Do barnacle geese pair assortatively? Lessons from a long-term study

Abstract

In the field, mate choice is usually studied by looking for non-random mating patterns in established pairs (Cooke & Davies 1983; Marzluff & Balda 1988; Reid 1988; Johnston & Johnson 1989). It has been recognized, however, that these patterns alone do not necessarily imply that active mate choice is taking place. Two main factors have been identified that can confound mate choice and produce nonrandom mating patterns: intra-sexual competition (Wishart 1983; Johnson 1988) and differential availability of mates during pair formation (Cooke & Davies 1983; Reid 1988). We propose another factor that needs to be investigated before interpreting non-random mating patterns. Traits that show lifetime variation (e.g. body weight, dominance rank, plumage, vocalization and behaviour) may change after pair formation and produce the observed non-random mating pattern. This is especially likely for traits that have a strong environmental component (e.g. body weight and rank) and for species in which mates remain together throughout the year. Thus when two geese pair up, they acquire the same rank (Lamprecht 1986; Black & Owen 1989). Similarly, traits that involve a learning process (e.g. vocalizations) may change over time (Mundinger 1970; Payne 1982). Here we use field data on a wild population of barnacle geese, Branta leucopsis, collected over 17 years to determine whether there is non-random mating for body size or body reserves and discuss whether this could be due to mate choice. We look for correlations in the body measurements of mates both before and after pair formation. Data were collected between 1973 and 1989 from a migratory population of barnacle geese that breeds in arctic Spitsbergen and winters on the Solway Firth in northern Britain. Birds were caught at regular intervals and fitted with individually coded plastic rings, which were readable with a telescope from a distance of up to 250 m. During