Reproductive performance of Ring-billed gulls in relation to nest location / by Patricia Lynn Somppi. --
Somppi, Patricia Lynn.
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Reproductive performance of Ring-billed Gulls (Larus delawarensis) in relation to nest location was investigated on Granite Island, northern Lake Superior, in 1976 and 1977. To determine whether any differences existed in the hatching and fledging success of central and peripheral areas of the colony in 1976 and 1977, only 3-egg clutches initiated early in the season were used. This eliminated two variables that may also affect success; they are clutch size and time of clutch initiation. In both years, 25 3-egg clutches in the center of the colony were exchanged with 25 3-egg clutches on the periphery. Twenty-five 3-egg clutches were marked concurrently in each area as controls. In both years, there was no significant difference in hatching success (86%), or fledging success (58%) between exchange and control nests. These results indicated the ability of the gulls to hatch eggs and raise young was not related to nest location. Ten clutches from each area which were artificially incubated, had essentially equivalent hatching success. To determine if there was a difference in attentiveness in the two areas, 25 nests in each area were monitored photographically for 6 days during the incubation period. In both years, over 90% of the gulls incubated a minimum of 90% of the time regardless of nest location. In 1977, histories were known for all nests in the study area. Hatching success and clutch size were not correlated with incubation attentiveness. In both the center and periphery of the colony, eggs laid before or at the peak of clutch initiation had a hatching success of 60%, 30% higher than eggs laid after the peak. Hatching success was significantly related to clutch size, 3-egg clutches being the most common (54%) and the most successful (76%) in both areas. On Granite Island, reproductive success was related to time of clutch initiation and clutch size rather than nest location. The possible reasons for this are discussed.