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

Abstract

Glucocorticoids (GCs) are often interpreted as indicators of disturbance, habitat quality, and fi tness in wild populations. However, since most investigations have been unable to examine habitat variability, GC levels, and fi tness simultaneously, such interpretations remain largely unvalidated. We combined a quantifi cation 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 fi tness in female tree swallows Tachycineta bicolor . Control females experiencing the higher early-season food resources of inland -pasture habitat laid larger clutches, but fl edged an equal number but lower mass off spring compared to those in riparian -cropland habitat. Despite these diff erences in reproductive success, females nesting in the two habitat types did not diff er in baseline GC levels at the early-or late-breeding stage. Feather-clipping reduced provisioning rate in both habitat types. However, baseline GC levels were aff ected in a habitat-specifi c way; only individuals in inland-pasture habitats showed an increase in GCs. Despite this diff erence in GC levels, the manipulation did not infl uence off spring mass, reproductive output, adult return rate (a proxy for survival) to the following year, or reproductive success in the subsequent year. Nonetheless, regardless of treatment, individuals with higher GC levels during the late breeding stage returned in the following year with higher GC levels at incubation, indicating a long-term eff ect on future GC levels. Our results indicate that environmental changes (e.g. foraging conditions) can have consequences for body condition, behaviour, and current and future baseline GC levels without concomitant infl uences on fi tness, and that diff erences in fi tness components between habitats may not be refl ected in baseline GC levels. Th ese results illustrate that baseline GCs may not simultaneously refl ect environmental quality and fi tness, potentially limiting their application in ecological and conservation settings.

On again, off again: Acute stress response and negative feedback together predict resilience to experimental challenges

“…During cold weather events, the availability of flying insects decreases drastically (Winkler et al, ), forcing tree swallows to spend more effort to obtain sufficient food. The high FR treatment appeared to increase the cost of foraging to a level similar to that seen during naturally occurring cold snaps (or from feather clipping experiments: Winkler & Allen, ; Ardia & Clotfelter, ; Madliger & Love, ). The reduction in body mass seen over the treatment interval is similar to that seen in tree swallows facing naturally occurring periods of cold weather (M. N. Vitousek, unpublished data).…”

“…Previous work in this population has found that baseline corticosterone levels are higher under poor conditions; thus, we predicted elevated baseline corticosterone in the High FR group. However, previous feather clipping experiments in tree swallows showed inconsistent effects on baseline corticosterone (decrease: Patterson et al, , increased: Madliger & Love, ). It is possible that in this study, low sample sizes at recapture, particularly in the High FR group, may have prevented detection of a change in baseline corticosterone.…”

Conservation implications of a lack of relationship between baseline glucocorticoids and fitness in a wild passerine

“…This would be a particularly important factor for manipulated females, whose foraging rates decrease (Madliger et al. ), but whose breeding success was comparable to controls (Madliger and Love ). Indeed, the total number of foraging trips to manipulated and control nests was equivalent (Madliger and Love ), indicating that males did compensate to ensure a certain overall foraging rate for their brood (Patterson et al.…”

“…A final explanation for a general lack of CORT-fitness relationships, particularly at the nestling provisioning stage when chicks begin to plateau in body mass (Quinney et al 1986, McCarty 2001, is the possibility that males could compensate for reduced foraging ability in females (Patterson et al 2011), leading to unaltered nesting success despite alteration in female CORT levels. This would be a particularly important factor for manipulated females, whose foraging rates decrease ), but whose breeding success was comparable to controls (Madliger and Love 2016). Indeed, the total number of foraging trips to manipulated and control nests was equivalent (Madliger and Love 2016), indicating that males did compensate to ensure a certain overall foraging rate for their brood (Patterson et al 2011).…”

“…This would be a particularly important factor for manipulated females, whose foraging rates decrease ), but whose breeding success was comparable to controls (Madliger and Love 2016). Indeed, the total number of foraging trips to manipulated and control nests was equivalent (Madliger and Love 2016), indicating that males did compensate to ensure a certain overall foraging rate for their brood (Patterson et al 2011). In particular, this highlights the possibility that GC levels fail to reflect fitness due to unmeasured variables (e.g., mate quality) and has implications for Notes: All values were calculated using models included in the 95% confidence sets, weighting by Akaike weights.…”