Abstract
Anthropogenic alterations of woodland habitat may influence stopover biology, which in turn could alter the stress physiology of migratory landbirds. Woodland stopover habitats are scarce in the Northern Prairie region of North America and consist of native riparian corridor woodlands (corridors) and smaller, more isolated woodlots of anthropogenic origin around farmsteads (woodlots). Corridor habitats have been greatly reduced since the time of European settlement, but woodlot habitats have appeared over this same time period. In this study, we compared stopover biology and stress physiology of migratory landbirds using natural and anthropogenic woodland habitats. We first tested for differences between birds in the two habitats for baseline corticosterone (CORTB) and the magnitude of the stress response for individual species, taxonomic families and foraging guilds. Plasma corticosterone increased significantly for all bird groups in both habitats following 30 min of restraint stress (CORT30), and neither CORTB nor the magnitude of the stress response (CORT30 - CORTB) differed significantly between birds in the two habitats. Secondly, because CORTB levels are often elevated and CORT secretion following a stressor is often suppressed for birds in poor body condition, we hypothesized that woodland migrants with higher fattening rates would show reduced CORTB and a robust stress response. We tested this hypothesis by assessing the relationships between plasma corticosterone and plasma metabolites associated with refuelling. We found that CORTB was negatively associated and the magnitude of the stress response positively associated with plasma triglycerides (an indicator of fat deposition), with opposite patterns for corticosterone and plasma β-hydroxybutyrate (an indicator of fat catabolism). These data suggest that both corridor and woodlot habitats serve as effective stopover habitat and that the reduction of corridor habitat and increased reliance on anthropogenic woodlots is not detrimental to the stress physiology of migrants in a region with limited woodland habitats.
Highlights
Migration is energetically expensive, and woodland migrant birds must refuel at stopover habitats along the migratory route (Moore and Woodrey, 1993; Yong and Moore, 1997; Moore and Aborn, 2000; Wikelski et al, 2003; Faaborg et al, 2010)
We found that CORTB was negatively associated and the magnitude of the stress response positively associated with plasma triglycerides, with opposite patterns for corticosterone and plasma β-hydroxybutyrate
Plasma CORT levels in birds may be related to body condition, and many studies suggest that fat birds may show lower levels of CORTB and a greater stress response than lean birds
Summary
Woodland migrant birds must refuel at stopover habitats along the migratory route (Moore and Woodrey, 1993; Yong and Moore, 1997; Moore and Aborn, 2000; Wikelski et al, 2003; Faaborg et al, 2010). Plasma CORT levels in birds may be related to body condition, and many studies suggest that fat birds may show lower levels of CORTB and a greater stress response than lean birds In southeastern South Dakota, the average size of corridor woodlands is more than three times larger than woodlots, which results in much greater amounts of woodland edge for woodlot habitats, and corridors harbour more diverse vegetation than woodlots (Castonguay, 1982; Gentry et al, 2006; Liu and Swanson, 2014) These differences in size, structure and vegetative diversity of corridors and woodlots may produce differences in predation pressure, competition among migrants and thermal microclimates (Ellis et al, 2012), all of which could potentially influence stress physiology. We examined whether stress physiology in migrants was correlated with refuelling rates in the two habitats, as measured by plasma metabolites (Liu and Swanson, 2014)
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