Food availability affects strength of seasonal territorial behaviour in a cooperatively breeding bird

Seasonal patterns in territorial behaviour are common in many species, and are often attributed to the adaptive benefits of increased defence or the provision of information to potential competitors or mates during the breeding season. However, because defence behaviour is likely to be costly in terms of time and energy, an alternative possibility is that decreases in the nonbreeding season are a consequence of reduced food availability. We studied territoriality in the pied babbler, Turdoides bicolor, a cooperatively breeding bird species that defends permanent territories. Groups interacted with rivals less and responded less strongly to an experimentally simulated intrusion of neighbours in nonbreeding periods compared to the breeding season. Foraging efficiency and biomass intake were significantly lower in the nonbreeding season, which resulted in birds being significantly lighter at this time of year. Finally, a feeding experiment in the nonbreeding season showed that groups given supplementary food significantly increased their response to a simulated territorial intrusion. These results indicate that the reduction in territorial behaviour during the nonbreeding season may be attributed, at least in part, to a reduction in food availability. We suggest that future studies on seasonal variation in territorial behaviour, especially those investigating species in which two or more individuals combine their defence, should take this potential constraint into account.