Interactions Between House Sparrows and Sparrowhawks C.J
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Interactions between House Sparrows and Sparrowhawks C.J. Barnard How does an efficient Sparrowhawk catch a sparrow? How does a wily House Sparrow avoid being eaten? ecent work on the development of feeding skills by young birds has Rhighlighted the fact that many are less successful than adults at captur• ing prey (e.g. Orians 1969; Buckley & Buckley 1974). Young Oystercatchers Haematopus ostralegus, for example, take three years to become fully efficient (Norton-Griffiths 1968). Many raptors, in particular, require considerable hunting skill in the face of sometimes elaborate anti-predator behaviour on the part of their prey. It would not be surprising, therefore, if inexperienced young showed a certain amount of ineptitude in catching prey that involved specialised hunting techniques. The observations presented here are admittedly few and mostly qualita• tive, as appears to be a general problem with raptor behaviour, but they nevertheless provide an insight into both juvenile hunting and the anti- predator behaviour of prey. The attacks took place at a farm near Oxford during February and March 1977, three involving adult female Sparrow- hawks Accipiter nisus and one a juvenile. In all cases, the prey were House Sparrows Passer domesticus, which were feeding close to hedges in open fields. continued... [Brit. Bints 72: 569-573, December 1979] 569 570 House Sparrows and Sparrowhawks Behaviour of the hawks All the hawks that I saw initiating attacks first approached their prey at high speed, flying close to the ground and close to the bottom of the hedge, but an important difference between the adults and the juvenile was that the former approached on the opposite side of the hedge to the prey, whereas the juvenile appeared to approach on the same side. In the last case, however, the confused behaviour of the fleeing sparrows made it difficult to be certain on which side of the hedge they had originally been sitting. The efficacy of the adults' attacks seemed to depend on a rapid 'flick' over the top of the hedge, during which they tried to snatch any bird still sitting on an outer twig. Although none of the observed attacks was successful, the adults came substantially closer to success than did the juvenile. Indeed, another reason for supposing that the juvenile approached on the same side of the hedge as the sparrows was the absence of a 'flick': the hawk struck lower down on the approach side of the hedge. A further important difference between the adults and the juvenile lay in the degree of attack persistence. After failure of the first attack, the adults immediately left and either searched another part of the hedge or left the area entirely; only one strike attempt was made during an attack. The juvenile, however, returned to make five further strikes after failure of the first. The adults' behaviour conforms to that recorded by Morse (1973) and is probably adaptive, since Sparrowhawks appear to depend on surprise for effective attack. The return strikes of the juvenile were all made from a perching position on the outside of the hedge, three being directed at one sparrow and two at another. The attempts were hopelessly unsuccessful and the hawk spent several seconds after each strike untangling itself from the hedge. Behaviour of the sparrows In common with many species of small passerine that feed in open fields, the sparrows did not venture far from cover. Furthermore, the length of their feeding bouts was very much determined by how far they were from cover (fig. 1): the farther they were from the hedge, the shorter was the duration of their feeding. They most often fed 18-20 m away from a hedge (fig. 2), which meant that most of their feeding bouts were very short. Feeding was punctuated by frequent returns to the hedge when the whole flock moved as a steady stream and gathered on the outer branches. After staying there for a few seconds, the sparrows filtered back to the feeding area. In all the, attacks, the sparrows were perched on the outside of the hedge when the hawks approached; that is, they were in between feeding bouts. To the human observer, they were markedly more conspicuous in this position, even when sitting still, than when they were out on the ploughed field, but it is not clear whether this was an important factor in the hawks' decision to attack. Certainly, hawks in the genus Accipiter do attack birds feeding on the ground (Goslow 1971) and are perfectly capable of selecting a target at ground level. House Sparrows and Sparrowhawks 571 Distance from hedge (m) Fig. 1. Mean duration of feeding bouts of House Sparrows Passer domesticus (in seconds) in relation to distance from hedge (in metres), based on 42 observations at farm near Oxford in February and March 1977 