Brood size manipulations within the natural range did not reveal intragenerational cost of reproduction in the Willow TitParus montanus

Abstract

To test for the existence of a reproductive cost, we manipulated brood sizes (‐2 and +2 nestlings) over 6 years in a northern population of Willow Tits Parus montanus breeding in natural holes. Possible effects were sought in subsequent survival and fecundity of the parents. Parents given extra chicks made more feeding visits than did parents with reduced and control broods. However, this was not reflected in differences in parental body‐weight between groups at the end of the nestling period. Brood size manipulation did not significantly affect female or male survival. In 4 out of 6 years, there was a weak and nonsignificant effect on male survival, consistent with a cost of reproduction. Female and male fecundity in the year following the experiment was not affected by the manipulations. Thus, the data do not give evidence of an intragenerational cost of reproduction in the Willow Tit. Parents appeared unwilling to increase their breeding effort to a level which jeopardized their own survival or future breeding success. It is possible that, because of the time constraints in northern latitudes, females work under their capacity and lay smaller clutches than would otherwise be most profitable. Thus, no costs to the parents would be expected as a consequence of manipulations. These results suggest that the current reproductive rate is not maintained by reproductive cost in the Willow Tit. However, the results do not rule out the possibility that selection has operated outside the current range of reproductive rates during evolutionary history of the species.

Energy expenditure, nestling age, and brood size: an experimental study of parental behavior in the great tit Parus major

“…We did not try to analyze the effect of the manipulation on adult survival because of the large data sets needed to demonstrate statistically small differences in survival probability (Graves, 1991;Moreno, 1993;Nur, 1988;Roff, 1992). Interseasonal fecundity costs have been revealed for some species (Gustafsson and Pärt, 1990;Gustafsson and Sutherland, 1988;Røstkaft, 1985), while they have not been found in others (Korpimäki, 1988;Lessells, 1986;Orell et al, 1996;Pettifor, 1993;Pettifor et al, 1988). In the great tit, a double-brooded species, an intraseasonal cost has been revealed (Smith et al, 1987;Tinbergen, 1987;Verhulst and Tinbergen, 1997).…”

Haematological health state indices of reproducing Great Tits: a response to brood size manipulation

“…The question to what extent (if at all) the increased reproductive effort actually confers deleterious effects upon parents, however, often remains unknown in such studies (e.g. Verhulst 1995;Orell et al 1996). To solve this problem, an assessment of the direct effects of brood size manipulations on parental health state is therefore necessary.…”

Unpredictable food supply modifies costs of reproduction and hampers individual optimization

“…Mauck and Grubb 1995;Orell et al 1996;Blondel et al 1998;Both et al 2000;Tinbergen and Verhulst 2000;Pettifor et al 2001). This might reflect the view that if one investigates a sufficient number of life history traits, both costs and optimization will be detected (Pettifor et al 2001).…”

“…It is also accepted that food abundance may seriously affect the costs of reproduction (Tinbergen and Dietz 1994;Blondel et al 1998). The fact that the optimal value of clutch size in birds may vary with environmental quality has been widely suggested by variation of selection differentials across seasons (van Noordwijk et al 1981;Gibbs 1988) and significant year effects on reproductive success in long-term studies (Pettifor 1993;Orell et al 1996;Blondel et al 1998;Pettifor et al 2001).…”