Egg sizes of nine passerine bird species in a subalpine birch forest

Egg measurements together with calculated volumes are presented for nine passerine species (Prunella modularis, Luscinia svecica, Phoenicurus phoenicurus, Turdus pilaris, Turdus philomelos, Turdus iliacus, Phylloscopus trochilus, Fringilla montifringilla, and Carduelis flammea), breeding in rich subalpine birch forests on a south-faced mountain slope in southern Swedish Lapland. There were no clear regional size trends in comparisons with egg collections from other parts of Europe. There was a significant increase in egg size in the latter part of the laying sequence. The ultimate egg is from 2.1% (F. montifringilla) to 6.5% (Ph. trochilus) larger than the mean of the clutch. According to several years’ measurements of T. pilaris and T. iliacus eggs there was no significant size variation between seasons. This constancy together with the relatively large egg sizes might be a consequence of the productive breeding habitat of the study area.