Stopover territoriality in songbirds crossing the Sahara: aggression and energetics during migration

Abstract

Stopover territoriality, i.e., the defense and broadcast of an ephemeral home range during avian migration, is rare or perhaps overlooked. However, settling on a territory may allow for easier access to resources and therefore promote faster refueling despite the added costs of territory defense. We examined the occurrence of stopover territoriality and its relationship to body condition and refueling rates. We color-ringed Western Subalpine Warblers (Curruca iberiae) mapped their home ranges and measured associated habitat characteristics during a spring stopover in the Sahara. We simulated territory intrusions via song playback to quantify territorial behavior. Males aggressively defended and broadcasted their stopover territories in response to intruders. Initial body condition in males was negatively related to home range vegetation density, suggesting that males in higher condition are selective for certain territory features. Low vegetation density does not support habitat selection for foraging, but perhaps does support social monitoring or predator vigilance as its functions. Fuel deposition rates in recaptured males were unrelated to home range habitat characteristics. Female Subalpine Warblers did not use repeated home ranges yet had similar refueling rates, body conditions, and stopover durations as territorial males. Therefore, while stopover territories were selected based on body condition, territory characteristics did not influence refueling rates. Subalpine Warblers expressed reduced migratory characteristics, making prolonged stopovers of up to 27 days with low refueling rates but maintaining body condition while defending territories. These results suggest that some individuals are staging for non-energetic reasons, such as pre-breeding preparations, or transitioning between a migratory and a breeding life history stage. Significance statement Territoriality in birds is commonly associated with breeding activities, less so with the wintering period and rarely with migratory periods. However, territoriality during migration may be energetically beneficial. During a trans-Saharan stopover, male warblers establish and defend temporary territories by singing and engaging in physical defense of home ranges, and those in better condition occupied territories with sparser vegetation. Females were not territorial but refueled at similar rates as males. Stopover territoriality did not increase access to food-rich patches, but perhaps aided in efficient social monitoring and preparations for the approaching breeding season. This is the first detailed documentation of birds defending ephemeral territories during migration, and its potential functions, costs and benefits may inform studies of full-annual cycles and life histories of many migratory species.