Breeding biology of the great grey shrike Lanius excubitor in W Poland

Abstract

A Great Grey Shrike population was studied in two large plots (220 km2 and 176 km2) in western Poland in 1999-2003. During the study period densities varied from 11.4 to 14.1 breeding pairs/100 km2 but numbers were stable. In all, 180 Great Grey Shrike nests were found - 114 (63.3%) in conifers, 66 (36.7%) in deciduous trees. This population's reproductive parameters were relatively high in comparison to those of other European populations: mean clutch size - 6.6, hatching success - 92.5%, mean brood size - 5.72, mean number of fledglings per pair - 4.1, mean number of fledged young per successful pair - 5.25. Eggs (mean 27.1 × 19.9 mm) were found to be larger than reported in the literature. Nesting success was similar in both study plots, but there was slight seasonal variability: 41.0%-52.6% from 99 nesting attempts in the first plot, 42.1%-43.7% from 37 nests in the second. Predation was the main cause of nest losses. Plastic string used as nesting material appeared to be the most important cause of partial failures: 13 (8.2%) of a total of 147 nestlings surviving to fledging perished as a result of becoming tangled up in it. Nestlings rarely starved. Nests in linear habitats suffered significantly higher breeding losses (78.6%) than those in non-linear habitats (50%). The high fitness values obtained from this population were probably due to traditional farming practices, the sparing use of pesticides and the good potential food source.