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

Abstract

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 (δ 13 C). 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 (δ 2 H). 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.

Wind‐associated detours promote seasonal migratory connectivity in a flapping flying long‐distance avian migrant

“…where the geographical structure between barrier-associated sites and breeding distribution estimated from isotope signature was generally weak, consistently weaker than earlier estimates of migratory connectivity between breeding and wintering sites (Cohen et al, 2019). The contrast to this study can likely be attributed to the general difference in distribution of land masses differs between the Old and New World migratory systems, as inter-population mixing generally decreases with increased land availability (Finch et al, 2017).…”

“…The relatively low migratory connectivity between breeding and main non‐breeding sites is commonly observed in avian long‐distance migrants and may in this case be related to the geography of the African continent (Finch et al, ). The opposite pattern of migratory connectivity across seasons was recently described in a group of Nearctic‐Neotropical species migrating through the Gulf of Mexico region (Cohen et al, ), where the geographical structure between barrier‐associated sites and breeding distribution estimated from isotope signature was generally weak, consistently weaker than earlier estimates of migratory connectivity between breeding and wintering sites (Cohen et al, ). The contrast to this study can likely be attributed to the general difference in distribution of land masses differs between the Old and New World migratory systems, as inter‐population mixing generally decreases with increased land availability (Finch et al, ).…”

“…The opposite pattern of migratory connectivity across seasons was recently described in a group of Nearctic-Neotropical species migrating through the Gulf of Mexico region (Cohen et al, 2019),…”

“…Knowledge of its extent and causes is essential to understand processes affecting populations of migrants throughout the annual cycle (Boulet & Norris, ; Hewson, Thorup, Pearce‐Higgins, & Atkinson, ; Webster & Marra, ), and to implement effective conservation efforts (Cresswell, ; Marra, Norris, Haig, Webster, & Royle, ; Vickery et al, ). Acknowledging that events outside the breeding season may affect individual fitness and thereby limit or regulate population numbers, studies on migratory connectivity focus on describing the spatial relationship between wintering and breeding locations (Finch et al, , but see Cohen et al, ; Delmore, Fox, & Irwin, ; Knight et al, ; Trierweiler et al, ). However, also decisions made during the migration period could influence fitness by affecting survival (Klaassen et al, ; Sillett & Holmes, ) and arrival timing to breeding sites (Tøttrup et al, ).…”

Integrating broad‐scale data to assess demographic and climatic contributions to population change in a declining songbird

“…Spatial structuring of migratory species may be maintained (strong migratory connectivity) or dissolved (weak connectivity) between breeding and nonbreeding seasons. Effectively linking environmental covariates to demography in these species requires understanding of spatial structuring throughout the annual cycle (Cohen et al, ; Hostetler et al, ; Ruegg et al, ) and consideration of potential “carry‐over effects,” whereby environmental conditions experienced in one season affect demographic rates and population changes in a subsequent season (Norris & Marra, ).…”