Development of hippocampal specialisation in two species of tit (Parus spp.)

Food storing birds have been shown to have a larger hippocampus, relative to the rest of the telencephalon, than do non-storers. A previous study reported that this difference in relative hippocampal volume is not apparent in a comparison of nestling birds, but emerges after birds have fledged. This conclusion was based on a comparison of a storing and a non-storing species in the corvid family. The present study compared another storer/non-storer pair of species in order to test whether the results of the previous study can be replicated in another family of birds. The volumes of the hippocampal region and remainder of the telencephalon were measured and estimates of neuron size, density and total number in the hippocampal region were made for nestlings and adults of the food-storing marsh tit Parus palustris and non-storing blue tit Parus caeruleus. Relative hippocampal volume did not differ between nestlings of the two species, whilst the relative hippocampal volume of adult marsh tits was greater than that of blue tits. The difference between adults arose because in marsh tits but not blue tits, adults had a significantly larger relative hippocampal volume than did nestlings. Neuron density was significantly higher in both species in nestlings than in adults and adult blue tits had fewer neurons than did adult marsh tits. The results of this study are largely consistent with the earlier study comparing a storing and non-storing species of corvid, suggesting that the observed patterns may reflect a general difference between storers and non-storers in the development of the hippocampal region.