Timing of Breeding and Reproductive Costs in Collared Flycatchers

Abstract

Many recent studies of birds have attempted to measure a cost of reproduction by correlating current investment with subsequent parental survival and reproductive performance (see Dijkstra et al. 1990, Roff 1992, Stearns 1992). Demonstrating a link between current and future reproduction provides an important test of within-generation life-history tradeoffs (Williams 1966, Gadgil and Bossert 1970, Charnov and Krebs 1974, Daan et al. 1990, Stearns 1992). Studies of birds have typically relied on experimental manipulations of brood size (e.g. Gustafsson and Sutherland 1988, Nur 1988) because brood size often is correlated with the amount of care provided by parents and with the number of offspring subsequently recruited into the population. Timing of breeding is another factor that may be significantly correlated with parental survival (e.g. lower survival for late breeders; Harvey et al. 1988, Wiggins 1991, Verhulst et al. 1995) and offspring recruitment (e.g. decreased recruitment with later breeding; Price et al. 1988). Although recent experimental work has demonstrated a direct link between the timing of breeding per se and annual reproductive success (Verhulst and Tinbergen 1991, Brinkhof et al. 1993, Norris 1993), such studies have not assessed the long-term effects of breeding-date manipulations on subsequent parental survival or reproduction. If breeding date per se is negatively correlated with parental survival or future reproduction (e.g. owing to time constraints related to initiation of molt or migration), then experimental delays in breeding should result in reduced parental survival or reproduction in subsequent years. The limited available evidence is equivocal: Verhulst et al. (1995) found that female quality, but not breeding date, was responsible for the seasonal trend of decreasing female survival in Great Tits (Parus major), whereas among Blue Tits (P caeruleus), delayed breeding led to poor adult survival and lower future reproductive success (Nilsson and Svensson 1996). Experimental manipulations of breeding date are difficult to achieve without inducing concurrent effects on parental body condition. The two most common methods used to alter breeding date, clutch removal and temporary clutch replacement with dummy eggs, may result in considerable increases in en-