The Food Searching Behaviour of Two European Thrushes. Ii: the Adaptiveness of the Search Patterns

Abstract

1) This study describes the searching behaviour of two thrushes foraging for both naturally occurring and artificial foods. I have paid particular attention to temporal and spatial changes in searching behaviour and have attempted to answer the question of whether the changes in searching behaviour were adaptive ones. 2) When thrushes captured an earthworm and continued searching, they showed a net change in patterning of the twelve turns following capture so that they probably searched the area surrounding the capture point more thoroughly than an area covering the same number of moves units before capture (Figs 4, 5; Table 1). This confirms N. TINBERGEN'S (1967) hypothesis that path changes sometimes follow prey capture in bird predators. Since earthworms on the study meadow were aggregated in distribution, the net change in path may have been of adaptive value to the thrushes, by concentrating their search where further prey items were most likely to occur. It was not, however, possible to relate the increased searching effort quantitatively to the size of the earthworm aggregates. (3) The thrushes were then presented with populations of cryptically-coloured artificial prey in random, regular and aggregated distributions at each of two densities (Fig. 7). The movements of the thrushes (mainly blackbirds) feeding on these prey were recorded as described by SMITH (1974). (4) The overall distribution of the search paths of the thrushes showed a correspondence with the distributions of the artificial food populations (Fig. 9). The thrushes made larger turns and showed less tendency to alternate left and right turns at the higher density (Tables 8, 11), thus helping them to maintain their search paths within the smaller confines of the high density populations. (5) The capture rates of the thrushes at the two prey densities were roughly proportional to the differences in prey density (Table 4). Although there were suggestions of differing capture rates, unforeseen errors in experimental procedure did not allow firm conclusions on the effects of thrush predation on the different prey distributions within each density. (6) The presentation of the cryptic artificial prey led to an increase in the average length of moves made by blackbirds (Fig. 10; Table 5). Further increases in move length followed the cutting of the grass on the study meadow (Fig. 13) and the introduction of more conspicuous artificial prey (Figs 11, 12). It is suggested that the increases in move lengths were an adaptive reaction by the blackbirds to increases in prey detectability. (7) Captures of single artificial prey were preceded by larger than average turns and were followed by larger than average turns in the opposite direction (Table 9). (8) The presentation of the artificial food led to an overall increase in the speed of movement of the blackbirds (Table 10). (9) After the capture of prey in the low density random and aggregated populations thrushes showed a net tendency to concentrate their searching in the area surrounding the site of prey capture (Fig. 16; Table 12), similar to that shown after the capture of earthworms. This was not shown after the capture of regularly distributed prey (Figs 16, 17 ; Table 12), nor was it clearly shown after the capture of prey in the high density random and aggregated distributions (Fig. 17; Table 12). (10) The overall speed of movement was greater over the ten moves preceding capture than over the ten moves following capture in the low density regular distribution (Table 13). (11) The changes in searching behaviour following capture are discussed with reference to N. TINBERGEN'S (1967) hypothesis that predators select for 'spacing out' in prey. (12) A method is presented which allowed the measurement of the wild blackbirds' abilities to detect the artificial prey supplied (Figs 18-20). The 'risk' to the artificial prey was high when the blackbirds were within 20 cm of the prey, but fell to near zero at a distance of 120 cm. (Fig. 21). The square root of the risk was linearly related to the distance between predator and prey (Fig. 22). The measures of the detection ability of the blackbirds are discussed in relation to their movement patterns. (13) The results are discussed in light of the general problem of whether predatory animals show adaptive flexibility in response to spatial and temporal variations in food supply. I have argued that at least some aspects of the thrushes' behaviour provide evidence for such flexibility. I have emphasised that the basic 'unpredictability' of the environment, which generates the need for behavioural plasticity, also makes it unrealistic to expect to find predators making optimally adapted responses to all variations in the environment.