Michael T. Hallworth

Miniaturized GPS Tags Identify Non-breeding Territories of a Small Breeding Migratory Songbird

For the first time, we use a small archival global positioning system (GPS) tag to identify and characterize non-breeding territories, quantify migratory connectivity, and identify population boundaries of Ovenbirds (Seiurus aurocapilla), a small migratory songbird, captured at two widely separated breeding locations. We recovered 15 (31%) GPS tags with data and located the non-breeding territories of breeding Ovenbirds from Maryland and New Hampshire, USA (0.50 ± 0.15 ha, mean ± SE). All non-breeding territories had similar environmental attributes despite being distributed across parts of Florida, Cuba and Hispaniola. New Hampshire and Maryland breeding populations had non-overlapping non-breeding population boundaries that encompassed 114,803 and 169,233 km2, respectively. Archival GPS tags provided unprecedented pinpoint locations and associated environmental information of tropical non-breeding territories. This technology is an important step forward in understanding seasonal interactions and ultimately population dynamics of populations throughout the annual cycle.

Habitat Selection and Site Fidelity of Canada Warblers (Wilsonia Canadensis) in Central New Hampshire

Abstract Habitat selection by migratory bird species affects their fitness. Studies of populations with marked individuals of known reproductive performance provide the greatest insight on the relationship between a species and the range of habitats it occupies. Canada Warblers (Wilsonia canadensis) have not been intensively investigated at the population level, despite significant regional declines in their numbers over the past 40 years (e.g., 4.6% year−1 in New Hampshire). We mapped 92 male territories over four years and quantified habitat structure and habitat composition for territories and nonterritory areas. Analyses revealed more shrubs (stems and foliage), more perch trees, and lower canopy height on territories. Birds returned at an average rate of 52.0 ± 2.96% (SE) and exhibited average between-year shifts of 32 m in territory locations. Over the four years, males had high pairing success (91.0 ± 5.61%). Three-quarters of paired males fledged at least one young. Pairing and fledging success did not influence site fidelity. Paired males had more shrub stems and song-perch trees. Territories of earlier-arriving males had more shrub stems. The high proportion of males pairing and successfully fledging young, together with the high average return rates, indicate that areas with dense subcanopy vegetation constitute prime Canada Warbler breeding habitat in New Hampshire. Timber harvest practices that result in high shrub density and residual standing trees, such as deferment or two-age harvest, have the potential to slow Canada Warbler population declines, but only if they are widely applied in forest management.

Do Archival Light-Level Geolocators and Stable Hydrogen Isotopes Provide Comparable Estimates of Breeding-Ground Origin?

ABSTRACT. Migratory connectivity for small migratory passerines has been quantified primarily with stable hydrogen isotopes in feathers (&dgr;2HF) 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 &dgr;2HF 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 &dgr;2HF 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 &dgr;2HF 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 &dgr;2HF 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 &dgr;2HF.

Does age influence territory size, habitat selection, and reproductive success of male Canada Warblers in central New Hampshire?

Abstract The Canada Warbler (Wilsonia canadensis) is currently in decline in the northeastern United States and basic demographic parameters remain to be described. We studied marked populations (76 ASYs, 14 SYs, and 2 of unknown age) of Canada Warblers on two study sites from 2003 to 2006. We mapped 92 territories (including males returning in multiple years) of 71 males using handheld GPS and ArcMap. We compared the pairing and fledging success of older and younger males on both sites, a red maple (Acer rubrum) swamp and a young forest intensively harvested in 1985 with ∼10% residual standing trees used by males as song perch trees. Both sites had a high proportion of ASYs (84% ASY for all territorial males, 77.5% of all males including non-territorial individuals). Both pairing (91%) and fledging (78%) success was comparatively high suggesting these two sites were of high value to this species. A higher proportion of SYs were transients. Pairing success was lower for younger males which established territories, but paired SYs fledged at least one young at a rate comparable to older males. This study corroborates the benefits of age and experience to reproductive performance. The results suggest that both red maple swamps and post-harvest forests with thick subcanopy vegetation and emergent trees provide high quality habitat for breeding Canada Warblers.

