Common cuckoo female host selection is not determined by host quality but can affect cuckoo nestling growth when parasitising Common redstarts

Common cuckoo (Cuculus canorus) females lay their eggs in the nests of other avian hosts, relying on parental care provided by parasitised hosts. Therefore, it would benefit cuckoo females to target high-quality individual hosts, able to provide optimal parental care. Attempts at testing cuckoo female host selection have so far shown mixed results. However, this might be because studies have rarely considered the host nests that are available in space and time during each cuckoo egg-laying event, as well as the implications of host choice on the cuckoo nestling growth. Here we combined long-term monitoring data with an experiment to test whether cuckoo females parasitising Common redstarts (Phoenicurus phoenicurus) target individual hosts to optimise their nestling[s] growth. Making use of data collected between 2013 and 2022 from 350+ nest boxes in Oulu (Finland), we first explored whether there is a range of available host nests for the cuckoo female to choose from. Second, we tested if hosts are targeted according to individual quality (using clutch size as a proxy). And third, we investigated the outcomes of cuckoo female host-selection on nestling growth (mass, tarsus length, and wing length) between 2014 and 2019. We conducted a cross-fostering experiment where we either left cuckoo eggs to hatch and be raised in the nest their mother originally chose for them or moved cuckoo eggs to non-parasitised nests. Additionally, we conducted an exploratory analysis to test the quality of the parents caring for the cross-fostered cuckoo nestlings. After accounting for how many host nests were available to the cuckoo female to choose from, we found that nests with bigger clutches were not more likely to be parasitised, and cross-fostering did not affect mass and tarsus length growth. However, both in the wing length growth and in our exploratory analysis of host parental care on cuckoo nestling growth we found that cuckoo nestlings that grew in the nest selected by their mothers, reached higher asymptotic growth at a slower rate. This suggests that while cuckoos may not choose redstart hosts based on their individual quality when parasitising common redstarts, cuckoo female host selection might improve cuckoo nestling growth and thus, have an adaptive significance.