An experimental study of feeding, vigilance and predator avoidance in a single bird

Summary

In an aviary experiment with captive Goldfinches (Carduelis carduelis L.) vigilance and foraging behaviour were recorded before and after the appearance of a predator. When foraging, individual Goldfinches had a low head jerk rate during the first minute, but each scan was of relatively expanded duration; the scanning rate increased with a shorter duration of each scan up to the time the predator, a Merlin (Falco columbarius), appeared. Thereafter the birds showed a low head jerk rate, which returned to the former level after ten minutes. The time spent feeding was low if head jerk rate was low and high if head jerk rate was high. There were significantly more scans of long duration after the predator was visible compared with undisturbed feeding. The intake loss of individuals due to increased vigilance after the appearance of the predator during the following minutes is calculated to be 53%. A linear, negative regression function is formulated relating the number of scans per time unit and total time spent scanning:

$$\begin{gathered} SCAN DURATION \hfill \\ = - 0.99 \times HEAD JERKS + 62.46 \hfill \\ (r = - 0.96,P < 0.001) \hfill \\ \end{gathered} $$

With increased number of scans the total time spent vigilant is decreased. From this is concluded that if birds ingest relatively large sized seeds and therefore only a few per time unit they gain a higher security against predators compared to feeding on small sized seeds and ingesting relatively more, because time not spent vigilant is increased with increased pecking rate.