Phylogeography of a migratory songbird across its Canadian breeding range: Implications for conservation units

Abstract The objectives of this study were to describe and evaluate potential drivers of genetic structure in Canadian breeding populations of the Ovenbird, Seiurus aurocapilla. We performed genetic analyses on feather samples of individuals from six study sites using nuclear microsatellites. We also assessed species identity and population genetic structure of quill mites (Acariformes, Syringophilidae). For male Ovenbirds breeding in three study sites, we collected light‐level geolocator data to document migratory paths and identify the wintering grounds. We also generated paleohindcast projections from bioclimatic models of Ovenbird distribution to identify potential refugia during the last glacial maximum (LGM, 21,000 years before present) as a factor explaining population genetic structure. Birds breeding in the Cypress Hills (Alberta/Saskatchewan) may be considered a distinct genetic unit, but there was no evidence for genetic differentiation among any other populations. We found relatively strong migratory connectivity in both western and eastern populations, but some evidence of mixing among populations on the wintering grounds. There was also little genetic variation among syringophilid mites from the different Ovenbird populations. These results are consistent with paleohindcast distribution predictions derived from two different global climate models indicating a continuous single LGM refugium, with the possibility of two refugia. Our results suggest that Ovenbird populations breeding in boreal and hemiboreal regions are panmictic, whereas the population breeding in Cypress Hills should be considered a distinct management unit.

Habitat Use of the Louisiana Waterthrush During the Non-Breeding Season in Puerto Rico

Abstract We used radiotelemetry to quantify habitat and spatial use patterns of neighboring Louisiana Waterthrush (Parkesia motacilla) along two streams in the Caribbean National Forest in Puerto Rico during 2005–2007. Home range sizes varied with younger birds having larger home ranges and core areas than older birds. All birds occupied some length of stream but a wide range of off-stream habitats were also used. Off-stream habitats included a range of disturbance from residential areas to small saturated pastures. Neighbors exhibited a wide range of overlap in home ranges (x¯  =  20%) and older birds had more overlap than younger birds. The greatest percent of foraging time was along streams (64.4%) followed by muddy substrate (26.5%), housing developments (7.4%), and roads (1.7%). The greater proportion of time foraging along streams indicates this is the preferred habitat for this species. Use of off-stream habitat indicates a strategy of exploiting food-rich ground substrates, and in particular those with high moisture.

The strength of migratory connectivity for birds en route to breeding through the Gulf of Mexico

The strength of migratory connectivity is a measure of the cohesion of populations among phases of the annual cycle, including breeding, migration, and wintering. Many Nearctic-Neotropical species have strong migratory connectivity between breeding and wintering phases of the annual cycle. It is less clear if this strength persists during migration when multiple endogenous and exogenous factors may decrease the cohesion of populations among routes or through time along the same routes. We sampled three bird species, American redstart Setophaga ruticilla, ovenbird Seiurus aurocapilla, and wood thrush Hylocichla mustelina, during spring migration through the Gulf of Mexico region to test if breeding populations differentiate spatially among migration routes or temporally along the same migration routes and the extent to which withinpopulation timing is a function of sex, age, and carry-over from winter habitat, as measured by stable carbon isotope values in claws (δ13C). To make quantitative comparisons of migratory connectivity possible, we developed and used new methodology to estimate the strength of migratory connectivity (MC) from probabilistic origin assignments identified using stable hydrogen isotopes in feathers (δ2H). We found support for spatial differentiation among routes by American redstarts and ovenbirds and temporal differentiation along routes by American redstarts. After controlling for breeding origin, the timing of American redstart migration differed among ages and sexes and ovenbird migration timing was influenced by carry-over from winter habitat. The strength of migratory connectivity did not differ among the three species, with each showing weak breeding-to-spring migration MC relative to prior assessments of breeding-wintering connectivity. Our work begins to fill an essential gap in methodology and understanding of the extent to which populations remain together during migration, information critical for a full annual cycle perspective on the population dynamics and conservation of migratory animals.