A Quarter Century of Variation in Color and Allometric Characteristics of Eggs from a Rain Forest Population of the Pearly-eyed Thrasher (Margarops fuscatus).

Abstract

Egg color, size, and shape vary considerably within and among female Pearly-eyed Thrashers (Margarops fuscatus). Results of a 25-yr study (1979-2004) are presented to provide comparative data. In a sample of 4,128 eggs, typical shape was prolate spheroid; but several variations were observed, depending on the age, stature, and physiological condition of the female, as well as environmental influences such as the availability of essential nutrients. From a subset of 3,867 eggs measured between 1979 and 2000, a speciesspecific egg shape, or elongation index of 1.44 was derived (mean = 1.44; SD = 0.07; range = 0.93-1.85). Pooling eggs from among completed clutches of 1-4 eggs in each (mode = 3), the average egg measured 32.40 mm in length (SE = 0.02; range = 18.13-38.97 mm) and 22.50 mm in breadth (SE = 0.01; range = 16.81-33.61 mm). Observed average fresh egg mass for a set of 3,581 eggs was 8.8 g. Of three standard formulae used to predict average fresh egg mass, Rahn et al. (1985) was the most accurate (8.7 g). Averaging five standard formulae resulted in an egg volume of ca. 7.0 cm3. In the Pearly-eyed Thrasher, egg size and mass tended to decrease just as it does in several non-passerine species. Light clutch mass relative to body mass is a preadaptation that increases Margarops’ potential of laying more clutches throughout its reproductive life, thus enhancing its lifetime reproductive success and ensuring its continual survival in the region.

Citation

ARENDT, WAYNE J. 2004. A Quarter Century of Variation in Color and Allometric Characteristics of Eggs from a Rain Forest Population of the Pearly-eyed Thrasher (Margarops fuscatus). Caribbean Journal of Science, Vol. 40, No. 2, 204-217,