Among the North Atlantic marine birds, only one species fills the
plankton-feeding niche (Plautus alle L., Alcidae) while in the North Pacific
and adjacent seas, no less than five alcids occupy it. A comparison of the feeding and nesting habits was made in order to understand how the food
resources and the nesting habitat were partitioned between three of these species,
the Crested auklet (Aethia cristatella (Pallas)), the Least auklet
(A. pusilia (Pallas)) and the Parakeet auklet (Cyclorrhynchus psittacula
(Pallas)), The study was made on St. Lawrence Island, Alaska, during the
summers of 1964- to 1966.
The two congeneric species differ markedly in size (pusilla 90 g; cristatella. 300 g) the Crested and the Parakeet auklets are of equivalent
body size. The three species have diurnal habits. The two Aethia are active
on the nesting colonies during the morning and the evening and feed at sea
in early afternoon and early morning. Cyclorrhynchus is present on the
colonies in the morning and early afternoon only and spends the rest of the
day at sea, feeding.
Aethia cristatella and A. pusilla exhibit the same type of response
to the food source : both have a diversified diet in May-July (caridean
larvae, hyperiids, mysids, gammarids) and restrict themselves to one dominant
prey item during the chick-rearing period (August-September). A. pusilla
then concentrates its feeding on Calanus finmarchicus and A. cristatella on
Thysanoessa spp. The two auklets have largely overlapping habits and share
the same feeding grounds. They differ markedly in diet, but more so in the
size of the prey organisms used and these differences can be accounted f or
by differences in bill-size alone.
Slightly over half of the diet of Cyclorrhynchus was made of carnivorous
macroplankton (large hyperiids, fish, etc.). The Parakeet auklet
differs from its possible competitor, the Crested auklet, by occupying a
slightly higher trophic level and by devoting more of its time to feeding.
The two species, however, are found on the same feeding grounds and are presumed
to utilize the same feeding depth-range. Differences in diet between the
two are provisionally attributed to differences in bill structure and bill
shape.
In Aethia the reversal to monophagy during the chick-rearing
period seems to reflect a sudden increase in the availability of palanus and
Thysanoegsa. This, in turn, is believed to have importance in determining
the timing of the breeding season. No obvious factor is responsible for
timing in Cyclorrhynchus. which breeds slightly later than either of the two
species of Aethia. The Crested and the Least auklets seem to depend upon
prey organisms that oscillate widely in abundance and availability. The
Parakeet auklet depends to a greater extent upon organisms whose supply is
more or less constant throughout the year.
Segregation in nesting is complete between the two genera. Cyclorrhynchus
is a cliff-nester while Aethia occupies talus slopes. In the latter
habitat, the marked difference in body size between the two species is again
responsible for segregation through the action of one principal factor,
the average rock diameter on the slopes. The density of A. criptatella
increases in early with increasing boulder size : the density of A. pusilia
decreases both with decreasing boulder size and with the decreasing abundance
of its large congener from a knowledge of the average size of the particles
in the nesting habitat, one can predict accurately the relative abundance of
the two species of Aethia.
The position of the plankton-feeders in the community of diving
marine birds is examined. Feeding adaptations (degree of tongue cornification,
palatal breadth, number and arrangement of palatal papillae) follow a gradient
or a regular modification throughout the family. On the basis of these variations,
which can be expressed as a ratio (Bill-width / Gape), a model is
constructed that gives a graphical representation of the breadth of the
ecological field occupied by the family. Two distinct levels emerge : the
fish-feeders (uria,. Alca) and the plankton-feeders (Aethia,. Plautps): an
intermediate level can be distinguished (Fratercula. Lundq) The species of
the latter group preserve adaptations that allow them to utilize plankton and
fish-foods. The model allows us to recognize and define special adaptations
with respect to other members of the family and to recognize the main
trends in evolution of body size and feeding adaptations within this taxonomic
group.
In comparison with younger families or orders, most members of the
Alcidae are discrete and well-defined ecologically. Among the three plankton feeders
studied, the overlap in requirements is very small and no sign of
competition for food or for nesting was found. / Science, Faculty of / Zoology, Department of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/41186 |
| Date | January 1967 |
| Creators | Bedard, Jean |
| Publisher | University of British Columbia |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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