Increasing search rate over time may cause a slower than expected increase in prey encounter rate with increasing prey density.

A previous experiment with birds searching for caterpillars in an aviary demonstrated a highly counterintuitive result, that the rate at which a forager encounters prey does not increase linearly with prey density. Here, I demonstrate that if search rate increases over time then this can produce exactly the observed type of behaviour. Further, I argue that declining perception of predation risk over time in the absence of reinforcement, coupled with a trade-off between anti-predator vigilance and searching ability (both widely reported in field and laboratory studies), could generate such a change in search rate over time. Hence, if my hypothesis is correct, the previous experimental results could have considerable generality, and invite reconsideration of our mathematical descriptions of predator-prey interactions.