Food consumption and roosting behaviour of Great Grey Shrikes Lanius excubitor wintering in south-western Finland.

For diurnal predators the short time of winter daylight may limit the possibilities to feed and hunt. In this field study, these aspects, as well as roosting behaviour,were evaluated in wild Great Grey Shrikes (Lanius e. excubitor). The main roost-sites were large junipers (Juniperus communis) and willows (Salix sp.). Daily mean amount of captured prey varied between 36.3-59.8 g and the dailymean food consumptionwas 44.1-62.6 g, both parameters being lowest in midwinter. In the diet, Microtus-voles dominated in the autumn and late winter whereas mice (mainly Harvest Mouse Micromys minutus) were most abundant in midwinter. Increased capture efficacy rather than increased hunting efforts enabled the shrikes to catch a fairly constant number of prey / day (2.1-3.1), independent on time available for hunting.With decreased amount of daylight the shrikes showed also signs of increased hunting activities in morning and evening twilight. The shrikes responded to shortened daylight time by decreasing the meal-to-meal interval and, as a result the portion of the daylight time used for feeding increased as the photoperiod shortened. Thus, the shrikes showed efforts to adjust their foraging and feeding behaviour in order to reach a stable prey capture rate and food consumption without increased energy costs.

S. Karlsson, Ornis Fennica 84:57-65. 2007