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Breeding behavior and feeding habits of the bald eagle (Haliaeetus leucocephalus L.) on San Juan Island, Washington Retfalvi, Laszlo

Abstract

The breeding behavior and feeding habits of the bald eagle (Haliaeetus leucocephalus L.) were studied during 1962 and 1963 on San Juan Island, Washington. The primary aim of the study was to acquire information which would relate to the general decline in bald eagle numbers. Thirteen bald eagle nests were found on San Juan Island. On the basis of the spacing of these nests, the density of breeding eagles was considered to be low. The number of bald eagles varied throughout the year; the highest numbers were present in February and the lowest numbers in October. The change in eagle numbers was caused by the fluctuating numbers of juveniles. Two active nests were kept under observation, one in 1962 and another in 1963. The breeding eagles showed hostility toward intruders such as adult and juvenile bald eagles and man in the vicinity of their nesting site. The birds showed indifference toward intrusions of red-tailed hawks, crows and gulls. The chronology of breeding activities on San Juan Island was approximated with the aid of local information and with findings of former investigators. Egg-laying occurs between March 4 and 19; hatching between April 8 and 14. The young spend 12 to 1 3 weeks in the nest during which time parental attention gradually decreases. During the first 5 weeks of the youngs' life the nest is constantly guarded by one of the parents. Parental attention markedly decreases after the young start their wing exercises at the age of 8 to 9 weeks. In general, the female parent spent three times as much time at the nest as did the male. Food was brought to the nest at irregular intervals by both the female and male parents. The young were fed mainly by the female during the first seven weeks of their life; later the young fed themselves. No appreciable change in the amount of food supplied to the young during their nest life was observed. Young of the same nest were similar in size and indications of maltreatment from the others or from the parents were not observed. Most of the food brought to the nest consisted of rabbit carrion. This type of food item was available throughout the year due to the rabbits' high mortality resulting from collision with automobiles on the roads. Fish was fed to the young during the first six weeks of their life; thereafter, mainly rabbit was consumed. Rabbit carrion was the primary source of food for young eagles in their post-nestling period. The destruction of breeding habitat by real estate developments is the major cause of decline in numbers of bald eagles on San Juan Island.

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