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Growth and Survival of Young Florida Scrub Jays

Glen E. Woolfenden
The Wilson Bulletin
Vol. 90, No. 1 (Mar., 1978), pp. 1-18
Stable URL: http://www.jstor.org/stable/4161020
Page Count: 19
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Abstract

Growth of young was measured in a marked population of Florida Scrub Jays that has been censused from 1969 to the present. Data were gathered mostly in 1973 when samples ranged up to 59 nestlings, which were the reproductive efforts of 28 pairs. Fresh eggs weigh 5.8 g, 7.6% of adult female weight, and lose about 13% of their weight during incubation. Newly hatched, unfed young weigh about 4.5 g, about 78% of a fresh egg. Based on growth curve computations, nestling growth is half completed at 8.2 days. The overall growth rate index of 0.335 is similar to that of Piñon Jays, and the young grow only slightly slower than expected for their body size. Development at fledging lags behind most passerines thus far measured, as is true also of Piñon Jays. Florida Scrub Jays do not attain "adult" weight during the first year. Growth of certain extremities also requires many months. Fluctuations in fledgling weights parallel those of "adults" from fall to early spring. Desertion and weather rarely cause nesting failure, and starvation of nestlings accounts for less than 10% of nestling losses. Predation is the major factor; it accounts for over 80% of all nestling losses. Breeding pairs with helpers produce more young, especially fledglings, than do unassisted pairs. Nestlings fed by relatively more feeders are heavier, but survival as fledglings does not correlate with nestling weight or the feeder index. Even nestlings half normal weight at day 11 appear to survive as fledglings as well as do heavier birds. Decreasing predation, especially on nest contents, is proposed as the major way that helpers increase reproduction. Elaborate active nest defense by breeders and helpers supports the suggestion. The suspected predators they may sometimes dissuade are certain snakes, Fish Crows, Blue Jays and Scrub Jay cannibals.

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