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Physical Development of Nestling Bald Eagles with Emphasis on the Timing of Growth Events

Gary R. Bortolotti
The Wilson Bulletin
Vol. 96, No. 4 (Dec., 1984), pp. 524-542
Stable URL: http://www.jstor.org/stable/4161989
Page Count: 19
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Physical Development of Nestling Bald Eagles with Emphasis on the Timing of Growth Events
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Abstract

Developmental changes in color, weight, body size, and the appearance of body contour feathers, are described for wild nestling Bald Eagles (Haliaeetus leucocephalus) in Saskatchewan. Chicks hatched with relatively large bills, large legs, and a small body. The growth of some body components (e.g., the legs) was complete about halfway through the nestling period, whereas the mature size of the bill and flight feathers was not reached until after the birds left the nest. The maximum absolute weight gain per day (180 g, a conservative figure) of Bald Eagles appears to be the greatest of any North American bird, but this is to be expected of a temperate zone altricial species of its body size. Weight growth was not correlated with body feather development or the rate of eighth primary feather growth, but was significantly correlated with the timing of the emergence of the eighth primary. There was a great deal of variation in the age at which body feathers unsheathed, yet little variation in the growth of flight feathers. Body feather growth and primary feather growth were largely independent. Males differed from females in being smaller, having earlier inflection points to growth curves and growing flight feathers at a younger age, but were not different in rate of growth. When body size was accounted for, the relative growth of the sexes was equal. The emergence of second down and flight feathers was delayed for the second-hatched chick in the nest compared to its older sibling. The age at which eaglets became homeothermic was estimated to be about 15 days, at which time a sharp decline in the nest attentiveness behavior of the parents was observed. Caution must be exercised when attempting to determine the adaptive significance of patterns of growth, for growth itself may not have been the primary adaptation. Several analyses presented here showed that the timing of growth events, rather than the rate at which they proceed, was the more important consideration in assessing development.

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