THE RETURN AND DEPARTURE OF SWIFTS APUS APUS AT OXFORD

• Published in: Ibis
• Main Author: Lack, David
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1958
• Subjects: Apus apus
• Online Access: http://dx.doi.org/10.1111/j.1474-919x.1958.tb07957.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1474-919X.1958.tb07957.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1474-919X.1958.tb07957.x

SUMMARY 1. The return in spring and departure in autumn of resident Swifts was studied in six years, primarily by means of a nightly check of roosting birds. 2. The mean date of return in spring has varied by nine days in different years. The return was delayed with a cold northerly airstream, but not otherwise. 3. Soon after its return in spring, a bird sometimes spent a night or two away from the tower. 4. Sometimes in spring, but hardly ever in autumn, a transient Swift spent one night in the tower; occasionally, three adults then roosted together. 5. The number of residents in the tower at the end of May was about 90% of that in the preceding late summer. 6. Members of the same pair usually returned and departed on different days. 7. The mean date at which the adults departed in autumn depends primarily on the date when breeding started, and hence on the weather in May. 8. The parents usually stayed for a few days after their young had left, presumably to put on fat. This interval was longer in those years in which the weather in July had been bad, and in one year it was longer for parents of larger than smaller broods, while it was shorter, irrespective of other conditions, late in the season. Occasionally one, but never both, parents departed before the last of their nestlings, chiefly in fine summers. 9. Adults which laid eggs which failed to hatch tended to leave before the parents of young, but non‐breeding pairs (mainly yearlings) tended to leave later than parent birds that raised young. 10. Autumn departures were held up by depressions, and occasionally by mist. 11. After their mates had departed in autumn, a few individuals “kept company” with other residents, courting and roosting with them. 12. In autumn the birds usually departed from the tower inconspicuously, but twice Oxford residents probably joined a large flock of passing migrants.