The Food Searching Behaviour of Two European Thrushes

Abstract

1. The movement path of a predator will clearly be an important determinant of its ability to encounter and subsequently attack suitable prey items. Previous work on this aspect of searching behaviour has been mainly carried out on invertebrates, while this study describes the movement paths of two bird predators, the european blackbird and the song thrush (collectively referred to as 'thrushes') on a grass meadow in central Oxford. 2. The paths of foraging thrushes are divided up into natural units consisting of a series of alternating moves and pauses. A method of mapping these moves is presented (Figs 2, 3). 3. The large scale movements of the thrushes on the meadow was not uniform and this was probably related to non-uniformity in the environment, possibly including factors influencing feeding success (Figs 4, 6). 4. The durations of the moves and pauses were measured from cine film and video tape records. Only between one sixth and one tenth of the total foraging time was spent in actual movement, the remainder probably being spent largely in scanning for, or attacking, potential prey objects (Figs 7, 8). 5. Measures of the individual move lengths and angles. turned between successive moves were taken from the maps (Figs 3, 9), and the average speeds of movement were calculated for each track. Song thrushes and female blackbirds made longer moves than male blackbirds, and female blackbirds moved across the meadow at a higher overall speed than male blackbirds or song thrushes (Tables 1-4). 6. Methods are presented for describing the 'rules of movement' of the thrushes across the study meadow. When feeding mainly on earthworms, thrushes tended to make sequences of three, or possibly more, moves that were similar in length, and to make pairs, triplets, and possibly bigger groupings of alternating left and right turns. The size of each move was independent of the sign of the preceding turn and vice versa. The tendency of successive turns to alternate in sign, combined with the relatively restricted size of turns between moves, produced an ongoing search path, thus avoiding the pitfall of searching the same ground twice in a short time.