|Abstract or Summary
- 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
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.