Abstract:
Breeding parameters of Great Cormorants (PkaZac/iOCOfiCLX CCUibo dCUtbo) and
Double-crested Cormorants (P. CLU/uXuA CMJhLtllb) were examined at two mixed
species colonies at Cape Tryon and Durell Point, Prince Edward Island from
1976 to 1978. Differential access to nests at the two colony sites resulted
in more complete demographic data for P. CCUibo than for P. CLUJiituA.
In 1911j P. CCtfibo was present at both colonies by 21 March, whereas
P. auAAJtuA did not return until 1 April and 16 April at Cape Tryon and
Durell Point, respectively. Differences in the arrival chronology by
individuals of each species and differences in the time of nest site
occupation according to age, are suggested as factors influencing the nest
site distribution of P. CXUtbo and P. aiVtituA at Cape Tryon.
Forty-eight P. dOJtbo chicks banded at the Durell Point colony between
19 74 and 19 76 returned there to nest as two- to four-year olds in 19 77 and
19 78. Unmarked individuals with clutch-starts in April were likely greater
than four years old as all marked two to four-year olds (with one possible
exception) in 19 77 and 1978 had clutch-starts in May and June.
Seasonal variation in the breeding success of P. dOJibo individuals was
examined at Durell Point in 1977. Mean clutch-size, hatching success and
fledging success exhibited a seasonal decline. Four- and 5-egg clutches
represented the majority (75%) of all P. CCUibo clutches at Durell Point in
1977 and had the highest reproductive success (0.48 and 0.43 chicks fledged
per egg laid respectively). Smaller clutches produced small broods with
significantly higher chick mortality while larger clutches suffered high
egg loss prior to clutch completion.
