Clutch investment in the Chilean Swallow shifts with time of breeding and position in the laying sequence

LILJESTHROM MARCELA; JUAN CARLOS REBOREDA; CAREN COOPER

THE CONDOR

COOPER ORNITHOLOGICAL SOC

Año: 2012 vol. 114 p. 377 - 384

0010-5422

Egg and clutch size vary within a population and the combination of both determines the reproductive investment by females. We investigated female investment in clutches of Chilean Swallows (Tachycineta meyeni) breeding in an extreme climate at the southernmost distribution of their range. We analyzed clutch, egg and yolk size variation in relation to female condition, laying date and position in the laying sequence. As predicted by energy/nutrient constraints, clutch size and yolk size decreased over the breeding season, though egg mass increased. Female investment strategy differed with clutch size with investment in eggs increasing with laying order in clutches of 4 and 5 eggs, and decreasing with laying order in clutches of 3 eggs. Eggs that were heavier for their size were more likely to hatch than eggs light for their size. Females may make a strategic decision, with high investment early in the laying sequence resulting in a short sequence (small clutch) and delayed investment resulting in a long laying sequence (large clutch). Alternatively, shift in clutch investment may simply be a consequence of physiology. Better estimates of female body condition are needed to tell apart these options. .