Predation and Starvation: Age-Specific Mortality in Juvenile Juncos (Junco phaenotus)

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Abstract
(1) Death rate of yellow-eyed juncos was highest in the first year, significantly higher than in adults; 11% of ringed nestlings were found the following spring. Nestlings, fledglings incapable of extended flight, and recently independent juveniles all suffered especially high mortality. (2) Predation was the main cause of death of nestlings and fledglings. Starvation was less important: it was rare for only one nestling in a brood to die, nestling weight was independent of brood size, and survival was independent of weight at fledging. (3) However, starvation was the major mortality factor of recently independent juveniles. These birds lost weight and their survival was significantly associated with weight during the first few weeks of independence. (4) Age and sex did not significantly affect survival during winter. Intraspecific competition for limited food may not usually cause differential mortality of age and sex classes but may do so in years with low rainfall and scarce food.