TERMINAL INVESTMENT AND A SEXUAL CONFLICT IN THE COLLARED FLYCATCHER (FICEDULA-ALBICOLLIS)

When the expectation of future reproduction is reduced by senescence, life-history theory predicts that reproductive effort will increase with increasing age. This idea was examined in the collared flycatcher by estimating whether reproductive costs increase with female age, comparing feeding rates and weight losses of old, "senescent" females (i.e., greater-than-or-equal-to 5 yr old) and middle-aged females (2-3 yr old) with the same breeding phenology and the same brood size, and testing whether feeding rate was correlated with daily energy expenditure and with weight loss of females during the nestling period. There was a negative relationship between fledgling production and subsequent survival among old females (greater-than-or-equal-to 5 yr old), but not among younger age classes, which suggests that reproductive effort increases with age. Also, old females fed their nestlings more often and lost more weight during the nestling period than did middle-aged females. Observed feeding rates were positively correlated with daily energy expenditure and weight loss. Since there was no evidence that individuals that survived to old ages were better at all ages, the results strongly suggest that old collared flycatcher females increase their reproductive effort at the cost of a decreased probability of surviving to the next year. However, the payoff of the increased reproductive effort of old females seemed to be small. We suggest that this is a consequence of a conflict between the sexes over the division of work, because old females generally are mated to younger males that probably have better future prospects. Data on male feeding rates in relation to female feeding rates support this idea.