The adrenocortical responses to stress in breeding Willow Warblers Phylloscopus trochilus in Sweden: effects of latitude and gender

Abstract

1. In the present study we tested the hypothesis that birds suppress their adrenocortical response to stress when breeding in a severe environment. 2. We measured increases in circulating levels of corticosterone following capture and handling in populations of Willow Warblers, Phylloscopus trochilus, breeding in a rich deciduous forest in southern Sweden and in Willow Warblers breeding in a subarctic environment in Swedish Lapland. Data from an 8‐year study on survival rates and nest predation in southern and northern Sweden are also included. 3. By comparing the changes in corticosterone levels immediately following capture, handling and restraint, and then up to 1‐h post‐capture across gender and latitude, we were able to assess possible ecological influences. 4. Willow Warblers had about the same survival rates in northern and southern Sweden (0·27 and 0·30, respectively). Depredated clutches were normally replaced in southern Sweden (72%), but not in northern Sweden (1%). 5. A reduced adrenocortical response to stress was found in Willow Warblers of both sexes breeding in northern Sweden where conditions were more severe and the nesting season shorter than in southern Sweden. At the southern site, males had a significantly more robust adrenocortical response to stress than females, whereas at the northern site changes in corticosterone following capture were identical in males and females. Maximum corticosterone levels generated by the capture stress protocol were significantly higher in males at the southern site than in all other groups. 6. There were marked individual variations in the patterns of corticosterone changes following capture in all groups, but there were no significant relationships of dynamics of corticosterone changes to body mass or fat score in males. However, in females at the southern site, the maximum corticosterone level and the rate of corticosterone increase during the capture stress protocol were positively correlated with body mass. In contrast, at the northern site females showed negative correlations of body mass with maximum corticosterone and rate of corticosterone increase. These differences may reflect different ecological bases for modulation of the adrenocortical response to stress with latitude, and with gender.

Reproduction and modulation of the stress response: an experimental test in the house sparrow

“…The magnitude of stress response was expressed as the maximum levels of corticosterone. We also calculated the rate of increase from baseline to maximum corticosterone levels per minute (ng ml K1 min K1 ; Silverin et al 1997;Lindström et al 2005). The latter measure of stress response was highly correlated with the maximum levels of corticosterone (rZ0.920, p!0.0001), and the statistical analyses of these variables gave nearly identical results, hence we report only the results for maximum corticosterone levels to facilitate the comparison with other published results.…”

Baseline and Stress‐Induced Levels of Corticosterone during Different Life Cycle Substages in a Shorebird on the High Arctic Breeding Grounds

“…This is consistent with observations on a reduction in the stress response in passerines caught on their breeding territory in extreme environments with short breeding seasons. It differs from nonterritorial passerines or passerines at lower latitudes with longer breeding seasons, which show relatively high sensitivity to environmental stressors, even during incubation (e.g., Wingfield et al 1992Wingfield et al , 1995Silverin et al 1997). Although an adrenocortical stress response induces temperate-breeding passerines to respond to stressful events in an adaptive manner, for example, by deserting the nest in response to bad weather or a predator (Silverin 1986(Silverin , 1990(Silverin , 1998Wingfield and Silverin 1986), the short arctic summer does not allow any time for repeat nesting attempts (Tulp et al 1998); such responses would result in failed reproduction.…”

“…A reduced sensitivity of the HPA-axis to environmental perturbations has been found in several bird species breeding in extreme environments (Wingfield et al 1992O'Reilly 1995;Silverin et al 1997; but see Astheimer et al 1994Astheimer et al , 1995. It has been suggested that such a reduced response to environmental stress may be functional in preventing birds from suspending reproductive activities during perturbations that are likely to occur within the short breeding season.…”

Stress Responsiveness in Nestlings: A Comparison of Two Sampling Techniques

“…In addition, in most studies similar amounts of blood are removed at each sampling period, but in others the initial blood sample may be larger, if additional hematological measurements are performed (e.g., Clinchy et al 2004). Finally, the same individual is sampled repeatedly in many studies (Wingfield et al 1992, Silverin et al 1997, Love et al 2003, while, in others, each bird is bled only once during the capture and handling period (Sockman and Schwabl 2001, Quillfeldt et al 2004, Brown et al 2005). …”

“…However, subtle, but possibly important, procedural differences occur among studies that use this protocol. For example, in many studies morphological measurements are taken after the last sampling period, so that the only time birds are handled is when blood samples are taken (Wingfield et al 1992, Silverin et al 1997, Blas et al 2005. In contrast, sometimes morphological measurements are taken in between blood samples, when the birds would otherwise be in a cloth bag (Meddle et al 2003, Adams et al 2005.…”