STABILIZING SELECTION ON BLUE TIT FLEDGLING MASS IN THE PRESENCE OF SPARROWHAWKS

Like British great tits, Belgian blue tits have a lower winter body ma ss when sparrowhawks are present. Since body mass affects manoeuvrabil ity in small birds, tits may balance the risks of starvation and the r isk of hawk predation by varying the amount of extra fat carried durin g winter. Predation pressure by sparrowhawks on young and inexperience d fledglings is at least as intense as that on the adults during winte r. We therefore expected that tit fledgling body mass could also be re duced in the presence of sparrowhawks. In the years after one pair of sparrowhawks settled in a study plot, the mean body mass of blue tit f ledglings was lower compared with that in years when there were no spa rrowhawks. Furthermore, the shape of the curve relating juvenile survi val to fledging mass changed, because the survival of the heaviest fle dglings was reduced, which altered the selection differential of juven ile survival as a function of body mass from directional to stabilizin g. Of seven published studies on the fledgling body mass-survival rela tion in tits, all three of the studies conducted in the absence of spa rrowhawks showed the highest survival rates for the heaviest young, wh ereas in all four studies with sparrowhawks present this was no longer the case.