Effects of body size on goose behavior: lesser snow goose and Ross's goose

Abstract

Body size is highly variable among geese, both at intra- and interspecific levels. Interspecific variation in several behaviors has been attributed to differences in body size in geese: incubation constancy, tendency to maintain family units, and time spent foraging. Body size has important physiological implications for birds, mostly because mass-specific metabolic rate is greater for birds of smaller mass. The Body-size Hypothesis predicts that smaller species deplete their energy reserves at relatively faster rates than do larger species. Hypotheses and conclusions concerning effects of body size on waterfowl behavior often are based on comparisons of species that confront different climates, habitat types, and food resources, and migrate variable distances with different energetic costs. Accordingly, I controlled for such variation by comparing the behavior and physiology of lesser snow geese (hereafter snow geese) and Ross's geese, which are closely related and highly sympatric throughout the annual cycle. I found that incubation constancies of both species averaged 99%. The defeathered ventral area was positively related to clutch volume and inversely related to prolactin levels in female Ross's geese, but not in female snow geese; moreover, prolactin levels and body condition were inversely related in Ross's geese, but not in snow geese. I documented that 5 of 5 female snow geese and 1 of 5 female Ross's geese possessed fully-developed brood patches. In winter, I documented that Ross's geese spent more time feeding than did snow geese. All these findings, except that for incubation constancy, were consistent with predictions of the Body-size Hypothesis. Finally, I studied effects of intraspecific body size variation on goose behavior by studying movements and behavior of snow geese in southwest Louisiana. I found that both adult and juvenile snow geese from coastal marshes had larger bodies and bills than did those from rice-prairie habitats. Adult snow geese from coastal marshes spent more time feeding than did those in rice-prairies, whereas the opposite was true for juveniles. I conclude that snow geese in southwest Louisiana segregate into coastal marsh and rice-prairie habitats by body morphometrics, but move too frequently between the 2 habitats to be considered separate populations.