Abstract
Many flying animals, like birds feeding their young, make commuting flights between a central place and foraging areas in the surroundings. Such central place foraging (CPF) represents a special case of foraging theory. We use simple geometry and trigonometry to analyse CPF flight performance (a round-trip cycle of outward flight from the central place and inward flight back along the reverse track) in relation to wind. In addition to considering the situation of a constant and uniform wind field, we take into account two factors that are likely to apply in many CPF situations under natural conditions: (a) that animals carry a load that will increase the energy expenditure during the inward flight and (b) that they may fly at different altitudes during out- and inward flights in the wind gradient above the ground or sea surface. Four main predictions emerge: (1) efficiency of CPF flights will be reduced with increasing wind speed, and foraging at the longest ranges is expected under low wind speeds. (2) A preference for CPF flights in crosswinds is expected in a constant and uniform wind field. (3) Carrying a load during the inward flight makes it optimal to fly with a small component of following winds during this flight while the outward flight will have a corresponding component of opposed winds. (4) With a steep wind gradient (e.g. over rough terrain) providing much shelter from wind at the lowest altitudes, predicted behaviour may change from crosswind preference to a preference for flights along the head/tailwind axis (at low altitude into headwinds and high altitude in tailwinds). Detailed tests of predictions for CPF flights in relation to wind will be important for understanding constraints and adaptations in animal responses to wind and for evaluating consequences of changing wind regimes in animal movement ecology.
Highlights
Many animals make foraging trips from a central place to which they return after each trip
In the ideal case of a central place forager making commuting flights over exposed terrain or the sea, wind will have an important influence on the economy of flights, since wind speeds will often amount to a large fraction of the birds’ airspeed
Adding the effect of extra flight costs associated with a load on the return flight (see “Effects of extra flight costs during return flights”) to the cases with surface layer (Fig. 6) will introduce an asymmetry in the relationship between central place foraging (CPF) flight efficiency and wind direction such that outward flights in the headwind sector in combination with return flights in the tailwind sector are more favourable than the reverse behaviour
Summary
Many animals make foraging trips from a central place to which they return after each trip. Taking these expectations into account when calculating Sout and Sin in the different wind direction intervals will give relative time/energy costs of CPF flights in relation to wind direction (Eq 4) as plotted in Fig. 6 for different gradient factors in comparison with the case without a wind gradient (k = 1) These results show that the first and second main predictions for cases without a wind speed gradient (see “Central place foraging flights in a uniform wind field”) are still likely to hold under strong winds over open sea/water and over very flat and open land where the wind gradient is less pronounced (k = 0.75).
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