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

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Abstract
Clutch size and the number of young, 8 days old, were determined for 23 individually ringed females of the willow warbler, P. trochilus, in a study plot in the subalpine birch forest (550 m [above sea level]) at Ammarnas (66.degree.N), 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 2nd replacemnt clutch after a 2nd exposure of predation. Size of 1st clutches was 6.4 eggs compared with 5.2 eggs of replacement clutches. The average production of young was 1.3 in 1st clutches compared with 2.6 with both 1st and replacement clutches included. 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 2 wk. The length of the breeding season does not allow 2 successful broods to be raised before the adult molt in late July and the decline of invertebrate food in Aug.

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