Abstract
The seasonal timing of avian reproduction is supposed primarily to coincide with favourable feeding conditions. Long-term changes in avian breeding phenology are thus mostly scrutinized in relation to climatic factors and matching of the food supplies, while the role of nesting mortality is largely unexplored. Here we show that higher seasonal mean daily mortality rate leads to a shift in the distribution of breeding times of the successful nests to later dates in an an open-nesting passerine bird, the red-backed shrike Lanius collurio. The effect appeared to be strong enough to enhance or counteract the influence of climatic factors and breeding density on the inter-annual variation in mean hatching dates. Moreover, the seasonal distribution of reproductive output was shifted to larger, or smaller, broods early in the season when the nesting mortality increased, or decreased, respectively, during the season. We suggest that population level changes in timing of breeding caused by a general advancement of spring and of the food supplies might be altered by the seasonality in nesting mortality. Hence, we argue that consideration of nesting mortality is of major importance for understanding long-term trends in avian phenology, particularly in species capable of renesting.
Highlights
Optimal timing of reproduction is important from both the lifetime and the seasonal perspective
We modelled nesting mortality in terms of daily mortality rate (‘‘DMR’’ hereafter)
Significance of Nesting Mortality on Breeding Time Here we showed that in red-backed shrikes, a species that readily establishes replacement clutches, higher seasonal mean nesting mortality would shift the overall distribution of hatching dates in a season towards later calendar dates
Summary
Optimal timing of reproduction is important from both the lifetime and the seasonal perspective. Animals may often benefit from an early time of breeding [1,2,3]. Experimental studies suggest that seasonal date causally affects fitness of reproducing animals [4,5,6,7]. In a wider geographical comparison, a main proximate driver is the seasonal prolongation of the photoperiod affecting timing of reproduction on the annual basis, especially at higher latitudes [8,9]. Food abundance has been recognized to play a primary role, while other factors have received less attention, though they may affect breeding recruitment of progeny to the subsequent population [1,5,17,23]
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