Population increase and breeding biology of the Dunnock Prunella modularis in southern Finland

Abstrakti

The breeding biology ofthe Dunnock (Prunella modularis) was studied on the basis of 517 nest cards gathered mainly from southern Finland during 1954-87. The habitats used included several kinds offorests, spruce and mixed forests being the mostcommon. Most nests were located in spruce bushes (62.5% of nests) or in juniper bushes (15.3%). Laying of the firstclutches started on averageon 16 May(SD=7.0 days, n=151) ; there was no difference between habitats, but northwards the laying was delayed. Due to the second and thereplacementclutches, the laying period continued until late July ; thelatest clutches were commenced on 23 July . Mean clutch size was 5.44 (SD=0.69, n=97) in June, but significantly lower in May (5 .11, SD=0.61, n=151) and July (4 .14, SD=0.62, n=21). Clutch size did notvary significantly between habitats, latitudinal zones, or cool and warm years . The probability of an egg giving rise to a fledgling was 0.37. The daily survival probability ofnestlings was significantly higher than that ofeggs. The breeding success increased with advancement of season . The overall survival probability was lower in cool (0 .20) than in warm (0 .46) Mays. The low survival of eggs during cool weather was attributed to increased predation of conspicuously coloured eggs . There was no difference in survival probability between habitats during the egg stage. The results are briefly discussed with respect to the recent population increase ofthe Dunnock in Finland.