Hunting in the kestrel, Falco tinnunculus, and the adaptive significance of daily habits

Summary

• 1. In an attempt to evaluate the importance of individual daily habits to a freeliving animal, foraging behaviour of kestrels was observed continuously for days in sequence in open country. Data obtained in 2,942 observation hours were used. Flight-hunting was the prominent foraging technique yielding 76% of all prey obtained.

• 2. Flight-hunting was impeded by rain, fog and wind speeds below 4 m/s and above 12 m/s (Fig. 3). Flight-hunting tended to be suppressed also in response to recent successful strikes and more generally by a high level of post-dawn accumulated prey (Figs. 4, 5). Flight-hunting had a tendency to be enhanced in response to recent unsuccessful strikes (Fig. 6).

• 3. Trapping results demonstrated a fine-grained daily pattern of common vole trap entries, with peaks at intervals of ca. 2 h (Figs. 7, 8). The interpretation of some of this pattern as representative of vole surface activity was supported by overall strike frequencies of kestrels hunting for voles (Fig. 9).

• 4. Detailed analysis of the behaviour of three individuals revealed significant peaks in hunting yield and frequency, coinciding with each other and with peaks in vole trapping (Fig. 11). It is suggested that the kestrels adjusted their flight-hunting sessions to times of high ‘expected’ yield. Vole activity peaks sometimes remained unexploited.

• 5. Meal frequencies culminated shortly before nightfall except in incubating females. The difference between the daily distributions of hunting and eating was due to some of the prey being cached in daytime and retrieved around dusk (Fig. 13). Caching behaviour is interpreted as a circadian strategy allowing separate optimization of hunting-adjusted to prey availability-and eating-adaptive by retaining minimum body weight in daytime flight and by thermo-regulatory savings at night.

• 6. Some kestrels showed remarkable constancy from day to day in the temporal distribution of specific behaviours (Fig. 16) and of spatial movements (Figs. 18, 19). In three 1–2 week sequences of observation analysed, flight-hunting frequency peaked 24 h after prey capture (Fig. 17). This is probably based on day to day correlations in flight-hunting frequency as well as on increased motivation for hunting in response to prey capture 24 h ago (Table 5).

• 7. In one individual with three distinct hunting areas, the tendency to return to an area again was maximal 24 h after prey capture in that area (Fig. 21, Table 6). A field experiment tested the effect of prey capture on the daily distributions of hunting and site choice in this individual (Fig. 22). A significant concentration of flight-hunting activity in the experimental feeding area was observed at the daily time of feeding (Fig. 23). Two alternative hypotheses are compatible with the result. Favoured is the one that the birds use “time memory” for the optimization of their daily patterns of flight-hunting and site choice.

• 8. By adjusting her daily flight-hunting to times of high yield, one kestrel saved 10–22% on her total time spent flight-hunting. Maximal efficiency, by concentration of all hunting activity in the hour of maximal yield, was not attained, presumably because of information constraints. The generality of the contribution of daily habits to survival is discussed.