Shortening day length as a previously unrecognized selective pressure for early breeding in a bird with long parental care

Marcin Podlaszczuk,Piotr Minias,Agnieszka Wojciechowska,Zbigniew Wojciechowski,Patrycja Podlaszczuk,Tomasz Janiszewski

doi:10.1007/s10336-014-1136-7

Abstract

Several different selective pressures have been suggested to explain an intense competition for early return to breeding grounds in birds. In this study we hypothesized that shortening day length during summer months may constitute additional selective force acting towards early breeding in avian species with long parental care. To test this hypothesis, we studied time budget and foraging activities of early-nesting and late-nesting white storks Ciconia ciconia from the Central-European population. We found that duration and distance of foraging trips increased significantly over the course of the reproductive season. The relative frequency of foraging trips increased at the expense of other activities, such as resting, plumage maintenance, and nest maintenance. Mean daily foraging duration increased with increasing day length in the early part of the season, with 0.68 h of foraging per individual per 13.16 h of day length in mid-April increasing to 7.42 h of foraging per individual during solstice (16.61 h of day length). Afterwards, mean foraging duration continued increasing in spite of decreasing day length, reaching 11.63 h of foraging per individual per 14.92 h of day length at the end of the season in mid-August, when storks were forced to continue foraging after sunset in order to meet energy requirements of fledglings. The results suggest that shortening day length during summer months may constitute a serious time constraint on food delivery rates to offspring for late-breeding pairs of white stork.

Highlights

In birds, there is an intense competition for early return to breeding grounds, and a multitude of different selective pressures have been identified to explain this pattern (Kokko 1999)
In this study we demonstrated that foraging of white storks may be limited by shortening day length at the end of the reproductive season
As such limitation in the time available for foraging coincides with the peak of the brood energy requirements, we suggest that it may constitute an important selective force acting towards early initiation of breeding in this species

Summary

Introduction

There is an intense competition for early return to breeding grounds, and a multitude of different selective pressures have been identified to explain this pattern (Kokko 1999). Early-arriving individuals gain better access to high-quality territories (Forstmeier 2002), which may provide fitness benefits in terms of larger food resources, availability of more favourable nest sites, or lower predatory pressure (Aebischer et al 1996; Lozano et al 1996; Cooper et al 2011). The timing of one event may determine the timing of the forthcoming events throughout the sequence of the life-cycle stages, which is known as the ‘‘domino effect’’’ (Piersma 1987). Under such circumstances, one may expect that the timing of arrival at breeding grounds will have profound consequences on the tightly regulated reproductive schedules of birds. It has not been explicitly recognized that in some avian species with long parental care, early nesting allows rearing chicks under a more favourable photoperiod

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