First pairing in Snow Geese Anser caerulescens: at what age and at what time of year does it occur?

In migratory birds, the place and time of pair formation are important parameters for population structure and dynamics. Geese are not only migratory but also exhibit long-term monogamy, and therefore the first pairing event in a bird’s lifetime is of particular importance. Through behavioural observations of young, known-age, marked birds conducted on the wintering grounds during three winter seasons we investigated two aspects of the timing of first pair formation in the Wrangel Island population of Snow Geese Anser caerulescens: (1) the age at which birds first form pair bonds, and (2) the seasonal pattern of first pair formation. Wrangel Island Snow Geese paired considerably later in life than Snow Geese from a low-Arctic population: almost none of the birds formed pairs in their second winter, and many were still in sibling groups for at least part of that season. The proportion of birds in pairs continued to increase until at least 5 years of age. Most pairing took place during the observation periods, and in general the proportion of birds in pair bonds increased gradually throughout the winter season. The amount of pairing during spring migration or summer varied annually and among cohorts, indicating that even very young birds may be able to form pair bonds quickly if conditions on the breeding grounds are unusually favourable. Pairing later and remaining in family groups longer may be a response to breeding conditions in this high-Arctic colony. Here, productivity is typically low due to harsh weather and predation, whereas Snow Geese breeding in the low Arctic are less restricted and form pairs and start to breed when younger. The fact that most, but not all, pairing takes place on the wintering grounds helps explain why a previous study found a certain amount of gene flow between the two Wrangel Island subpopulations with separate wintering grounds.