Blood parasites affect the repertoire size, but not the phrases duration in the thrush nightingale song

During two years, we analyzed the relationship between the song repertoire sizes, the duration of selected song phrases, parasitic load, and the level of chronic stress, using a thrush nightingale's ( Luscinia luscinia ) as an example. The size of the individual repertoire was 11.3 ± 1.9 (from 9 to 16) song types in 2023, and it was 10.6 ± 2.0 (from 7 to 14) song types in 2024. The population repertoire consisted of 33 song types in 2023 and 29 song types in 2024. Over the two-year period, 40 song types were identified. Both the individual repertoire and the population repertoire have changed to a limited extent between the two years. Blood parasites were found in 61 of 68 nightingales, indicating a high prevalence of 89%. Haemoproteus sp. was present in all infected birds, while Plasmodium sp., Leuсocytozoon sp., Trypanosoma sp., and Microfillaria sp. were found in 26.5%, 4.5%, 2.9%, and 2.9% of individuals, respectively. According to GLM modeling, the level of parasitemia as well as the combination of an individual's age and the year of observation both affect significantly the size of a nightingale's repertoire. The average H/L ratio did not show significant associations with any of the variables used in the study. There was no significant effect of parasitemia on the duration of two target phrases. Thus our results are partly consistent with the hypothesis that there may be a negative correlation between sexually selected traits and parasite load, as suggested by Hamilton and Zuk (1982).