Differential investment and costs during avian incubation determined by individual quality: an experimental study of the common eider (Somateria mollissima)

Abstract

Individuals of different quality may have different investment strategies, shaping responses to experimental manipulations, thereby rendering the detection of such patterns difficult. However, previous clutch-size manipulation studies have infrequently incorporated individual differences in quality. To examine costs of incubation and reproductive investment in relation to changes in clutch size, we enlarged and reduced natural clutch sizes of four and five eggs by one egg early in the incubation period in female common eiders (Somateria mollissima), a sea duck with an anorectic incubation period. Females that had produced four eggs (lower quality) responded to clutch reductions by deserting the nest more frequently but did not increase incubation effort in response to clutch enlargement, at the cost of reduced hatch success of eggs. Among birds with an original clutch size of five (higher quality), reducing and enlarging clutch size reduced and increased relative body mass loss respectively without affecting hatch success. In common eiders many females abandon their own ducklings to the care of other females. Enlarging five-egg clutches led to increased brood care rate despite the higher effort spent incubating these clutches, indicating that the higher fitness value of a large brood is increasing adult brood investment. This study shows that the ability to respond to clutch-size manipulations depends on original clutch size, reflecting differences in female quality. Females of low quality were reluctant to increase investment at the cost of lower hatch success, whereas females of higher quality apparently have a larger capacity both to increase incubation effort and brood care investment.

Cost of reproduction in a long-lived bird: incubation effort reduces immune function and future reproduction

“…We did not include any control clutches where clutch size had not been manipulated. However, a previous clutch size manipulation study conducted in the same colony found no differences in mass loss, nest desertion rates or brood abandoning rates between one group where no manipulation took place, and another group where one egg was swapped between nests without altering clutch size, suggesting that there is no effect of exchanging eggs between nests in eiders (Hanssen et al 2003a). Owing to nest losses caused by desertion and predation, 24 females in the low demand and 30 females in the high demand group were initially included in the analysis of incubation costs (table 1).…”

“…Thus, it is unlikely that the longterm effects of our experiment are caused by factors other than the altered costs of incubation. 1993; Moreno & Sanz 1994;Siikamäki 1995;Hanssen et al 2003a), and also costs to young after hatching (Heaney & Monaghan 1995;Cichó n 2000;Ilmonen et al 2002). A recent study on the Grindøya apopulation of eiders showed that enlarging clutch size by one egg led to reduced hatching success in low-quality females and increased mass loss in high-quality females (Hanssen et al 2003a).…”

“…In eiders, nest desertion rates are reduced when clutch size, and thus clutch value, is experimentally increased (Hanssen et al 2003a). In addition, large incubation cost in eiders may be compensated for by abandoning the brood to the care of other females after hatching (Bustnes & Erikstad 1991;Hanssen et al 2003b) at the cost of lower survival of the adopted ducklings (Bustnes & Erikstad 1991;Eadie & Lyon 1998).…”

“…In contrast, incubating eggs has traditionally been considered less costly than brood rearing (Monaghan & Nager 1997;Thomson et al 1998). However, studies which have experimentally increased clutch size during incubation have documented various short-term costs to the incubating parents (Biebach 1981(Biebach , 1984Haftorn & Reinertsen 1985;Coleman & Whittall 1988;Moreno & Carlson 1989;Smith 1989;Moreno et al 1991;Székely et al 1994;Reid et al 2000a;Engstrand & Bryant 2002;Hanssen et al 2003a) and the offspring (Moreno & Carlson 1989;Moreno et al 1991;Siikamäki 1995;Heaney & Monaghan 1996;Reid et al 2000a,b;Engstrand & Bryant 2002;Ilmonen et al 2002;Hanssen et al 2003a;Larsen et al 2003). Increased physical demand on the parent from increased incubation effort could also lead to delayed reproductive costs such as lower adult survival or reduced future fecundity.…”

“…However, only one of the existing studies of incubation costs, known to us, has strongly indicated reduced survival (Visser & Lessells 2001), and no studies have shown reduced fecundity. One of several possible reasons for not finding such costs may be that parents are reluctant to increase effort in response to brood or clutch size manipulation, thereby jeopardizing the viability of eggs or young in enlarged broods (Siikamäki 1995;Hanssen et al 2003a). That is, increased demand does not lead to the increased current parental investment that, in turn, could lead to future fitness costs for the parent (Hõ rak 2003).…”