SK S7N 3H5, Canada 1

Received 19 January 2015; accepted 30 May 2015 ABSTRACT. Declining numbers of Blackpoll Warblers (Setophaga striata) have been documented at long-term migration monitoring sites as well as in breeding areas. However, the “loop migration” of Blackpoll Warblers makes it difficult to ascribe population change at migration monitoring sites to specific breeding populations. Individuals from all populations across the breeding range of Blackpoll Warblers concentrate in fall along the Atlantic coastline of eastern North America prior to initiating a transoceanic flight to wintering areas. In spring, Blackpoll Warblers return along a different route, moving north into the southeastern United States where birds from eastern and western breeding populations then diverge during migration to reach their respective breeding areas. To monitor breeding populations outside of breeding areas and identify factors potentially affecting those populations, we must be able to identify where birds captured during migration breed and map seasonal variation in population-specific flyways. To “map” population-specific migration movements of Blackpoll Warblers, we used feather deuterium (!2 Hf ) values and a spatially explicit model to assign molt origins of 289 Blackpoll Warblers moving through sites in the Gulf of Maine (GOM) region and at three locations further west and south (northern Great Lakes area, Pennsylvania, and Florida). The assignment method was validated with feather samples from 35 birds captured during the breeding season at Churchill, Manitoba, Canada. As predicted, the spatial pattern of movement within and between seasons reflected “loop migration.” Blackpoll Warblers captured during fall migration in the GOM region included birds from across their breeding range, whereas birds captured during the spring were exclusively from northeastern populations. During fall migration, Blackpoll Warblers captured at two sites west of the GOM were from breeding areas further northwest than those from western Canada that were captured in the GOM. Blackpoll Warblers captured in eastern Florida during spring migration were assigned exclusively to breeding areas in the northeast, suggesting that eastern and western populations diverge soon after entering the United States. Finally, most Blackpoll Warblers sampled at Manomet Bird Observatory originated from breeding populations in Alaska and western Canada that have shown a similar (70–90%) decline over the same period. Our results, therefore, not only document the “loop migration” pattern of Blackpoll Warblers, but, by mapping patterns of connectivity between breeding and non-breeding areas, may help target conservation efforts for breeding populations of Blackpoll Warblers where most needed. ´ ´ RESUMEN. Evidencia isotopico (!2 Hf ) de un “circuito de migracion” y el uso de una ruta migratoria por el Golfo de Maine por ambas poblaciones reproductoras occidentales y orientales de Setophaga striata En los sitios de monitoreo de la migración a largo plazo, y también en las a´reas de reproducción, se han documentado la disminución del número de Setophaga striata. Sin embargo, el “circuito de migración” de S. striata hace dif´ıcil atribuir los cambios de demográficos a poblaciones reproductoras espec´ıficas en los sitios de monitoreo de migración. Antes de iniciar un vuelo transoceánico a las a´reas de invernada, individuos de S. Striata, de todas las poblaciones de todo el a´rea de reproducción, se concentran en el otoño a lo largo de la costa atlántica del este de América del Norte. En primavera, S. striata regresan por una ruta diferente, moviéndose al norte, entre el sureste de los Estados Unidos donde las poblaciones reproductoras del este y oeste se divergen durante la migración para llegar a sus respectivas a´reas de reproducción. Para monitorear poblaciones reproductoras fuera de las a´reas de reproducción e identificar los factores que puedan afectar a esas poblaciones, debemos ser capaces de identificar donde las aves, que fueron capturadas durante la migración, se reproducen y a mapear la variación estacional en las rutas migratorias espec´ıficas a cada población. Para mapear los movimientos migratorios de cada poblacion de S. striata, utilizamos los valores de deuterio (!2 Hf ) en las plumas y un modelo espacialmente expl´ıcito para asignar or´ıgenes de muda de

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Corresponding author. Email: [email protected]

