AN EXPERIMENTAL STUDY OF THE COSTS OF REPRODUCTION IN THE KITTIWAKE RISSA TRIDACTYLA : COMMENT

The cost of reproduction is a central concept in life history theory, encompassing any reduction in future survival or fecundity caused by a reproductive event (Roff 1992, Stearns 1992). The cost of reproduction has been measured by a variety of techniques, some of which have been criticized (Reznick 1985, 1992). One technique, phenotype manipulation (including clutch size manipulation), has been successfully used to demonstrate costs of reproduction in natural populations (Gustaffson and Sutherland 1988, Nur 1988, M0ller 1993). If the costs of reproduction are expressed as reduced survival, then accurate survival estimates of adults are required to show such costs. Unfortunately, in several cases, the evidence of decreased survival caused by high reproductive effort is based on improper survival estimates (see Pollock et al. 1990, Lebreton et al. 1992, 1993, Clobert 1995, Kendall and Nichols 1995, and Martin et al. 1995 for recent reviews). Jacobsen et al. (1995) recently reported experimental results on the costs of reproduction. In their experiment on Kittiwakes (Rissa tridactyla), clutch size was manipulated and adult survival responses were reported. Observing higher return rates for females in the reduced-brood group than for those in the control and enlarged-brood groups, Jacobsen et al. (1995) suggested that an increase of one offspring represented a significant increase in parental reproductive effort and, thus, resulted in a lower probability of survival to the subsequent season. However, considering the methods used to estimate survival probabilities and some other issues related to the experimental manipulation of brood size,