The relationship between measures of stress and condition in nestling European starlings

Abstract

Maintaining an effective immune system is energetically costly. During times of stress, the immune system and an individual’s physical condition can become compromised. Elevated white blood cell (WBC) counts, and high heterophil/lymphocyte (H/L; types of WBCs) ratios are two reliable indicators of stress in birds. The objective of this study was to determine whether nestling condition was negatively correlated with stress in European starlings (Sturnus vulgaris). I predicted that nestlings in poor condition would have higher stress and therefore higher overall WBC counts and H/L ratios than nestlings in good condition. The best and worst condition nestlings were chosen from each nest when 14 -15 days of age (n = 16 nests), as measured by regressing mass against tarsus length. Blood smears were made for each of these nestlings with a small blood sample. Smears were then fixed in methanol, stained using Hema III and examined with microscopy to estimate WBC count and H/L ratio per 10,000 erythrocytes. Condition differed significantly between the two nestling groups (best and worst condition). Mean WBC counts per brood tended to be positively correlated with mean H/L ratios per brood. However, counter to my prediction, no correlation existed between mean WBC count and mean nestling condition per brood or mean nestling H/L ratio and mean condition per brood. Differences in H/L ratio, however, were positively correlated with differences in nestling condition, suggesting that nestlings in similar condition are similarly stressed. The lack of significant relationship between H/L ratio and condition found in nestlings contrasts with results from another study done on adult starlings in this same population. Nestlings at 14-15 days of age are likely still developing their immune response, and so WBC counts and H/L ratios are not good measures of stress.