When the approaching hawk was spotted, as it was in each attack, one or more of the sparrows gave the characteristic predator alarm call (see Summers-Smith 1963) and the whole flock retreated deeper into the hedge. During attacks, the sparrows kept up an incessant, high-pitched chatter, stopping only several seconds after the hawk had left. This is fairly typical behaviour of small passerines, and similar observations have been made by Marler& Hamilton (1966), Ficken & Witkin (1977) and others. The three attacks by the adults all occurred along one particular hedge which, unfortunately, was too dense for me to be able to observe the sparrows (flocks of over 25 in all cases) while they were inside the hedge. Distance from hedge (m) Fig. 2. Frequency of feeding bouts of House Sparrows Passer domesticus at different distances Irom hedge (in metres), based on 98 observations at (arm near Oxlord in February and March 1977 572 House Sparrows and Sparrowhawks The juvenile Sparrowhawk, however, attacked a small group of 12 sparrows sitting in a particularly thin hedge, where they were visible for the duration of the attack. This provided a rare opportunity to watch their responses during the subsequent strike attempts in which target sparrows exhibited quite remarkable behaviour, which, so far as I am aware, has not been recorded before. Instead of frantically moving away when the hawk first alighted in its vicinity, the target sparrow froze and appeared to become silent. It remained in this attitude until the hawk struck, whereupon it quickly dodged a short distance (4-5 cm) to either side of the line of strike and froze again; this behaviour was repeated during all strike attempts. Although the hawk and target were only a matter of 30 cm or so apart, the target was unobtainable so long as it remained in the hedge. Discussion The behaviour of the juvenile hawk illustrates two points worth comparing with that of the adults. First, its approach was almost certainly carried out on the 'wrong' side of the hedge so far as surprise attack was concerned, and this allowed plenty of time for the sparrows to retreat deep into the hedge before it could make a strike. Secondly, having failed (as did the adults) in its first attempt, the juvenile returned and made five fresh attempts to catch sparrows, even though they were unobtainable in the hedge. Two factors may explain this. If the juvenile was repeatedly inept at catching prey, hunger may have affected its decision to persist. Also, the sparrows were visible in the hedge and thus potentially available. Nevertheless, the juvenile wasted several minutes and risked injury from thorns, briars and branches while trying to reach the sparrows. The advantage to the sparrows of retreating into the hedge is quite clear, and several authors have discussed similar behaviour by sparrows and other passerines (e.g. Summers-Smith 1963, Ficken & Witkin 1977). What is worth noting here, however, is the extent to which the sparrows steadfastly remained in the hedge when apparently in severe danger of capture. It would presumably have been possible for them to attempt to escape while the hawk was disentangling itself, but many avian predators are particularly adept at taking prey in the air (Rudebeck 1950-51) and Sparrowhawks themselves are more effective in the open (Brown 1976). Moreover, by remaining in the hedge, very little energy needs to be expended in avoiding capture, as the dodging behaviour I have described illustrates. It seems that thin cover is all that is necessary (the potential prey need not be concealed from view) for effective protection and to minimise the effort of predator avoidance. From the behaviour of the juvenile hawk, it appears that awareness of this and surprise-approach tactics both have to be learned at some initial cost. Acknowledgements I should like to thank Dr J. R. Krebs, Jon Erichsen, Peter Garson and Richard Cowie for reading this paper in an earlier form. The observations were made while I was supported by a grant from the Science Research Council. House Sparrows and Sparrowhawks 573 Summary A juvenile Sparrowhawk Accipiter nisus was inefficient in its hunting behaviour when compared with adults, because it approached on the 'wrong' side of the hedge and so failed to achieve surprise, and persisted in fruitless strike attempts when its prey was unobtainable. Persistence may have been brought about by hunger or deceptive prey availability. Retreating into cover provided not only immediate safety for the prey species (House Sparrow Passer domesticus), but also minimised the cost of further predator avoidance. Cover did not have to be extensive to provide this protection. Juvenile Sparrowhawks may have to learn efficient approach tactics and the costs (in terms of time and energy wastage and risk of injury) of attack persistence. References BROWN, L. 1976.