TERRITORY SIZE, PREY RENEWAL AND FEEDING RATES: INTERPRETATION OF OBSERVATIONS ON THE PIED WAGTAIL (MOTACILLA ALBA) BY SIMULATION

SUMMARY (1) We have previously shown how pied wagtails defend winter feeding territories along a river. Owners walk a regular circuit round their territories, exploiting a renewing food supply (insects washed onto the river banks). (2) We use a simulation model to investigate the effect of territory size on the owner's feeding rate. Larger territories can yield more food because the longer the foraging circuit the greater the time for resource renewal between successive visits to the same stretch. But larger territories cost more to defend. (3) Simulation shows that the territory size that maximizes feeding rate or energetic gain varies with the rate of food renewal, smaller territories being better when renewal is fast and larger ones when it is slow. (4) Real wagtails did not track these short-term optima by changing territory size, but maintained a territory size of about 600 m throughout the winter. (5) We show that this territory size may promote an owner's long-term chances of over-winter survival. If territory size is less than 600 m, then many days would yield a feeding rate inadequate for survival, while defending a territory larger than 600 m would not increase the number of profitable feeding days. (6) We use simulation to explore how changes in intruder pressure, walking speed and ability to detect intruders affect feeding rate; owners would achieve only a slight gain in feeding rate by improving their detection ability beyond the observed values.