| dc.description.abstract | Female-female pairs of Ring-billed Gulls were studied on
Granite Island, northern Lake Superior, during the breeding
seasons of 1979 and 1980. In 1979 the colony consisted of approximately
2400 pairs with 99 nests (4.1%) containing five to
seven eggs. In 1980 the colony had increased in size to 2600
pairs and contained 71 (2.7%) superclutches. Extrapolating from
superclutches in my observation areas, where all attendants were
known, approximately 85% of the total ntmber of superclutches
were attended by female-female pairs. The difficulty distinguishing
these nests from those of polygynous groups laying in a
single nest-cup or nests receiving dump eggs is discussed. Because
of these difficulties, all superclutches were monitored
and treated as a single sample.
Eggs laid in superclutches were slightly smaller than those
from normal clutches, but did not differ in shape. The incubation
period differed significantly for eggs of the two clutch
types, with eggs in superclutches taking Ionger to hatch than
those in normal clutches. Ihis may be because the eggs toward
the outside edge of nests containing superclutches received
significantly less heat. Nests containing superclutches were
larger than those containing normal clutches but were of similar
quality and were not differentially located by substrate,
density or colony location. Nearest neighbour distance was also
similar for the two clutch types.
Nest success did not differ significantly between superclutches
and normal clutches. Hatching success for superclutches was 33% in
1979 and 29% in 1980, while normal clutches had 77% and 61% in 1979
and 1980, respectively. Significantly more eggs rolled from the nest
or were destroyed or abandoned in superclutches compared to those from
normal clutches.
Egg fertility rates were 87% and 94% for eggs in superclutches
while normal clutches had 100% and 99% fertility in the two years of
study. Chicks from normal clutches had a significantly higher fledging
success than those of superclutches. Chicks from the latter had a
higher rate of mortality due mainly to a higher death rate during the
week following hatching. During both years of my study chicks from
superclutches hatched at significantly lighter weights than those from
normal clutches, but their weights did not differ after the first week
post hatch. Tarsal and culmen measurements followed a similar pattern
to that of weight. Food types brought to chicks from the two clutch
types did not differ. Their diet consisted almost totally of fish and
insects.
Females of female-female pairs were of normal size, weight, age
and condition compared to heterosexually paired females. Levels of 13
blood chemistry parameters were measured for female-female pair members and heterosexually paired birds. The only consistent differences were low cholesterol and elevated progesterone levels in members of female-female pairs. Intra-pair size differences suggest that the females
assertively paired by size. They exhibited mate fidelity and to a lesser
extent nest-site tenacity.
Behaviour of female-female pairs is compared to heterosexual
pairs during courtship, the incubation period and rearing of the
brood. Females involved in female-female pairs cooperated well. Four
polygynous groups were monitored, and their behaviour and reproductive
success is reported. Existing hypotheses regarding the origin(s) of
female-female pairing and their biological significance are discussed. | |
