British Birds 86:358-369 August 1993

Within a restricted area of about 9 ha in Skurup, South Sweden (10 km from the south coast), I observed Common Swifts Apus apus roosting in the foliage of trees and on a lattice-work mast. The behaviour was concentrated in August, when Common Swifts migrate southwards in Sweden. A few observations (one in May, eight in June, and three in September) were made outside August.

During the Augusts of the 11 years 1982-92, I spent 244 evenings in the study area. On 75 evenings, swifts made fly-ins or perched; 48 swifts roosted in the foliage of trees of various kinds, and 23 swifts roosted on the mast. A total of 108 swifts, in addition to the ones which were seen roosting, made typical fly-ins towards trees or the mast.

The swifts mostly had to make several (in some cases up to 20 or 30) fly-ins towards different trees or the mast before they succeeded in perching. They arrived and perched from 10 minutes to 40 minutes after sunset, earlier in cloudy weather and later in clear weather. In the morning, the swifts always left early if the weather was unfavourable, but in good weather they sometimes stayed until well after sunrise.

In trees, the swifts roosted by hanging from thin twigs or leaves on the outside of the foliage, so the bird's position was vertical. Often, the swift hung more freely, in some cases from just one leaf. Then, its position was more crooked, with the back almost downwards and with the wings pointing downwards (plate 121).

Newly fledged young Common Swifts can easily be recognised by their white 'face' and light fringes on flight and body feathers. When the swifts perched, it was mostly too dark to see those characters with certainty. Of the 71 roosting swifts, I could age 33 (46%), mostly through a telescope, in good light in the morning. All except one were newly fledged young birds.

The number of observations has varied considerably between different years. In 1992, roosting frequency was unusually high (26 swifts roosted in foliage or on the mast, 19 swifts made fly-ins), which may have been due to high breeding success in good weather that summer over great areas. On many occasions, I have seen a tendency by the swifts to keep company. On three mornings in 1992, flocks of swifts (numbering about 30, 50, and 60) gathered near and over the study area soon after those roosting in foliage or on the mast had left their roost. A possible conclusion would be that all these swifts had spent the night in foliage or in similar situations.

In early years, I knew of only six similar observations mentioned by Lack (1956). Now I know of 34 observations in Europe (Britain nine, Denmark five, Finland four, Germany seven, Sweden five, Switzerland four). Lack hinted at the possibility that roosting in foliage could be practiced by Common Swifts coming from the northern parts of the species' range, where the most northerly populations regularly nest in holes excavated by woodpeckers (Picidae). The behaviour is easily overlooked, but methodical watching anywhere (such as the British coast) where migrating Common Swifts concentrate could reveal interesting results: the first recorded observation of this behaviour was made in Scarborough (Gyngell 1897).

Jan Holmgren, Rödhakevägen 23, S-274 33 Skurup, Sweden

REFERENCES

Gyngell, W. 1897. Common Swift roosting in tree. Zoologist (Series 4) 1: 468-469.

Lack, D. 1956. Swifts in a Tower. London.