Blood parasites of migrating Willow Warblers (Phylloscopus trochilus) at a stopover site

This is the authors' abstract. We don't add key points for this paper.

Populations are often difficult to separate on the basis of morphometric data. However, since blood parasites have shown significantly different prevalences and parasitaemias in natural host populations, such discrepancies may provide an opportunity of studying population differences in migrating birds. We obtained blood samples from 312 Willow Warblers (Phylloscopus trochilus) at the Tauvo Bird Observatory, Gulf of Bothnia, Finland. Approximately one-fifth of the birds were parasitized by at least one out of four different haematozoans. Most of these birds (88%) were infected with either Haemoproteus belopolskyi (Hb) or Leucocytozoon phylloscopus (Lp). These parasite species were predominant in the samples at different times, Hb earlier and Lp later. Based on tail and wing lengths, body mass, and wing length - tail length ratio, Hb-infected juvenile males were significantly larger than Lp-infected birds. Different dipteran vectors transmit these haematozoa, and in conjunction with morphological evidence, the observed difference in time of main occurrence of these two parasites may indicate that the birds had become infected in different areas. Thus, our samples may include different (sub)populations of Willow Warblers. In general, blood parasites may provide a means of differentiating among individuals originating from different areas.