Opposite but analogous effects of road density on songbirds with contrasting habitat preferences

Abstract

Roads are a widespread and growing form of habitat alteration, making their effects on wildlife a major conservation concern. We investigated effects of roads on occupancy patterns and stress physiology of the golden‐cheeked warbler Setophaga chrysoparia, an endangered songbird that depends on large tracts of old‐growth forest for breeding. We compared those effects in the white‐eyed vireo Vireo griseus, a common songbird that prefers to breed in scrubby habitat that is common along forest roads. In warblers, older, socially dominant males were more likely to occupy territories, but had higher baseline plasma concentrations of corticosterone, in less‐roaded habitats. In vireos, older males were equally likely to occupy territories, but, opposite to warblers, had lower baseline corticosterone concentrations, in less‐roaded habitats. In both species, the corticosterone response to acute stress was unrelated to road density, and body condition and fat stores either increased or were sustained across road density groups during the breeding season, suggesting that breeding near roads did not cause chronic stress. Our results suggest that roads can cause increased male–male competition for territories in nearby roadless habitat, and may result in higher corticosterone concentrations in dominant individuals defending preferred sites near or far from edge, depending on species habitat preferences. Increased competition and chronically elevated corticosterone levels may be of concern if they alter parental care or immunity of individuals breeding in the best habitat.