Measurement of growth rates: the effect of sampling time on weight in a seabird.

In most field studies on growth rates of birds, chicks are weighed in the "morning" or whenever it is convenient. I found that weight of laboratory-reared young herring gulls (Larus argentatus) varied significantly depending on the time of day it was measured. Chicks were always lighter at daybreak before they had eaten. However, mean weight at other times of the day was less variable. These results suggest that it is important to always weigh chicks at the same time each day, and to appreciate that daily or treatment differences may reflect sampling time differences unless the timing and amount of chick feeding is considered. Further weight gain during the day was directly correlated with the amount of food eaten first thing in the morning, and was directly correlated with weight loss overnight.