Graduate Thesis Or Dissertation
 

Resource allocation in four syntopic species of marine diving birds

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  • During 1969-1971 I studied patterns of resource allocation and behavioral interaction among Brandt's Cormorants, Pelagic Cormorants, Common Murres and Pigeon Guillemots in the area of a breeding colony on Yaquina Head on the central Oregon coast. Extensive studies of the breeding colony were combined with detailed observations of the distribution, abundance and behavior of these birds at sea. The nesting areas differed considerably among the four species. Brandt's Cormorants and Common Murres were the most similar, both nesting in colonies on flat or gently sloping, unvegetated surfaces of the same offshore rock. In contrast, Pelagic Cormorants nested on the face of precipitous cliffs on the mainland and offshore rocks, singly or in loose aggregations; Pigeon Guillemots also nested singly or in loose aggregations in nooks, crevices and sometimes burrows on mainland and offshore cliffs. Breeding phenologies overlapped almost completely, with egg laying beginning in late May and the first young fledging in mid-July. Murres initiated egg laying as early as May 6 and had an abbreviated nestling period. At an age of 2-3 weeks, chicks left the breeding rock, and were accompanied at sea by the adult male. Dispersal from the breeding colony was primarily longshore rather than offshore. The energetic and competitive advantages of this type of parental care pattern in murres and other alcids are discussed. I suggest that precociality has evolved in some alcids as the result of a gain in subaqueous flight capabilities at the expense of aerial flight. The greater underwater maneuverability permits those species with precocial young to exploit highly mobile midwater fish populations at great distances from the breeding colony. However, their decreased ability to ferry food from the offshore feeding areas to the breeding colony has resulted in selection for early abandonment of the colony by adult and chick. The four species differed in the average length and range of dive times. The ratio of dive time/rest time, however, was very similar when all dive times or a specific dive time were considered. Pelagic Cormorants had the lowest average dive time, while Brandt's Cormorants, Pigeon Guillemots and Common Murres took increasingly longer dives. The same ranking of species was observed for offshore distribution, with Pelagic Cormorants foraging closest to the beach and Brandt's Cormorants slightly further offshore, followed by Pigeon Guillemots with Common Murres being the most pelagic in their foraging distribution. Pigeon Guillemots fed exclusively on bottom-dwelling species while Common Murres took primarily midwater fish. The cormorants fed upon prey occurring in both areas. In addition, both species of cormorants and the murres as well as Western Gulls fed together in mixed-species feeding flocks. These flocks foraged primarily upon schooling fish of the families Engraulidae and Osmeridae, which represent a patchily distributed but locally abundant food source. The three diving species which participate in mixed species flocks thus differed in their offshore distribution and took slightly different prey items, although overlapping extensively in their utilization of the more abundant species, such as anchovies and juvenile rockfish. This zonation results in less predation pressure per unit area on the less common and more predictably distributed prey species, but allows the entire offshore area to be searched effectively for abundant but patchily distributed fish schools. The behavioral interactions between members of the mixed species flocks allow rapid communication of prey school locations and permit each species to maintain contact with the school for a much longer time. The temporal and spatial overlap in the occurrence of breeding activities may contribute to a more effective coverage of the area surrounding the colony during the time of year at which food supplies are abundant.
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