Territory settlement in common loons: no footholds but age and assessment are important

• We tested the ‘foothold hypothesis’ for territory settlement in the common loon.
• Young loons often failed to claim territories where they had intruded.
• Loons produced as many young by settling early as by waiting to settle.
• Young males and females increased greatly in fighting ability with age.
• Intruders appeared to assess fighting ability of territory owners.

Many animals face the task of locating and settling on a territory where they can produce offspring. Over the past 36 years, theoretical and empirical studies have provided growing support for the ‘foothold hypothesis’, which attempts to explain territorial settlement of long-lived animals. The hypothesis maintains that a young prebreeder lives within or intrudes into a cluster of breeding territories, accumulates site-dependent dominance there, then outcompetes other prebreeders for a territory within the cluster when it becomes available. We examined patterns in territorial intrusion and settlement among prebreeders of known age and natal origin to test the foothold hypothesis in the common loon, Gavia immer. We tested two other hypotheses for territory settlement: the maturation hypothesis, which posits that animals await physical and/or behavioural maturity before territory acquisition; and the assessment hypothesis, which maintains that prebreeders intrude into territories to assess fighting ability of territory owners, one of which they ultimately evict. We found no evidence for footholds in loons: prebreeders focused their intrusions within roughly 10 clustered territories, but intruded infrequently into the lake on which they later settled. Furthermore, prebreeders that waited years to usurp a territory had reproductive success no different from those that settled more rapidly on a vacant territory. The maturation hypothesis, in contrast, was supported in both sexes: prebreeders showed a sharp increase in fighting ability with age, and males exhibited age-related increases in mass and tendency to confront territory owners. The assessment hypothesis also gained support because intruders interacted extensively with owners and intruded frequently after territorial turnovers. Our study adds to a small but growing number of studies that fail to support the foothold hypothesis for territory settlement and support the conclusion that modes of territory acquisition might be more varied than previously thought.