Does foraging behaviour explain the poor breeding success of great tits Parus major in northern Europe?

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Abstract
Great tits Parus major have generally much poorer breeding success in northern Finland than in mid‐ and western Europe. The aim of this study is to find out whether the poor success is linked to foraging behaviour. This was studied by monitoring great tits' foraging behaviour and food abundance in different substrates during the breeding cycle in two populations, N Finland (65°N) and Latvia (56°N). It was shown that breeding success, from eggs to fledglings, was significantly poorer (56% vs. 93%) and caterpillar abundance considerably lower in the northern population. The general patterns in foraging behaviour were the same in N Finland and Latvia (especially the preference of birches Betula spp.) indicating that great tits used basically the same species‐typical foraging strategy in both populations. However, thin branches of birch were preferred in the north but avoided in Latvia, which may suggest that northern great tits have changed their foraging niche towards the outer parts of canopy, a niche typically occupied by the blue tit. This shift is theoretically advantageous, since the outer parts of the canopy are the richest caterpillar source. In practice, however, the poorer success of great tits indicates that this is not a beneficial strategy. Primarily, great tits in the northern population seem to be food limited since they lay too large clutches in relation to accessible food resources. This may be because great tits are not adapted to lower caterpillar production in the northern margins of their distribution and cannot change their narrow diet (3/4 caterpillars), like e.g. willow tits can. In search for caterpillars, food accessibility is further limited because the great tits' normal foraging behaviour, with wide search radius, may not function properly in the denser, outer parts of the canopy. Great tits may also be too heavy to forage efficiently on leafed twigs.