The effect of mate choice on speciation in snow petrels

Abstract

Although snow petrels, Pagodroma nivea, show a tendency to assortative mating by size morph, the existence of mixed pairs producing viable young reveals that reproductive isolation between the two size morphs is imperfect. However, the degree of isolation depends on breeding locality. A given area can harbour either (1) only small birds, or (2) only large birds, or (3) small bird colonies close to large bird ones, or (4) mixed pairs, the percentage of which varies from one locality to another. We investigated the mating and nesting patterns that result in mixed pairings and can explain the speciation process considered to be in progress by modern authors. Data from a 34-year demographic study in Terre Adélie, Antarctica, showed that mate and nest fidelity were very high. Nest changes, but not divorces, generally led to higher fecundity. Snow petrels did not seem to choose their mates on the basis of age or experience, and divorcees tended to form new pair bonds with neighbours; nevertheless, there was evidence for active mate fidelity. Pairs involving mates of the same size morph and mixed pairs had similar fecundity. Despite its heterogeneity, the breeding habitat of snow petrels was relatively predictable. Ice repeatedly made some nests unsuitable for breeding. Obtaining a nest that was not frozen was therefore the primary requirement for breeding. Strong competition for nests may explain high fidelity rates and, combined with the absence of reproductive costs in mixed pairs, may have promoted decreased choosiness during mate choice, preventing total reproductive isolation between the two morphs.