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Breeding biology of the American Robin (Turdus migratorius) Kemper, Dorothy Lynne

Abstract

The American Robin (Turdus migratorius) in southwestern British Columbia was studied in regard to timing of reproduction, breeding biology, gonadal changes with time, and the relationship between photoperiod and gonadal condition. The basic breeding biology is similar to that found for other robins in the eastern and midwestern United States. The only marked difference is the very high nesting success rate of the robins which I studied. The overall success rate was 86.6 percent with 87.8 percent of all eggs laid hatching and 98.6 percent of these fledging. The onset of the breeding season was marked by the increase in territorial aggressive behaviour of male robins. This increase is a gradual process taking place over a two month period from early February until late March. The histological pattern of testes development and regression in the robin is the same as that for other temperate zone passerines. The length of the daily photoperiod has a definate effect on the timing of testicular recrudescence. The average testes weights of robins kept on eight, 12, and 16 hour photoperiods for the period of the annual cycle when testicular development occurs in the wild were 9.4 mg., 14.6 mg., and 200.5 mg. respectively. As well as stimulating gonadal development increasing photoperiod, as opposed to short day lengths, affects migratory behaviour as evidenced by Zugunruhe or night restlessness.

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