Laying of Replacement Clutches in the Willow Warbler Phylloscopus trochilus in Lapland, Sweden

Abstract

Clutch size and the number of young, 8 d old, were determined for 23 individually ringed females of the Willow Warbler Phylloscopus trochilus in a study plot in the subalpine birch forest (550 m a.s.l.) at Ammarndis (660N), Swedish Lapland. Predation caused loss of nests and eggs for 17 of the females. Eleven of them laid a replacement clutch. Two of the latter laid a second replacement clutch after a second exposure of predation. Size of first clutches was 6.4 eggs compared with 5.2 eggs of replacement clutches. The average production of young was 1.3 in first clutches compared with 2.6 with both first and replacement clutches included. The study shows that the Willow Warbler is normally ready to replace lost clutches also at this high latitude and altitude, at least before hatching, delaying breeding up to two weeks. However, it is also shown, by argument, that the length of the breeding season does not allow two successful broods to be raised before the time of adult moult in late July and the decline of invertebrate food in August.