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S. striata en movimiento a través de sitios en la región del Golfo de Maine (GOM) y en tres lugares más al oeste y al sur (norte de la zona de los Grandes Lagos, Pensilvania y Florida). El método de asignación fue validado con muestras de plumas de 35 aves que fueron capturadas durante la temporada de cr´ıa en Churchill, Manitoba, Canadá. Como se predijo, el patrón espacial de movimiento dentro y entre temporadas refleja el “circuito de migración.” Individuo S. striata capturadas durante la migración de otoño en la región del Golfo de Maine incluyo las aves de toda su a´rea de reproducción, en cambio, las aves capturadas durante la primavera eran exclusivamente de las poblaciones del noreste. Durante la migración de otoño, S. striata capturadas en dos sitios al oeste del Golfo de Maine eran de zonas de cr´ıa más al noroeste que las aves de Canadá occidental, lo cual fueron capturados en el Golfo de Maine. S. striata capturadas en el este de la Florida durante la migración de primavera fueron asignados exclusivamente a las zonas de reproducción en el noreste, lo que sugiere que las poblaciones orientales y occidentales se divergen apenas después de entrar a los Estados Unidos. Por u´ ltimo, la mayor´ıa de las S. striata muestreados en el Observatorio de Aves Manomet originaron de las poblaciones de cr´ıa en Alaska y el oeste de Canadá que han mostrado una disminución de población (70–90%) similar en el mismo per´ıodo. Nuestros resultados, por lo tanto, no sólo documentan el patrón de un "circuito de migración” de S. striata, pero, por los patrones de asignación de conectividad entre a´reas de reproducción y no reproductores, puede ayudar a orientar los esfuerzos de conservación a las poblaciones reproductoras de S. striata donde más se necesitan. Key words: conservation, deuterium, loop migration, Setophaga striata

Like many species of Neotropical migrants, populations of Blackpoll Warblers (Setophaga striata) have experienced precipitous declines over the past few decades (Sauer et al. 2014). Trends for different breeding populations vary widely, however, across the United States and Canada, with some populations apparently remaining stable and others either increasing or decreasing (Crewe et al. 2008). Blackpoll Warbler population declines have been most apparent in western North America, with Alaskan Breeding Bird Survey (BBS) data indicating a 2.9% average annual decline from 1980 to 2005, and an overall decline of as much as 54% (Sauer et al. 2005, as cited in Alaska ADF&G 2006). Like many species, the number of Blackpoll Warblers observed during migration at longterm monitoring sites has also declined (LloydEvans and Atwood 2004, Dunn et al. 2006). Without knowing where and when specific breeding populations move during migration, determining the factors driving these trends is difficult (Osenkowski et al. 2012, Hobson et al. 2015). The Gulf of Maine (GOM) region, which extends from the southern coast of Nova Scotia to Cape Cod Bay in Massachusetts, has long been known as a major migration flyway for many bird species, including Blackpoll Warblers (cf. McClintock et al. 1978, Richardson 1978). From 1970 to 2002, however, numbers of Blackpoll Warblers captured during spring and fall migration at Manomet Bird Observatory in coastal Massachusetts have declined by !70% (LloydEvans and Atwood 2004, Miller-Rushing et al. 2008). Too few banded birds have been

recovered to reveal the breeding or wintering origins of Blackpoll Warblers on stopover at Manomet Bird Observatory, but such connectivity information is needed to link declines in numbers of Blackpoll Warblers observed during migration with events occurring during other stages of the annual cycle (Butler 2000, Marra et al. 2006, Hobson et al. 2014). Identifying breeding (or wintering) origins of birds during migration not only provides a greater understanding of within and across-species variation in migration strategies and overall life history evolution, but also allows management efforts to be directed more effectively throughout a population’s life cycle (Hobson et al. 2014). Our objective was to identify breeding/natal origins of Blackpoll Warblers moving through the GOM region (coastal Maine, New Hampshire, and Massachusetts) and, opportunistically, at three locations outside the region (northern Great Lakes region, Pennsylvania, and Florida) during spring and/or fall migration to better define population-specific and seasonally distinct migratory routes. To do so, we used stable hydrogen isotope ratios in feathers (!2 Hf ), as markers of biogeographic origin (Hobson and Wassenaar 2008), collected from birds captured at banding sites; we also used carcasses of birds killed by flying into buildings or towers. We applied a spatially explicit likelihood-based assignment model (Wunder 2008, 2010) to create a map representing probable breeding origins for each group of samples collected at each site (Hobson et al. 2009, Van Wilgenburg and Hobson 2011). We predicted that the spatial pattern of different breeding populations of Blackpoll

