Ecological factors that determine Ixodes ricinus tick burdens in the great tit (Parus major), an avian reservoir of Borrelia burgdorferi s.l.

Although bird-tick systems affect the human risk of tick-borne diseases, very little is known about the ecological factors that shape the spatio-temporal variation of tick infestations in terrestrial songbirds. We present a risk model that explains the levels of infestation of Ixodes ricinus, the main vector of Borrelia burgdorferi s.l., during the breeding season of the great tit (Parus major), one of Europe's most abundant avian reservoir hosts of B. burgdorferi s.l. Tit tick burden were modelled as a function of variables summarising vegetation, climate, proxies for mammal abundance and characteristics of individual birds and their nests. Tick loads were positively associated with the relative humidity prior to capture of the bird and the cover of bracken inside its territory. The number of cold winter days prior to the bird's breeding season showed a negative association with tick loads. None of the proxies for mammal abundance correlated with tick loads. Tick loads decreased with age in female tits, whereas they increased with age in male tits. Tick burdens in the parental tits were positively associated with their brood size and negatively correlated with the average nestling body weight. Possible mechanisms include: how tit foraging influences tick encounter rates, host tick resistance mechanisms and the environmental conditions that simultaneously affect tick exposure risk and brood characteristics. We believe this study provides the first detailed insights into the ecological factors that shape tick burden in a terrestrial songbird.