Intersexual niche differentiation in the blue tit (Parus caeruleus)

The intersexual niche differentiation hypothesis, according to which sexual size dimorphism (SSD) has evolved to facilitate niche separation between t he sexes, has received equivocal support in studies of birds. One explanati on for the inconsistency of the results in that other factors crucial for r eproductive success, such as variation in food abundance among territories, may obscure food niche differentiation between parents. We studied the eff ects of SSD, food availability and food demands of broods on niche differen tiation in the blue tit (Parus caeruleus). Parents attending experimentally enlarged broods delivered the same amount, but more diverse food items, to their offspring than parents attending reduced broods, although this effec t was significant in only one of the two study years. The time-lag (deviati on from the peak food abundance in habitat in days) and squared time-lag we re the most important determinants of the diversity of food items delivered to young and their condition (fitness surrogate). SSD, as measured by prin cipal component scores of morphological traits, did not explain any variati on neither in the diversity or amount of food provided, nor in offspring co ndition. These results suggest that synchronization of breeding with peak f ood availability is the most important determinant of diversity of food del ivered to young and, more importantly, their condition. No evidence for int ersexual food niche differentiation was found; at best, it is likely to con stitute only a weak selection pressure on the maintenance or enhancement of SSD in this system. (C) 2000 The Linnean Society of London.