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Warblers detected at our sampling sites within and beyond the GOM region would reflect the pattern of “loop migration” common among Neotropical as well as Eurasian–African migrants (Newton 2008, Tøttrup et al. 2012, La Sorte et al. 2014). Specifically, we predicted that birds sampled during fall migration in the GOM region would show greater diversity in breeding or natal origin than birds sampled at the same sites in spring. This pattern would reflect the fact that all populations of Blackpoll Warblers concentrate along and depart from the eastern United States during fall migration before making a transoceanic flight to South America (Baird 1999). We also expected that birds sampled during fall migration at locations west of the GOM would carry isotope signatures indicative of exclusively western populations because birds originating in the east should remain east in fall to reach coastal departure areas. In spring, however, Blackpoll Warblers move north from South America through the Caribbean, entering the United States primarily through Florida (see DeLuca et al. 2013). If eastern and western populations diverge soon after entering the United States, representing the most efficient flight paths for the different breeding populations (e.g., Alerstam and Lindström 1990), we predicted that not only would we detect few, if any, western Blackpoll Warblers moving through the GOM region during spring migration (in contrast to fall), but that samples collected from Blackpoll Warblers along Florida’s east coast would consist primarily of birds assigned to eastern populations. A final objective was to identify the breeding/natal origins of Blackpoll Warblers captured during migration at Manomet Bird Observatory to determine if they were from breeding populations known to be experiencing declines. METHODS

We collected 263 feather samples (a single rectrix number 3 or 2–3 crown or back feathers, see Pyle 1997; both feather types yield similar keratins and, thus, origin-derived isotopic signatures, see Mazerolle and Hobson 2005) from adult and young (Spring: Second Year = SY; After Second Year = ASY; Fall: Hatching Year = HY, After Hatching Year = AHY; Breeding: AHY, newly fledged young = HY) Blackpoll Warblers (sexes pooled) captured (and

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subsequently released) at the (1) Appledore Island Banding Station (hereafter Appledore Island, 2010) and Metinic Island (2009–2010), both located !10 km from the mainland (New Hampshire and Maine, respectively) and 157 km apart, (2) Manomet Bird Observatory (hereafter Manomet, 2008–2009) on the southern coast of the GOM in Manomet, Massachusetts, (3) Powdermill Avian Research Center (hereafter Powdermill, 2007–2008) in Rector, Pennsylvania, in the southwestern corner of the state, and (4) South Florida Bird Observatory (hereafter South Florida, 2009–2010) located in the Bill Baggs Cape State Park in Key Biscayne, Florida (Table 1). Additional feather samples were obtained from 26 Blackpoll Warblers killed in collisions with communication towers or buildings during fall migration at locations within 100 km of Boston, Massachusetts (1972–1974), and within 250 km of Eau Claire, Wisconsin (1967–1972) (Table 1). All birds were aged by the degree of skull ossification, flight feather wear, and/or molt limit patterns, as seasonally appropriate, and few birds were not assigned to a seasonally appropriate age class (Pyle 1997). To validate our assignment method, we collected feathers from 35 Blackpoll Warblers (adults and newly fledged young) during the breeding season at Churchill, Manitoba, Canada (2003 and 2008). We were able to include samples collected across the broad time frame (1967– 2010) represented in our collection because !2 H values are obtained from the stable, nonexchangeable H portion of the feather keratin and, therefore, are stable over extensive time periods (Leyden et al. 2006, Hobson et al. 2010). Stable isotope analysis. Prior to laboratory analyses, feathers were cleaned to remove surface oils using a 2:1 chloroform:methanol solvent rinse and prepared for !2 H analysis at the Stable Isotope Laboratory of Environment Canada, Saskatoon, Canada. We employed the comparative equilibration approach described in Wassenaar and Hobson (2003) to determine the !2 H value of the non-exchangeable hydrogen of feathers by using two calibrated keratin hydrogen-isotope reference materials (CBS: −197‰, KHS: −54.1‰). We performed hydrogen isotopic measurements on H2 gas derived from high-temperature (1350ºC) flash pyrolysis of 350 ± 10 "g feather subsamples in a Eurovector elemental analyzer (Milan, Italy) coupled with a VG Isoprime mass spectrometer

