Laying and clutch size of the Treecreeper Certhia familiaris in south-western Sweden

Abstract

Breeding data were collected on an unringed Treecreeper population nesting in artificial nest sites (nest pockets) of two sizes, erected in predominantly deciduous forests in southwestern Sweden. From 1982 through 1992, 459 breeding attempts were recorded. The earliest and latest dates of laying were 29 March and 26 June. The distribution of first egg dates showed three peaks: one in April and early May (first clutches), one from late May through June (probable second clutches), with a narrow peak in between (replacement clutches). Clutch size was related to laying date in a curved fashion peaking in early May with a mean of 5.93. Overall mean clutch size was 5.48 with a range of 3-7. About a third of the successful breeding attempts (n=37) were followed by another attempt in a nest pocket nearby, considered to be genuine second clutches. The sizes of these first and probable second clutches were 5.46 and 5.03, respectively. Clutch size in large and small nest pockets did not differ although the large pockets were preferred as nest sites.