Feeding Ecology of Marine Cormorants in Southwestern North America

-The diets of Pelagic, Brandt's and Double-crested cormorants (Phalacrocorax pelagicus, P. penicillatus and P. auritus) were compared. Data were based on 1,695 pellets and 71 chick regurgitations analyzed by us and information on 34 stomach samples published in the literature. A total of 19 sites between Kodiak, Alaska and central Mexico is represented. The three cormorants often fed in the same areas at the same time using the same technique. They exploited different microhabitats as defined by prey behavior; the prey species overlapped substantially between Brandt's and Pelagic cormorants, but those of the Double-crested Cormorant were quite different. Double-crested Cormorants fed on schooling fish usually occurring well above flat bottoms; Pelagic Cormorants fed on solitary prey on or concealed in rocky substrates; and Brandt's Cormorants fed on prey on or just above the bottom in rocky areas and in areas of flat sand or mud. The latter species fed over flat bottoms more in the northern part of their range than in the southern part where they fed almost exclusively in or near rocky habitat. Double-crested and Pelagic cormorants showed no geographic shift in their feeding habits. The feeding ecology of marine birds has been defined in five major ways: the distance offshore at which a species feeds (inshore, offshore or pelagic; Wynne-Edwards 1935), the depth to which it dives (bottom, mid-water or surface; Balz and Morejohn 1977), the prey species consumed (Ainley and Sanger 1979), prey size (Ashmole 1968), and the methods of food capture (Ashmole 1971). Except for diving depth, little has been said about differences in feeding habitat. This contrasts markedly with land birds, whose use of microhabitats for feeding is a well-known phenomenon. In the present article we attempt to define much more narrowly the feeding habitats of three sympatric, closely related seabird species: the Pelagic, Brandt's and Double-crested cormorants (Phalacrocorax pelagicus, P. penicillatus and P. auritus). Heretofore, all have been regarded as feeding similarly, by diving beneath the surface, from the bottom to mid-depths, in inshore waters. So similar has their feeding ecology appeared that Cody (1973) was led to postulate that, in areas of sympatry, breeding seasons have become staggered as a means to distribute demand on available food resources. We obtained information on prey from the analysis of fish otoliths and invertebrate hard parts in pellets. Pellets, which cormorants apparently produce almost daily, have received very little attention from seabird researchers, yet they prove invaluable in a study such as ours. In cormorants, these pellets are equivalent to stomach samples in terms of what they indicate about diet (Jordan 1959, Schlatter and Moreno 1976; Ainley, unpubl. data). Birds do not have to be c ll cted for their stomachs nor harassed in order that they regurgitate their stomach cont nts; thus, large samples and time series of s mples can be gathered. The attainment of adequate samples to allow valid comparisons between species has posed problems in studies of marine birds (Balz and Morejo n 1977). Jordan (1959) gave a detailed description of cormorant pellets and the mechanism and frequency of their production. Why cormorants produce pellets is not known, but the fact that pellets often contain many nematodes indicates that they may aid in parasite control (Jordan 1959; Ainley, unpubl. data). They are produced only by adult and subadult cormorants; younger birds begin producing them at about the time they are able to fly. In forming the pellet, the stomach contents are enveloped in mucus secreted by the stomach wall. Upon drying, the pellet becomes very hard. A cormorant regurgitates one pellet during the night, usually just before dawn, when it departs for daily feeding. Regurgitation before daylight not