The pressure to breed: intraspecific interactions of different goose species during spring stopover reflect mate guarding and proximity to nesting dates

Wild geese form large flocks, benefiting from shared vigilance against predators which increases feeding time. As spring progresses, males invest more in mate-guarding to protect paternity, so we hypothesised that intraspecific aggression would rise with flock size and proximity to first egg dates. We tested this using field observations of four goose species staging in northeast Poland, February–April: one local breeder (nesting in March) and three Arctic-nesting species (laying from late May). Of 662 observed aggressive episodes, 643 (97%) were intraspecific, and aggression in all species increased with flock size. Locally breeding Greylag Geese Anser anser and Arctic-nesting Barnacle Geese Branta leucopsis showed consistently high probabilities of aggression (60–80%) throughout, although Barnacle sample sizes were too small for firm conclusions. The other two Arctic-nesting species (Greater White-fronted Anser albifrons and Tundra Bean Goose A. serrirostris ) showed aggression rising initially from zero to similarly high levels in the three weeks before departure for nesting areas. No differences were found between geese feeding on grassland versus arable fields (which offered higher food intake). We conclude that intraspecific aggression reflects changing mate-guarding investment relative to first egg dates, not competition for food.