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Table 1. Locations, sampling periods and numbers (including age class) of Blackpoll Warblers sampled during migration and on the breeding grounds. Sitea SFBO AIS MEIS

Latitude 25.69°N 42.99°N 43.88°N

MBO PARC BOS WISC CH

41.57°N 40.16°N 42.35°N 44.57°N 58.78°N

Longitude N (Age groupb ) 80.16°W 56 (33 SY, 23 ASY) 70.62°W 23 (14 SY, 9 ASY) 69.13°W 25 (19 SY, 6 ASY) 41 (25 HY, 16 AHY) 70.40°W 64 (29 HY, 29 AHY, 6 U) 79.27°W 54 (46 HY, 8 AHY) 71.06°W 15 (U) 87.49°W 11 (U) 94.18°W 35 (6 HY, 28 AHY, 1 U)

Season Spring Spring Spring Fall Fall Fall Fall Fall Breeding

Dates 19 April to 10 May, 2009–2010 18 May to 1 June, 2010 15 May to 2 June, 2010 6 September to 13 October, 2009–2010 6 September to 4 November, 2008–2009 11 September to 11 October, 2007–2008 1972–1974 1967–1972 31 May to 23 August, 2003, 2008

Sites ‘BOS’ and ‘WISC’ represent areas of carcass collections within 100 km of Boston, MA and 250 km of Eau Claire, WI, respectively; actual dates of carcass collection are unknown. b HY, hatching year; AHY, after hatching year; SY, second year; ASY, after second year; U, Unknown. South Florida Bird Observatory, FL (SFBO); Appledore Island, NH (AIS); Metinic Island, ME (MEIS); Manomet Bird Observatory, MA (MBO); Powdermill Avian Research Center, PA (PARC) and Churchill, Manitoba (CH). a

(Manchester, UK) using continuous-flow isotope-ratio mass spectrometry (CFIRMS). Within-run replicate measurements (N = 5) of keratin standards indicated an analytical error of ±2‰. All results are reported for nonexchangeable H expressed in the typical delta notation, in units per mil (‰), and normalized on the Vienna Standard Mean Ocean Water-Standard Light Antarctic Precipitation (VSMOW-SLAP) standard scale. Data analyses. We had samples for both spring and fall from only one site (Metinic Island, Maine). Thus, prior to all subsequent analyses, we used the Metinic data to test for differences in !2 Hf between spring and fall (within site) migration and within the fall migration season (across sites) using robust regression with the “MASS” package (Venables and Ripley 2002) within the R statistical computing environment (R Core Team 2012). We constructed five a priori candidate models, including an interceptonly model, as well as main effects for season, date within season, and the interaction between date and season. We did not include “age class” in the model because of low or uneven sample sizes for each age class across seasons and sites. In addition, we wanted to avoid the confounding effect of season on age class, with the cohort of “hatching year” birds in fall becoming the “second year” cohort in spring. We were similarly not able to look for year × season effects in our analyses because we did not have samples from all sites for all years. Prior to analysis, dates

