SYNCHRONY IN THE LESSER SNOW GOOSE (ANSER CAERULESCENS CAERULESCENS). II. THE ADAPTIVE VALUE OF REPRODUCTIVE SYNCHRONY.

Abstract

Social animals cluster with conspecifics. Such clustering can occur in space or in time. Insect swarms, fish schools, ungulate herds and the vast breeding colonies characteristic of social birds are conspicuous instances of spatial clustering. Synchrony, on the other hand, is an expression of the tendency of individuals to cluster in time (Gochfeld, 1980). Reproductive synchrony refers to the tendency of individuals to carry out some stage of the reproductive cycle (e.g., pair formation, territory establishment, copulation, etc.) simultaneously with neighboring conspecifics. Synchrony has been investigated in several bird species (Smith, 1943; Hailman, 1964; Patterson, 1965) as well as anurans (Arnold and Wassersug, 1978), ungulates (Wilson, 1975), rodents (Hoogland, 1981) and even tropical trees (Janzen, 197 1). These studies have focused on both the proximate determinants and the ultimate benefits of reproductive synchrony. Proximate synchronizing mechanisms include endogenous (Immelmann, 197 1), ecological (Perrins, 1970; Veen, 1977), meteorological (Hall, 1970; Sealy, 1975) or social (Darling, 1938, 1952; Hailman, 1964) factors. The ultimate benefits of synchronous breeding are usually assessed in terms of the selective advantage accrued to synchronous individuals. If, for example, synchronous individuals enjoy the greatest reproductive success, centripetal selection would maintain synchrony by eliminating asynchronous phenotypes. Because selection acts on genetic variation, any hypothesis concerning the evolution of synchrony presupposes that the phenomenon has a genetic component. Thus, before we can answer the question Has synchrony evolved as an adaptive mechanism?, we must demonstrate that (1) phenotypic variation in individual synchrony reflects underlying genotypic variability and (2) synchronous phenotypes enjoy the greatest reproductive success. This paper represents the second part of a study of breeding synchrony in the colonial-nesting Lesser Snow Goose (Anser caerulescens caerulescens). In an earlier paper (Findlay and Cooke, 1982) we showed that females inherit the tendency to hatch their clutches at a particular time relative to the colony mean. In this paper we document the extent of synchronized breeding in the colony, and assess its adaptive value. The intent here is to determine the average effects of synchrony on reproductive success, pooled over several seasons of observations. Given the extreme temporal variability of the arctic environment, we might expect synchrony to be more important in some years than others. While obscuring annual differences, a pooling procedure provides a generalized model (based on large sample sizes) with which we can compare annual trends, and hence, assess temporal variability. For this reason, a pooling approach will be followed initially. Inter-year variation will be dealt with later in the report.