Do stable isotopes reflect nutritional stress? Results from a laboratory experiment on song sparrows

Stable isotope analysis is an increasingly valuable tool in ecological studies and shows promise as a measure of nutritional stress in wild animals. Thus far, however, the only studies on endotherms that have conclusively shown changes in delta N-15 and delta C-13 values in response to nutritional stress were conducted on fasting animals and animals growing under extreme levels of food restriction. We conducted a laboratory experiment to test whether delta N-15 and delta C-13 values provide a general index of nutritional stress. We compared the isotopic composition of whole blood, liver, muscle and feathers between two groups of juvenile song sparrows (Melospiza melodia) hand-reared in captivity under identical conditions except for feeding regime. To verify that our experimental treatment induced a biologically meaningful level of nutritional stress, we simultaneously measured the effects on physiology, growth and development at multiple scales. While food-restricted birds were physiologically stressed, physically smaller, and showed poorer growth and brain development compared to ad libitum-fed birds, there was no effect of feeding regime on either delta N-15 or delta C-13 values in any tissue. Instead of a continuum where the level of change in N-15 or C-13 contents corresponds to the level of nutritional stress, we suggest there may be a threshold level of nutritional stress below which such isotopic changes are likely to be negligible.