Browsing by Author "Sillett, T. Scott"
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Item Do Archival Light-Level Geolocators and Stable Hydrogen Isotopes Provide Comparable Estimates of Breeding-Ground Origin(The American Ornithologists' Union, 2013-04-01) Hallworth, Michael T.; Studds, Colin E.; Sillett, T. Scott; Marra, Peter P.Migratory connectivity for small migratory passerines has been quantified primarily with stable hydrogen isotopes in feathers (δ²HF) because, until recently, we lacked the technology to track small organisms over long distances. Direct tracking of small passerines throughout the annual cycle is now possible with archival light-level geolocators. Our objective was to evaluate whether δ²HF and geolocators produce similar breeding-origin assignments for the same individual birds sampled during the non-breeding season. We estimated breeding origin with geolocators and δ²HF and validated those estimates using a population of Ovenbirds (Seiurus aurocapilla) from a known breeding location at Hubbard Brook Experimental Forest, New Hampshire. We also deployed geolocators on Ovenbirds in Jamaica and Florida during March 2010–2011. We performed stable hydrogen isotope analysis on feathers of birds whose geolocators we recovered (Jamaica: n = 9; Florida: n = 3). Probabilistic assignments of δ²HF that accounted for regional variation in feather-isotope discrimination predicted breeding origins that agreed with kernel density estimates of origin derived from geolocators. By contrast, assignments of δ2HF using the common assumption of a consistent feather-isotope discrimination across space predicted breeding origins that overlapped minimally with those from geolocators. Finally, Bayesian analyses that incorporated prior information of Ovenbird abundance across the breeding range yielded more accurate assignments for both site-independent and site-specific discrimination factors. Our findings suggest that creating more detailed feather isoscapes by increasing the number of validation locations and sampling underrepresented portions of species distributions could increase the accuracy of geographic assignments using δ²HF.Item Hierarchical Distance Sampling Reveals Increased Population Size and Broader Habitat Use in the Endangered Bahama Oriole(2021-01-10) Rowley, Michael G.; Stanley, Richard C.; Antalffy, Janine M.; Christhilf, Jennifer L.; Stonko, Daniel C.; Johnson, Scott B.; Cant-Woodside, Shelley; Sillett, T. Scott; Fagan, Matt E.; Studds, Colin E.; Omland, KevinCaribbean is home to over 20 passerine species listed by the International Union for Conservation of Nature as vulnerable, endangered or critically endangered. The Bahama Oriole (Icterus northropi) is listed as critically endangered and is now restricted to Andros, a single island complex in The Bahamas. We investigated this species’ habitat use and produced the first statistically robust estimate of population size. We conducted point counts during the Bahama Oriole’s breeding season in May and June 2017, extensively surveying the northern part of North Andros over a study area covering 713 km². Hierarchical distance sampling models estimated 1269-2765 individuals within our study area, a substantially larger population on North Andros than was indicated by the previously published estimates. Earlier studies, which disproportionately sampled anthropogenic and coppice habitats, likely underestimated this species’ abundance in pine forest. We found that the Bahama Oriole is widespread, most abundant in pine forest, and not dependent on developed habitats during the breeding season. These findings provide a better outlook for the species’ persistence and indicate that conserving pine forest would benefit this critically endangered species. Our results also emphasize the importance of rigorously evaluating habitat use when developing conservation plans for endangered species. Systematic population counts and statistical analyses that account for detection probability are needed for endangered and vulnerable endemic birds across the Caribbean, especially in the face of increased hurricane strength and sea level rise due to climate change.Item Non-breeding season habitat quality mediates the strength of density-dependence for a migratory bird(The Royal Society, 2015-07-22) Marra, Peter P.; Studds, Colin E.; Wilson, Scott; Sillett, T. Scott; Sherry, Thomas W.; Holmes, Richard T.Our understanding of when natural populations are regulated during their annual cycle is limited, particularly for migratory species. This information is needed for parametrizing models that can inform management and conservation. Here, we use 14 years of data on colour-marked birds to investigate how conspecific density and habitat quality during the tropical non-breeding period interact to affect body condition and apparent annual survival of a long-distance migratory songbird, the American redstart (Setophaga ruticilla). Body condition and survival of birds in high-quality mangrove habitat declined as density increased. By contrast, body condition improved and survival did not vary as density increased in adjacent, lower quality scrub habitat, although mean condition and survival were almost always lower than in mangrove. High rainfall enhanced body condition in scrub but not in mangrove, suggesting factors such as food availability outweighed consequences of crowding in lower quality habitat. Thus, survival of overwintering redstarts in mangrove habitat, disproportionately males, appears to be regulated by a crowding mechanism based on density-dependent resource competition. Survival of individuals in scrub, mostly females, appears to be limited by density-independent environmental factors but not regulated by crowding. The contrasting effects of density and food limitation on individuals overwintering in adjacent habitats illustrate the complexity of processes operating during the non-breeding period for migratory animals, and emphasize the need for long-term studies of animals in multiple habitats and throughout their annual cycles.Item The role of tropical rainfall in driving range dynamics for a long-distance migratory bird(PNAS, 2023-12-18) Dossman , Bryant C.; Studds , Colin; LaDeau , Shannon L.; Sillett, T. Scott; Marra, Peter P.Predicting how the range dynamics of migratory species will respond to climate change requires a mechanistic understanding of the factors that operate across the annual cycle to control the distribution and abundance of a species. Here, we use multiple lines of evidence to reveal that environmental conditions during the nonbreeding season influence range dynamics across the life cycle of a migratory songbird, the American redstart (Setophaga ruticilla). Using long-term data from the nonbreeding grounds and breeding origins estimated from stable hydrogen isotopes in tail feathers, we found that the relationship between annual survival and migration distance is mediated by precipitation, but only during dry years. A long-term drying trend throughout the Caribbean is associated with higher mortality for individuals from the northern portion of the species’ breeding range, resulting in an approximate 500 km southward shift in breeding origins of this Jamaican population over the past 30 y. This shift in connectivity is mirrored by changes in the redstart’s breeding distribution and abundance. These results demonstrate that the climatic effects on demographic processes originating during the tropical nonbreeding season are actively shaping range dynamics in a migratory bird.