Parental care strategies in Eurasian penduline tit are not related to breeding densities and mating opportunities

Population size and density may influence various aspects of breeding systems. For example, the number of potential mates may influence the mating system and parental care strategies. The breeding system of Eurasian penduline fits, Remiz pendulinus, involves sequential polygamy by both sexes and uniparental care by either male or female. Additionally, about one third of the clutches is deserted by both parents. Mating opportunities and breeding behaviour were studied at two populations: a low-density population (The Netherlands) and a high-density population (Hungary). We expected that higher breeding density is associated with higher incidence of polygamy. However, despite the substantial differences in breeding density and mating opportunities between Hungary and The Netherlands, there was no difference in the frequency of parental care types. We further investigated the plasticity of the breeding system of penduline tits using data from five breeding populations in Europe, and found that care patterns did not differ between these populations. We conclude that the type of parental care is not related to local breeding density. Proximate mechanisms of breeding system variation (e.g., gene expression) may be inflexible and, thus, not adjusted to local conditions. Alternatively, long-distance dispersal of penduline fits may mix the gene pool, so that local adaptation cannot possibly occur.