Morphological Differences Relative to Ecological Segregation in Petrels (Family: Procellariidae) of the Southern Ocean and Tropical Pacific

Abstract

-We compared eight morphological characters (wing span, wing area, aspect ratio, tail length, bill length, bill depth, tarsus length, and mass of subcutaneous/ mesenteric fat) among petrels (family Procellariidae) of tropical versus southern polar avifaunas. Relative to body mass, tropical species have larger wings, bills, and tails, and lighter fat reserves than do polar species. We attributed these differences primarily to adaptations for feeding in markedly different pelagic environments. Larger wings, bills, and tails of tropical species enable them to make use of relatively light winds when foraging over wide ocean expanses to exploit sparse and highly mobile and /or volant prey. In contrast, the smaller wings, bills, and tails of polar species enable them to cope with strong winds to exploit highly abundant, less-mobile prey. Greater fat reserves among polar species probably are an adaptation for surviving extended periods when rough weather (rarely experienced by tropical species) precludes feeding, or for thermoregulation. The most consistent and marked differences between avifaunas are in wing structure and fat load-characters that are directly related to adaptations to physical factors such as wind and climate. Species-specific differences within avifaunas are mostly related to specializations for different foraging habits (i.e. feeding behavior, prey composition, and prey size). Morphological differences and within-species character variances indicated that the tropical ocean is used by a more generalist, migratory group of petrels, whereas the Southern Ocean is used by a more specialized, resident group of petrels. Received 20 October 1997, accepted 17 April 1998. No SPECIES comprising the tropical and polar avifaunas of the South Pacific occupy both habitats (Ainley and Boekelheide 1983, Ribic and Ainley 1989), suggesting specialization among members of each avifauna for existence in their respective regimes. Such specialization should arise from differences in selective factors, including differences in breeding and feeding habitat, predators, and weather/ climate (Mayr 1963, Bock 1974, Ainley 1977, Krebs and Houston 1989). Whatever these adaptations are, and few have been studied in detail (Warham 1990, 1996), they should be reflected by differences in morphology as noted for other avian groups (James 1982). Thus, a morphological comparison of species within the two avifaunas should help to identify the degree of specialization and factors responsible for their distinctness. Within the family Procellariidae, we compared morphology of species of petrels that predominate in the eastern tropical Pacific (ETP; nine species) with those that predominate in polar latitudes of the Southern Ocean (seven species). The 16 species share three fun' E-mail: harveyecology@worldnet.att.net damental life-history traits: (1) all are strictly pelagic throughout the year, (2) they have similar breeding habits, and (3) the overlap in prey species is extensive within each avifauna (Imber 1985, Ainley et al. 1991, 1992; Rau et al. 1992, Ainley and Spear unpubl. data). Therefore, we assume that differences in morphology should reflect mostly, if not entirely, adaptations for exploiting marine environments that differ both physically and biologically, and/or for existence in different climates. To identify environmental features that are likely to structure morphological adaptations of the two avifaunas, we compared the length of the wings, tail, and legs; bill shape (length and depth); wing area (relative to body mass); and aspect ratio (wing shape) between and among species of the two avifaunas. We also compared differences in the amount of fat reserves as an indicator of climatic stress (see Lima 1986), and as a factor affecting body