MORTON, M.L.*; MACDOUGALL-SHACKLETON, E.A.; PEREYRA, M.E.; HAHN, T.P.; UC Davis; U Western Ontario; UC Davis; UC Davis: Survival is Unrelated to Natural Variation in Reproductive Effort in Mountain White-crowned Sparrows

In theory, mortality can ensue in parents if they devote excessive resources toward reproduction. Such a trade-off between reproductive effort and survival has been found in some studies of hole-nesting passerine birds; return rates of breeding adults in the following season were reduced when investigators added extra eggs or nestlings to their nest boxes. Open-nesting passerines are less amenable to these types of manipulations but they do exhibit a wide natural range in reproductive effort due to variations in fecundity and to intervening environmental factors that can cause nest failures and extend the breeding season. These efforts and their potential costs can be evaluated by comparing subsequent survival rates to differences in numbers of eggs laid per clutch or per season, number of fledglings produced, frequency of double brooding, fledging date, and number of nesting attempts (first nest of the season plus replacements, if necessary). We explored this last variable, number of nesting attempts, as amplified by predation and storms, in a long-term study of a migratory passerine by examining nest-loss dynamics, behaviors and energy balance during the renesting sequence(s), and by compiling rates of survival in relation to the number of nests constructed and utilized per season by breeding pairs. In no analysis of these variables was a cost of reproduction evident; survival did not vary with reproductive effort. This is in agreement with results from the only other study of an open-nesting passerine, the Song Sparrow, by J. N. M. Smith, and it indicates that the time, energy, and nutrients devoted to reproduction in passerines are usually not so excessive as to compromise survival, even when environmental conditions are most challenging.