Parasite-induced maternal response in a natural bird population

1. The timing and mechanism of the maternal response to ectoparasites is investigated in a host-parasite system consisting of great tits and a haematophageous ectoparasite, the hen flea. It has been demonstrated previously that a maternal response to this parasite enhances survival and fertility of the offspring. This may have arisen by either a maternally transferred protection via the egg, or by a parental response affecting the common rearing environment after hatching. Two experiments aimed to differentiate between the two possibilities are reported here. 2. First, mothers were either exposed to or kept free of ectoparasite during egg production, and subsequently the newborn nestlings were cross-fostered between the two treatments. The experimental design discriminates as to whether the maternal effect arises before or after hatching. Within the same nest, the nestlings originating from previously exposed mothers grew faster than nestlings of unexposed mothers. 3. Secondly, we tested for the transfer of parasite-induced immunoglobulins (IgG) via the egg. Mothers were kept free of ectoparasites until they had laid the first egg and were then either exposed to or kept free of ectoparasites to the end of laying. The IgG-concentration significantly increased from the first to the eighth egg of exposed mothers, but not in eggs of unexposed ones. 4. In summary, the first experiment shows that ectoparasites can induce a beneficial maternal response at egg laying, and the second experiment suggests that the maternal effect is due to immunoglobulins transferred via the egg. Maternal responses to other parasites, e.g. blood parasites, are known for chicken in captivity, In natural populations of birds both the timing and mechanism of the response are poorly understood, despite their relevance for behavioural and population ecology.