during migration were standardized to 1 April = Day 1 for spring and to 1 September = Day 1 for fall. We used Akaike’s Information Criterion with second-order bias correction (AICc) and model weights to select among competing models (Burnham and Anderson 2002). Where model uncertainty occurred, we used AICcbased model averaging to derive model-averaged parameter estimates (Burnham and Anderson 2002). AICc model selection and model averaging was conducted using the “AICcmodavg” package (Mazerolle 2013). In addition to the aforementioned analyses, we also used robust linear models to compare !2 Hf among sampling locations. We calculated robust standard errors (‘sandwich’ estimators) to overcome problems with heteroscedastic errors using the “lmtest” (Zeileis and Hothorn 2002) and “sandwich” (Zeileis 2006) packages within the R statistical computing environment (R Core Team 2012). This analysis was split into two separate analyses: a comparison of !2 Hf among sites sampled during spring migration (Appledore Island, Metinic Island, and South Florida), and a comparison of !2 Hf among sites sampled during fall migration (Boston, Manomet, Metinic Island, Powdermill, and Wisconsin). Finally, we used a Mann-Whitney U-test to examine possible differences in !2 Hf values between adult and juvenile Blackpoll Warblers sampled in their breeding/natal grounds at Churchill, Manitoba.

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Assignments to molt origin. We depicted the putative breeding or natal origins of Blackpoll Warblers sampled on migration using a likelihood-based assignment technique (Hobson et al. 2009, Wunder 2010, Van Wilgenburg and Hobson 2011). We created a map (hereafter isoscape) of predicted !2 H in feathers (!2 Hf ;) by applying algorithms presented in Hobson et al. (2012) to rescale the precipitation amountweighted growing season !2 H in the precipitation map (!2 Hp ) of Bowen et al. (2005) into equivalent feather values. Specifically, we applied the regression equation !2 Hf = −17.57 + 0.95 !2 Hp , based on data collected from multiple species of Neotropical migratory birds, to rescale the feather isoscape, and used a spatial “mask” operation to extract only those areas of the continent falling exclusively within the species’ breeding range based on a digital breeding-range map (Ridgely et al. 2007). Following isoscape calibration, we estimated the likelihood that a cell (i.e., pixel) within the isoscape represented a potential origin for a given sample by comparing the observed !2 Hf against the isoscape-predicted !2 Hf using a normal probability density function (Hobson et al. 2009, 2014, Wunder 2010, Van Wilgenburg and Hobson 2011). Following previous work (Hobson et al. 2009, 2014, Wunder 2010, Van Wilgenburg and Hobson 2011), individuals were assigned to likely origins within the isoscape by selecting the raster cells consistent with the upper 67% of estimated “probabilities of origin” for each individual, coding those as 1 and all others as 0 consistent with the 2:1 odds of being correct versus incorrect. We subsequently summed the results of the assignments over all individuals by addition of the surfaces (Hobson et al. 2009, Van Wilgenburg and Hobson 2011). Finally, we validated our choice of isoscape rescaling algorithm and assignment approach using the sample of 35 known-origin Blackpoll Warblers (28 AHY, 6 newly fledged = HY, and one unknown age class individual, sexes pooled) collected at Churchill, MB (58.78°N, 94.19°W), from 31 May to 23 August 2008. All geographic assignments to origin were done using functions within the R statistical computing environment (R Core Team 2012) using scripts employing the “raster” package (Hijmans and Van Etten 2012).

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Table 2. Parameters estimates from robust regression models examining variation in !2 Hf in feathers collected from Blackpoll warblers captured during spring migration at Appledore Island (AIS), Metinic Island (MEIS) and South Florida Bird Observatory (SFBO), and during fall migration at Manomet Bird Observatory (MBO) and Powdermill Avian Research Center (PARC) as well as those collected at tower or building collision sites during fall migration near Eau Claire, WI (WISC), and near Boston (BOS). Season Parametera B SE Z P Spring Intercept −92.86 2.59 −35.9