Abstract:
The reproductive biology of the Ring-billed Gull (Larus delawarensis)
was studied on Gull Island, Presqu'ile Provincial Park, Ontario, in 1976
and 1977. Early started clutches (comprising the majority of clutches on
Gull Island) in 1977 produced more chicks per nest (2.20 ± 0.09) than late
started clutches (0.86 ± 0.13) as a result of reductions in mean clutch
size, hatching success and fledging success with date of clutch initiation.
Seasonal changes in mean clutch size, hatching success and fledging success
also resulted in early clutches, initiated at the peak of clutch starts,
producing more chicks per nest (2.34 ± 0.11) than either pre-peak
(2.13 ± 0.20) or post-peak (1.82 ± 0.29) clutches. Possible reasons for
these trends, including the observed predominance of immature plumaged,
breeding gulls in late started areas, are discussed.
Clutches were deserted at night for varying lengths of time from at
least 15 April until 10 May, 1977. It is suggested that this nocturnal
desertion behaviour resulted in the enhancement of inter- and intra-clutch
hatching synchrony in early started areas and further, that this may in
part explain the existence of the behaviour in terms of its adaptive
significance.
