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Pace-Setting Mechanisms of the Nesting Cycle in the Ring-Billed Gull

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Records of the incidence of sitting, provisioning and other attentive activities by parent Ring-billed Gulls at their nests were obtained for successive daily stages of the breeding cycle. The normal progression of stimulus features at 112 nests was experimentally delayed, advanced, arrested, accelerated and reversed by exchanging eggs and chicks of various ages for information on the effect of such manipulations on the progress of the breeding cycle. A series of undisturbed nests provided control data. The incidence of sitting activity at control nests (two parents alternating) was 99+%, when eggs were present, dropped to 90% at hatching and remained at a plateau at that level for 3 days, then declined at about 13% per day. Full time (egg-level) sitting was extended by experimentally postponing the introduction of chicks, shortened by early introductions and restored by returning to eggs after as much as four days with chicks. The 90% level of the early post-hatching period was induced by replacing eggs with chicks regardless of the time of the introduction (up to 8 days early or late) or the age of the introduced chicks (up to 6 days of age). The post-plateau decline was delayed and attenuated by arresting the normal nest progression at early chick stages, and advanced and accelerated by accelerating the progression. Provisioning (feeding the young) activity appeared with the introduction of chicks at any time, early or late in the cycle, and ceased abruptly with a return to eggs. The level of provisioning increased over the first two days to an incidence level of about 7%, then gradually declined after the fifth day. Exchanges of eggs and chicks were generally well tolerated by parent gulls in the delaying, advancing, arresting and accelerating experiments but not where the progression was reversed from advanced chicks to eggs or early chicks. The data suggest: a - The pace-setting mechanisms of the breeding cycle switch from internal (hormonal) to external (embryo and chick development) regulators early in the incubation phase of the cycle. b - Changes in parental activity at hatching are quantitatively uniform and are triggered by non-specific stimuli associated with cliicks. c - Changes in parental activity after the second day are quantitatively regulated by changing stimuli associated with growing chicks.

Affiliations: 1: University of Wisconsin, Madison, Wisc., U.S.A.


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