Tree Sparrow Passer montanus freezing in the presence of a Sparrowhawk Accipiter nisus

Abstract

Cryptic behaviour is a common example of a passive defence in animals enabling prey to reduce the likelihood of being detected by a predator (e.g. Edmunds 1974). One such common behaviour enabling birds to be cryptic is freezing behaviour, which is commonly exhibited by birds in response to the presence of a predator (Slater 1999). A striking example of freezing behaviour in Tree Sparrows Passer montanus was observed at Tovetorp Zoological Research Station in south-eastern Sweden on 28 February 2002. I observed a Sparrowhawk Accipiter nisus launching an attack, at 14.45 in the afternoon, towards a feeding station located underneath a birch tree where Great Tits Parus major, Blue Tits P. caeruleus, Tree Sparrows and Greenfinches Carduelis chloris where foraging. The Sparrowhawk attack was not successful, and as a result all birds at the feeder escaped apart from one Tree Sparrow. Tits and Tree Sparrows escaped towards protective cover approximately ten meters away whereas the Greenfinches escaped into midair and perched in a birch tree some distance away. After the unsuccessful attack the Sparrowhawk perched in the birch approximately two meters above ground well within the branches keeping it quite concealed, and only three meters away the remaining Tree Sparrow stayed on top of one of the feeders. The Tree Sparrow started to freeze when it detected the Sparrowhawk, and it remained motionless in a crouching position holding the body in a horizontal stance. Ten minutes later, at 14.55, the first bird, a Great Tit, made the first approach towards the feeder. When the tit was flying towards the feeder the Sparrowhawk took off and intercepted it. After three to four twists and swirls the hawk managed to catch the tit which tried to outmanoeuvre the hawk by turning back towards the protective cover. The Sparrowhawk then flew into a nearby forest with the tit in its feet. The Tree Sparrow was still freezing and there were no other birds at the feeder. At 15.05 Great Tits and Blue Tits flew into the top of the birch tree, approaching the feeding station and the first birds started to feed at 15.10. Five minutes later, at 15.15, the activity was normal at the feeding station with plenty of birds foraging. The Tree Sparrow had remained motionless the whole time and it had not started to move yet. At 15.17 the Tree Sparrow started to move its head and one minute later, 15.18, it flew down to the closest feeder and resumed its feeding. The Tree Sparrow froze for a total of 32 minutes and despite being perched only three meters away from the sparrowhawk for ten minutes it avoided detection. This could also be explained by the Sparrowhawk not having a completely clear view towards the Tree Sparrow since some branches may have obstructed the view in that direction. Nevertheless, the freezing behaviour by the Tree Sparrow was probably a very appropriate response minimising the risk of being detected by the Sparrowhawk. This behaviour is also often observed as a response to alarm calls in birds (Ficken & Witkin 1977, Ficken 1990) and it is also often displayed after predators disappear leaving birds behind with incomplete knowledge of the predator’s location (Kullberg & Lind 2002).