| dc.description.abstract | Nesting Ring-billed Gulls (Larus delawarensis) were studied on
Granite Island, northern Lake Superior, during the breeding seasons of
1983 and 1984 to determine if there was a relationship between food
supply and the time of breeding, body condition, and egg
characteristics affecting reproductive success.
Eggs laid in 1984 were heavier than those laid in 1983. This may
have been related to smaller pre-breeding food abundance in 1983. In
each year. Ring-billed Gulls nesting later in the season laid fewer and
smaller eggs that had poorer hatching success. Within clutches, third
laid eggs were smaller, lighter and produced fewer young than first or
second eggs.
Chemical analyses of eggs revealed that eggs laid in 1984 contained
more albumen, and proportionately more yolk lipid, than eggs laid in
1983, Eggs laid later in each year contained less albumen and fewer
nutrients (lipid and protein) than earlier laid eggs. Within early and
late clutches, c-eggs had decreased levels of albumen and nutrients.
For all eggs analyzed, an average of 35% of fresh egg weight was yolk,
with a fresh yolk to albumen ratio of 0.63. Water accounted for 69% of
fresh egg weight. Through the laying sequence, approximately 8% of
fresh egg weight was yolk lipid. The energy and nutrient content of
eggs remained proportionately constant through the laying sequence each
year. Egg weight was strongly correlated with egg volume and water content.
Chick weight at hatching was positively correlated with fresh egg
weight, though this correlation was weak. There was no difference in
hatch weights within clutches. Male and female eggs did not differ in
size or weight. There was no relationship between chick sex and egg
sequence in either year. The secondary sex ratio was skewed in favour
of female chicks in 1983. The overall secondary sex ratio did not
differ from unity, and the ratio of unisexual and bisexual 3-egg
clutches laid early in 1984 did not differ from 1:2:1.
In each year, females nesting later in the season had a lower body
weight and condition index than earlier nesting females. The indexed
body condition of females nesting early in 1983 was smaller than in
1984. Both male and female indexed condition decreased through the
breeding season in 1984. The body weight of females was correlated
with egg weight but not nutrient content. Egg size and composition
were not satisfactory predictors of female body weight and condition.
Chemical analyses of post-laying females revealed that late nesters
contained proportionately more body water and protein, but less lipid
than early nesters. Between years, there were no differences in the
body weight or nutrient content of post-laying females. The energy
content of the body was most strongly correlated with lipid levels in
the body. Both the body weight and indexed condition of early nesting
females were correlated with body lipid and energy levels. Females with larger stored nutrient levels (i.e. those nesting earlier each
year) laid heavier eggs which contained more nutrients.
Pre-breeding adult males and females tended to weigh more and
contain more energy per unit body weight than early nesters. I suggest
that reduced body condition, and hence egg quality, for those birds
nesting in 1983 and late nesters, were primarily related to differences
in food abundance and acquisition efficiency during the pre-breeding
period. | |
