Abstract:
Patterns of intra-clutch egg size variation and intra-clutch hatch
intervals in the Ring-billed gull (Larus delawarensis) were documented
during the peak nesting period of two consecutive breeding seasons, at a
colony near Port Colborne, Ontario. Egg size decreased with laying order;
third laid eggs were significantly smaller than first laid eggs. Hatching
of the third egg was delayed from that of first and second eggs. Intraclutch
egg size differences established initial size disparities among
chicks at hatch. Hatch intervals further exaggerated size disparities
during the early post brood completion period. Competitive asymmetries
among chicks were associated with increased mortality rates among third
hatched chicks despite the lack of evidence of a sibling feeding hierarchy.
Fledging success in 1987 was greater than in 1988. A "brood reduction
strategy" appears to have enabled parents in 1987, to obtain an extra unit
of reproductive fitness, while in 1988 parents were often unable to raise
the entire brood and third chicks likely represented insurance
reproductive value.
Experimental broods (1988) were created in which hatch intervals
were double those of natural intervals. The size disparities among chicks
were significantly greater than in control broods, and the pattern of
mortality among chicks suggested that first chicks benefited at a cost to
second and third chicks. Parents of peak experimental broods achieved a
fledging success rate similar to that of control broods.
Characteristics of chick adoptions were also recorded. In each study
year, 9 chicks abandoned their natal territories, 6 of which were adopted.
Chicks consistently established themselves into broods where they were
older than resident chicks. No direct evidence of cost to foster parents,
or benefits to adopted chicks was obtained, although fledging success of
adopted chicks was high.
