A northern population of Willow Tits Parus montanus did not store more food than southern ones

Abstract

For Willow Tits hoarded food is an important part of the winter diet . Alarger supply of stored food should then be required in areas with long and cold winters than in areas with short and mild winters. Since the storing of winter supplies occurs mainly during a few autumn months, it is then plausible that hoarding intensities at that time should be higher in northern than in southern populations. Foraging and food hoarding intensities in Willow Tits were sampled simultaneously with standardized observation methods in the southern, central and northern parts of the species range during the main hoarding period in autumn. However, hoarding intensities were highest in the central, and not as expected in the northern area . The southernmost birds may have stored almost as much food as the birds in northern populations, although the intensity peak came later in autumn. The observed differences in hoarding intensity were probably due to differences in the supply of storable food items, since Willow Tits encountered food at a higher rate in the central than in the other two areas. Eating intensities were similar in all three areas, around 0.8 items per minute, and it seems likely that Willow Tits stored supplementary food after satisfying a maintenance requirement of this magnitude. In the northern area subordinate birds hoarded at a higher rate than dominants, whereas this was not the case further south. It is not intuitively clear why this should be so, but foraging at high intensities may be costly . Subordinate birds have secondary access to food and high mortality during cold winters and might therefore work relatively harder in cold areas to build up winter food supplies .