Interannual variation in greater flamingo breeding success in relation to water levels

Ecology, Jan, 1995 by Frank Cezilly, Vincent Boy, Rhys E. Green

INTRODUCTION

In most wetlands, fluctuations in water depth appear to influence breeding success of waterbirds (Ogden et al. 1980, Frederick and Collopy 1989, Bildstein et al. 1990, Hafner et al. 1994) by strongly affecting food availability. Reduced breeding, or even abstinence from breeding, and low reproductive success during periods of drought have been reported for a number of bird species and explained by the effect of fluctuating water levels on the birds' food supply (Dusi and Dusi 1968, Ogden et al. 1980, Manry 1985; Kushlan 1986; Frederick and Collopy 1989, Bildstein et al. 1990, Johnson et al. 1991). However previous studies have only considered the relationships between water levels, the number of breeding pairs, and average nest productivity.

The number of chicks raised to fledging per nesting pair is an important parameter of the demography of colonial species. However, in most cases it is difficult to estimate (Erwin and Custer 1982, Nisbet et al. 1990). In addition, even when a reliable measure of fledging success can be obtained, this index may not be closely related to recruitment to the next generation. One way to improve the measurement of overall breeding performance is to consider variation in the body condition of fledglings simultaneously with productivity. Variation in the physical condition of nestlings and fledglings has been observed in various species of birds (Coulson and Porter 1985, Hochachka and Smith 1991, Magrath 1991, Ferrer 1992) and has been shown to have consequences in terms of natal dispersal (Ferrer 1992) and survival (Garnett 1981, Coulson and Porter 1985, Smith et al. 1989, Tinbergen and Boerlijst 1990, Hochachka and Smith 1991, Schmutz 1993, Williams et al. 1993).

An important issue, particularly in species that lay single-egg clutches and thus have no opportunity to trade off the number of offspring against offspring quality, is whether parents are able to buffer their offspring from environmental fluctuations. If this is the case, fledgling condition may not vary with environmental conditions. Alternatively, in poor years the average fledgling condition should be decreased, indicating that adults do not (or only partially) buffer offspring from environmental fluctuations. Although parents may work harder to provide sufficient resources to their offspring, the associated cost in terms of reduced survival may limit the ability of parents to maintain offspring quality under harsh conditions, especially in long-lived species (Pugesek and Diem 1990, Saether et al. 1993). Therefore it is important to produce estimates of both fledging success and fledgling body condition within demographic studies of colonial waterbirds.

In this paper, we examine variation among years in colony size, fledging success, and chick condition at fledging in the Greater Flamingo (Phoenicopterus ruber roseus) in relation to local hydrological conditions in the Camargue (southern France) to determine whether variation in water levels, through potential decrease in food availability and increased competition between foragers, affects the number of breeding pairs and their breeding success. We use data on water levels of the local hydrological network, the Vaccares system, to test this hypothesis. First, we analyze the variation in the number of breeding pairs in relation to the water levels in potential foraging sites at the time when breeding takes place over eight consecutive years (1984-1991). Second, we analyze the variation in colony productivity and in the body condition of young in relation to the extent of flooded area used by breeding flamingos for foraging around the colony. We also consider differences in recruitment between two cohorts as a potential consequence of variation in the average body condition at fledging in a given year.

METHOD

Study area and species

Flamingos have bred intermittently in the saline lagoons of the Camargue for centuries (Gallet 1949, Johnson 1983). In every year since 1974, they have successfully bred on an artificial island constructed for them in the Etang du Fangassier, part of the large complex of commercial salt pans of Salin de Giraud [ILLUSTRATION FOR FIGURE 1 OMITTED]. Depending on years and colony size, birds may also nest on a nearby dike. Flamingos begin to visit the breeding site in March. The first eggs (one per clutch) are usually laid in April and the period of egg laying can last 30-74 d. Both sexes incubate (Johnson 1983, Cezilly 1993) and feed the chick. Incubation lasts 29 d. When they are about 3 wk old, the chicks form a creche (assemblage of chicks not yet independent) in the vicinity of the breeding island where they remain until fledging, 75-90 d after hatching.

Water levels and the Vaccares system

Two major foraging habitats are used by breeding flamingos in the Camargue [ILLUSTRATION FOR FIGURE 1 OMITTED], the commercial salines (a network of shallow lagoons and ponds varying in salinity from that of the sea through to salt-saturated brine) and a complex of brackish lagoons and temporarily flooded "sansouire" (low-lying steppe with sparse halophytic vegetation) in the south of the Vaccares system (Johnson 1983; G. Hirons, unpublished data). The salines cover a large area ([approximately equal to]12 000 ha) and provide both nesting and foraging habitat for the flamingos (Johnson 1983, Britton and Johnson 1987). Management of the salines by the salt industry results in the maintenance of fairly constant water levels. However during the study period a strike in 1989 delayed by 10 d the pumping of water, resulting in a late flooding that year of many lagoons, including the Etang du Fangassier.