Do baseline glucocorticoids simultaneously represent fitness and environmental quality in a declining aerial insectivore?

Glucocorticoids (GCs) are often interpreted as indicators of disturbance, habitat quality, and fitness in wild populations. However, since most investigations have been unable to examine habitat variability, GC levels, and fitness simultaneously, such interpretations remain largely unvalidated. We combined a quantification of two habitat types, a manipulation of foraging ability (feather-clipping just prior to nestling rearing), multiple baseline plasma GC measures, and multi-year reproductive monitoring to experimentally examine the linkages between habitat quality, GCs, and fitness in female tree swallows Tachycineta bicolor. Control females experiencing the higher early-season food resources of inland–pasture habitat laid larger clutches, but fledged an equal number but lower mass offspring compared to those in riparian–cropland habitat. Despite these differences in reproductive success, females nesting in the two habitat types did not differ in baseline GC levels at the early- or la...