Seasonal changes in mass and histology of the spleen in Willow Tits Parus montanus

Seasonal changes in the mass and histology of the spleen of juvenile and adult Willow Tits Parus montanus throughout the year were examined. The results showed that the avian spleen is a highly dynamic structure able to undergo dramatic changes in size, mainly due to changes in the volumes of the lymphoid components. Twelve-day-old nestlings had small undifferentiated spleens with no germinal centres (GCs), but consisted of a homogeneous lymphocyte-containing tissue. One-month-old Willow Tits (July) had significantly heavier spleens than any other age group. Their spleens had a well developed red pulp and large numbers of small GCs and intensely basophilic cells (IBCs), probably reflecting an increased antibody production initiated by exposure to new antigens. The question why not all young tits develop large active spleens at this time is discussed, and it is suggested that the lymphoid activity of the spleen might be a factor of importance for successful establishment in an autumn/winter flock. Adult birds had small spleens in July. Although there was a very large inter-individual variation, most adult and juvenile tits had very big spleens in August and September, a period when tits are moulting and finishing flock establishment. This inter-individual variation most likely reflects differences in degree of infections. It is hypothesised that a large spleen, with a high lymphoid activity, in a moulting bird is the result of growing feather follicles being broken, and that the resulting wounds have become infected. Once moult and hock establishment an finished (October), spleens in both age groups remain small until next spring, containing few or no GCs and only weakly developed IBCs and red pulp. Small spleens during late autumn and winter may reflect low parasite infections or a high sensitivity to stress.