HORN NFAM TEN & TINBERGEN JM (2014) Body orientation of Great Tits Parus major sleeping in nestboxes. LIMOSA 87 (4): 209-215.

Great Tits in the Netherlands sleep in nestboxes in winter, standing on one leg with the head tucked in the shoulder feathers. The body feathers are raised to form an insulating feather ball. When the nest box is approached quietly the sleeping bird is hardly disturbed and it is possible to register its body orientation (Fig. 1). We registered these body orientations, as well as sex and age of the sleeping tits, over seven winters (1984/85-1990/91). Monthly visits in October-March to 74 nest boxes (of which 24 were made of wood, 48 of woodcrete and 2 of pvc) yielded 532 undisturbed observations. We hypothesised that Great Tits might sleep away from the entrance hole to avoid predation by martens or to shelter from wind entering through the hole. The alternative hypothesis was that the birds would sleep in random positions. We found that the orientation of the birds differed between box types. In the round woodcrete boxes the birds slept away from the entrance hole. In the square wooden boxes they tended to orient along the diagonal of the box, also with a preference to sleep away from the entrance but less strongly so than in the woodcrete boxes (Rayleigh's test, r=0.39 for wood and 0.72 for woodcrete, Fig. 3). No differences between sexes or ages were found in orientation. These observations are consistent with both the predation and the shelter hypothesis, but these alone cannot explain that the birds in the wooden boxes also orientated along the diagonal towards the flight hole. Perhaps, given the small size of these boxes, this is the best way to stay free from the walls and prevent feather damage. More detailed measurements on microclimate in the box and registrations of how body orientation affects predation risk using webcams will further narrow down the explanation of non-random body orientation of Great Tits sleeping in nest boxes.

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