Age-specific reproduction in three species of European ducks

-We tested the effects of female age on reproductive performance of Northern Shovelers (Anas clypeata), Tufted Ducks (Aythya fuligula), and Common Pochards (A. ferina) based on a long-term study (1958 to 1995) at Engure Marsh, Latvia. Yearling females initiated nests later than older (-2 years) females in each species, but female age explained little of the remaining variation in reproductive performance. Clutch size and brood size of all species declined seasonally. Age-specific increases in reproductive performance were most consistent between 1 and 2 years of age and were more apparent in the diving ducks than in Northern Shovelers. Clutch size, brood size, and duckling mass of yearling Tufted Ducks, and brood size and duckling mass of yearling Common Pochards, were smaller than those of older females. In Northern Shovelers, only duckling mass increased with age of the female. Nesting and hatching success did not vary by age for any species. Among 2-year-old females with and without previous breeding experience, experienced females nested five to six days earlier than inexperienced females in all species. Experienced, 2-year-old Tufted Ducks also had larger clutches and broods than inexperienced females. Effects of breeding experience generally lasted for only a single breeding season. Nesting date and duckling mass of yearling Northern Shovelers did not differ from that of inexperienced 2-year-olds. However, inexperienced 2-year-old Tufted Ducks and Common Pochards nested earlier and produced larger ducklings than did yearling females. Brood size of inexperienced, 2-year-old Common Pochards also was larger than that of yearlings. We found no evidence that age-specific increases in reproductive performance were related to differential survival of good breeders. Reproductive performance was constrained by past breeding experience in Northern Shovelers and by age and experience in Tufted Ducks and Common Pochards. Received 19 August 1996, accepted 22 May 1997. REPRODUCTIVE PERFORMANCE varies with parental age in many species of birds (Sather 1990). Young birds commonly nest later in the season, produce smaller clutches, and have lower fledging success than older conspecifics (Saether 1990, Forslund and Pirt 1995, Martin 1995). A better understanding of whether age influences reproduction is important for studies of demography and life-history evolution (Clutton-Brock 1988). Hypotheses proposed to explain age-specific variation in reproduction have focused on the concepts of constraint and restraint, which are not mutually exclusive (Curio 1983, Rohwer 1992). The constraint hypothesis proposes that young individuals lack some skill essential for high reproductive performance. Developmental or behavioral constraints, for example, may re3Present address: Gaylord Memorial Laboratory, School of Natural Resources, University of MissouriColumbia, Puxico, Missouri 63960, USA. E-mail: gaylord@sheltonbbs.com duce foraging success of young birds (Marchetti and Price 1989), thereby lowering reproductive performance (Desrochers 1992a). The restraint hypothesis suggests that the young are not less skilled than adults, but rather that they increase their probability of future reproduction by limiting current reproduction. This idea is based on life-history theory; i.e. residual reproductive value declines with age because of lower agespecific survival and reproduction (Curio 1983, Stearns 1992). In essence, reproductive events become more valuable with age; hence, older individuals invest more in reproduction than do younger individuals, causing age-specific differences in reproductive performance. Increased age-specific breeding performance also may result from differential survival of the more successful breeders (Clutton-Brock 1988, Rohwer 1992). We use data from a long-term study of breeding waterfowl on the Engure Marsh in Eastern Europe to examine age-specific (1 to 6 years old) differences in reproductive performance of