THE BEHAVIOUR OF THE WILLOW WARBLER PHYLLOSCOPUS TROCHILUS IN CONTINUOUS DAYLIGHT

• Published in: Ibis
• Main Author: Brown, R. G. B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1963
• Subjects: Finnmark
• Online Access: http://dx.doi.org/10.1111/j.1474-919x.1963.tb02475.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1474-919X.1963.tb02475.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1474-919X.1963.tb02475.x

Summary. Willow Warblers Phylloscopus trochilus at Wytham, Oxford (51° 46' N., 1° 15' W.), start to sing around sunrise, in late April to early June. In the continuous daylight at Neiden, East Finnmark (69° 42' N., 29° 25' E.), they do not sing all the time, but in late June to late July start singing between 22.45 and 01.45 hrs., the start becoming progressively later as the season draws on. At Neiden, nest visits start some time after song (01.00–02.30), and this is probably also true at Wytham. The influence of environmental variables on the starts of the two cycles is discussed. It is concluded that song starts at Wytham when light intensity has risen above a certain threshold level, and at Neiden, when it has reached a minimum value or has started to rise again. It is suggested that the Neiden birds react to the closest approximation of their light intensity cycle to that at dawn further south. The end of daily song at Neiden may be controlled by internal factors. It is suggested that the start of the Neiden nest‐visit cycle is controlled by light intensity, but modified by temperature effects. The possibility that the environment affects the bird indirectly, via its brood or food, can be ruled out. The functional significance of the retention of activity lulls in the continuous daylight of the Arctic summer is discussed. The daily song‐starts of the Garden Warbler Sylvia borin , Brambling Fringilla montifringilla , and Yellowhammer Emberiza citrinella , follow the same pattern as those of the Willow Warbler. The Redwing Turdus musicus often has no daily song lull; its nest‐visit lull begins and ends well before midnight. The significance of this is discussed.