Within bird populations, eggs vary extensively in size, shape, and color, but individual females tend to lay eggs that are relatively consistent in most traits. We measured all eggs laid by individual female Savannah Sparrows (Passerculus sandwichensis) breeding on Kent Island, New Brunswick, comparing subsequent clutches within the same year and in successive years. Repeatability was high for egg size and shape (0.78 and 0.72, respectively). Conversely, the repeatability of intensity of spotting was only moderate (0.46), and the background color of eggs varied from clutch to clutch and even within a clutch. Egg size increased slightly within a year from a female's first to subsequent clutch, but egg size was independent of the female's body size, the age or size of her mate, or weather conditions during clutch formation. Ranking of individual females' eggs by size and shape remained almost the same from clutch to clutch. Although laying eggs that are relatively consistent and individually distinctive in size, shape, and spotting could be an adaptation to allow females to discriminate their own eggs from those of brood parasites, this explanation does not apply on Kent Island, where neither intra- nor interspecific brood parasitism occurs. We found no evidence of short-term effects of egg size on the size of 7-day-old nestlings or of long-term effects on adults' survival or lifetime reproductive success. High repeatabilities suggest a substantial genetic component to variation in certain egg traits, which could limit females' ability to modify their eggs in response to environmental changes.
Vol. 114 • No. 2
Vol. 114 • No. 2