Geographical variation in body size of a pelagic seabird, the streaked shearwater Calonectris leucomelas

Aim: In general, within temperate latitudes, races of endotherms tend to show larger body size at higher latitudes, while smaller bodied ones tend to be found at lower latitudes, a pattern known as Bergmann's rule (or James's rule for intra-specific patterns). The adaptive basis of this geographical variation in body size is explained mostly by the heat conservation hypothesis. Application of Bergmann's rule has been confirmed mostly among terrestrial taxa and studies of marine taxa are rare. Our aim was to examine whether the intra-specific variation in body size of a pelagic seabird species (streaked shearwater Calonectris leucomelas) follows Bergmann's/James's rule. Location: Eight different breeding colonies of the streaked shearwater, ranging from 24–39° N in latitude (approximately the full latitudinal range of the species) and 123–142° E in longitude. Methods: We measured morphological traits of 454 adult streaked shearwaters in total. Principal components analysis was used to create a composite body size index for each sex from each colony using the measured traits, and the value of first principal component was compared. The relationships between body size index and latitude, longitude and the averaged mean air temperature were examined using generalized linear models. Results: Body size was positively correlated with latitude and longitude, and negatively correlated with the mean air temperature. Latitude and longitude were highly correlated with air temperature. Nonetheless, between colonies, populations that are considered to forage over large areas appeared to exhibit smaller body size than those of lower latitude populations. Main conclusion: The overall trend of the geographical variations in body size of streaked shearwaters followed Bergmann's/James's rule, suggesting that this rule may be applicable to seabirds. However, in addition to thermoregulatory adaptation, intra-specific differences in foraging characteristic in relation to local marine environment might also affect the body size in highly mobile seabirds