Commensal Foraging Between Hairy and Pileated Woodpeckers

A Source of Variation in Avian Growth Studies: undigested food.--In studies of avian biology, standardized growth curves are often obtained by weighing young at regular intervals. What is measured, in fact, is the bird plus the unassimilated food in the gut, some of which will be eventually assimilated but some of which will be excreted. Food in the gut is a source of variation in weight studies, though large samples minimize problems of excess. With small samples the potential for problem increases. For most, if not all birds, there is a long period of inactivity each day during which the chicks are not fed (e.g., for diurnal birds this occurs during the night). At the end of this period one would expect measured weight to reflect the weight of the bird without food in the gut. By comparing this weight with the weight of chicks after feeding has begun, one can get an idea of the degree of variation due to food in the gut. In 1979, I studied this question in Common Terns (Sterna hirundo) on Great Gull Island, Long Island Sound, New York. Common Tern chicks remain in the parental territory until they are about 23 days or older (Hays pets. comm.) and chicks reach their maximum weights at between 17 and 20 days of age (LeCroy and Collins 1972, Auk 89:595-611; LeCroy and LeCroy 1974, Bird-Banding 45:326-340; pets. ohs.). The chicks are fed at variable rates from once every few minutes to once every few hours depending on feeding conditions for adults, weather, etc. (Hays pets. comm.; pets. ohs.). At ages of 15 and 16 days I weighed chicks at sunrise (ca 0445) before they had been fed and at 0800, after feeding had been going on for some time. I compared the two weights each day and average growth increments calculated from the increments of individual birds. On day 15 at 0445 chicks weighed a mean of 99.6 -+ s.e. 2.03 g and at 0800 they weighed 108.0 _+ 2.23 g (t-test, one-tailed, n = 10, P • 0.001). On day 16 the earlier weights were again lighter but not significantly so: 107.5 _+ 1.93 g and 111.9 -+ 2.26 g (ttest, one-tailed, n = 11, 0.10 >P > 0.05). More importantly, the average daily weight increments were different as calculated with weights taken at the two times on the subsequent days. Weight increase, based on 0445 weights, was 5.7 -+ 1.35 g and was significantly different from the increase based on 0800 weights, 0.2 -+ 2.04 g (t-test, two-tailed, n = 9 and 10 for 0445 and 0800 increments, respectively, P • 0.05). Unassimilated food, then, can contribute a small, though significant amount of variation to measured weights and weight increments. In studies with large sample sizes unassimilated food is likely to contribute only a small amount of variation. I am grateful to the Frank M. Chapman Memorial Fund for a fellowship which provided support during this study. Helen Hays generously provided facilities and gave permission to work on Great Gull Island. The Long Point Bird Observatory kindly provided a place to write. J. DiCostanzo, E. H. Dunn, J. Farrand, Jr., H. Hays, and M. LeCroy made helpful comments on the manuscript. This is contribution number 58 of the Great Gull Island Project.--MALCOLX• C. COULTER, Dept. of Ornithology, AmeTict2n Mgtsegt?n of Nt2tural History, Central Park West at 79th St., New York, NY.10024. Received 30 Mar. 1980; accepted 29 Sept. 1980.