Growth and Development of Eastern Bluebirds

Descriptive data on avian growth rates have been used for aging (Hamel, 1974) and sexing (Holeomb and Twiest, 1970), for analyzing differential wing and leg growth (Olendorff, 1973), for examining the effects of variations in clutch size (Lack and Silva, 1949; Rieklefs, 1968; Holeomb, 1970) and hatching order .(LeCroyand LeCroy, 1974), and for retrospective dating of events in the breeding season, especially hatching (Williams, 1959).Conclusions can be drawn on such matters once the normal growth rate is established for any given species.The reliability of these conclusions for individual birds is dependent upon the normal variation in development and may vary interspecifically and for different growth parameters (Ricklefs, 1968).Little information is available on development in the Eastern Bluebird (Sialia sialis).Hamilton (1943) weighed three broods of S. sialis, and Donahoe (1939) and Smith (1937) gave general accounts of bluebird nestling development.This paper describes development for Eastern Bluebirds in southern Michigan.Parameters chosen were: (1) body and plumage measurements for young of various ages, (2) overall changes in the physical appearance, (3) ages of young that leave the nest undisturbed, (4) ontogeny of behavior, and (5) body weight changes.Characteristics of normal development in this species are then used to assess the effects of season and brood size on growth rate, and to make comparisons with other New World thrushes.METHODS "Free-living" Eastern Bluebirds were examined in Macomb County, Michigan from 1970 to 1975.Bluebirds nested there in nesting boxes similar to those used by Laskey (1940).Two areas of investigation, postfledging development and diurnal weight variation of nestlings, were explored using captive birds reared by "xvild" (= not tame) adult bluebirds that bred in large aviaries (Pinkowski, 1975a), were offered unlimited food (insects and fruit) and water, and were subjected to natural photoperiods.Observations on postfledging development and frequent weighing of free-living birds were nearly impossible or unduly disturbing, and the results obtained on the captive birds are considered typical because development in these birds was normal in other respects (fledging age, weight increases, age for attainment of independence, all approximating the corresponding data obtained for free-living birds).Individual nestlings were marked with a felt-tipped marker on the tarsi immediately after hatching and were banded at the end of the first week for identification.All birds were weighed and measured at the same time each day, usually at 1800-2000, whenever possible.All measurements were plotted and interpolated 273