The relationship between phenological traits and brood size of the white stork Ciconia ciconia in western Poland

Abstract

Changes in the spring arrival dates of migrant birds have been reported from a range of locations and many authors have focused on long-term trends and their relationship to temperature and other climatic events. However, there may be consequences (costs) for changes in arrival dates which long-lived birds may have to repay in the future. In this paper we take the opportunity provided by a detailed monitoring scheme to examine several features of the timing of arrival in relation to chick production of white storks Ciconia ciconia in Poland during the period 1983–2003. Longer occupied nests were associated with greater productivity than shorter occupied ones. Early arriving pairs were more productive than later ones. Individual birds and the whole population could achieve greater productivity by earlier arrival. However, high chick production in one year tended to be followed by a slightly lower productivity and delayed arrival in the following year. Hence, the cost of early arrival at breeding sites may have to be paid for in the following, rather than in the current, year.

Introduction

In migratory birds, the earliest individuals to arrive on their breeding grounds typically occupy the best territories and consequently have the highest reproductive success (Potti, 1998, Kokko, 1999, Forstmeier, 2002). Moreover, in some species, such as the white stork Ciconia ciconia which builds large, “perennial” nests and where the number of potential nest sites is sometimes limited (e.g. Creutz, 1985, Profus, 1991), advanced arrival increases the opportunities to maintain occupation of good nests and save the energy needed for nest construction. Early arrival also provides the opportunity for an earlier start to egg laying (Møller, 1994, Brown and Brown, 2000; but see Both and Visser, 2001), which strongly influences breeding capacity. Therefore, a strong selection is expected for early arrivals from wintering grounds located in tropical Africa (see Kokko, 1999, Morbey and Ydenberg, 2001 for more general models). This view is supported by changes in recent decades, when migratory birds have tended to return earlier from wintering grounds to breeding places as a consequence of climate change (reviews in: Walther et al., 2002, Parmesan and Yohe, 2003, Lehikoinen et al., 2004). This tendency has also been noted in the white stork (Ptaszyk et al., 2003, Tryjanowski et al., 2004). Moreover, previous papers have clearly indicated a link between arrival date and timing of breeding (Tryjanowski et al., 2004), suggesting a strong evolutionary pressure on earlier migration.

On the other hand, birds that arrive early on their breeding grounds can suffer the consequences, and in extreme cases, death (Whitmore et al., 1977, Brown and Brown, 2000), mainly due to the influence of and/or changes in the food supply. It can be argued that the costs of early arrival would always be greater for individuals in poorer condition (Møller, 1994, Kokko, 1999, Forstmeier, 2002). However, to date, such results have only been presented for relatively short-lived passerines. The situation for long-lived species may differ for a number of reasons. Firstly, one poor year of reproductive output following adverse weather on the breeding grounds (Tryjanowski et al., 2004) may not be too detrimental to lifetime reproductive success. Secondly, the cost of reproduction in long-lived species may result in a reduction in future fecundity, rather than reduced survival (Linden and Møller, 1989, Hanssen et al., 2005).

In this study we start to answer the classical questions on the relationship between arrival date and chick production. Secondly, we examine how arrival date and productivity are linked to particular nest sites and if any pattern is discernible between years. Finally, to the best of our knowledge for the first time in phenological studies, we try to show the potential cost to the females of early arrival on the breeding grounds and how it affects their potential reproductive performance. From earlier work, we anticipated a negative relationship between arrival date and the number of chicks fledged in a particular year (Tryjanowski et al., 2004). We also anticipate that, for a long-lived species like the white stork, early arrival and greater chick productivity may result in higher reproductive costs although the consequences for the population may be delayed (Hanssen et al., 2005, Sæther et al., 2005).