Protocalliphora larvae: moderate but lasting carry-over effects on yearling and mother coloration

Parasites play a significant role in the evolution of their hosts, influencing a wide range of traits, including animal signals such as coloration. While the direct effects of parasitism on coloration are well-documented, it remains unclear whether these effects persist beyond the parasitized phase, leading to direct carry-over effects. Additionally, the impact of parasitism on parental investment may have indirect consequences for the parents themselves, potentially affecting their coloration. We analyzed a 20-year dataset to estimate both direct and indirect long-term effects of the ectoparasite Protocalliphora sp. on blue tit (Cyanistes caeruleus) and their plumage color. This ectoparasite only affects the nestling stage but is also known to increase parental care. We measured the impact of ectoparasites on nestlings' condition, and then quantified their long-term effects on the colorations produced during the subsequent molt in i) parasitized individuals at their yearling stage, and ii) parents that had reared parasitized nestlings. Our analyses focused on a carotenoid-based (yellow patch) and a structural (blue patch) plumage coloration. Our findings revealed that parasite load was negatively correlated with nestling condition. We also found long-term effects of Protocalliphora larvae on yearling coloration, reducing the brightness of the yellow patch in individuals from large broods. In parents that reared parasitized nestlings, we observed lasting consequences in coloration, reducing brightness of the yellow patch in both parents but decreasing blue patch intensity only in mothers. This study supports the hypothesis that early life parasitism can have a lasting effect on ornaments of parasitized individuals, as well as carry-over indirect effects on parent ornaments.