Habitat effects on physiological stress response in nestling blue tits are mediated through parasitism

Abstract We investigated determinants of the physiological stress response mediated by stress proteins in blue tit Cyanistes caeruleus nestlings growing up in oak forests in central Spain, resulting from different forest management practices. We assessed circulating levels of the heat-shock protein HSP60 as an integrated physiological measure of the conditions experienced by nestlings during postnatal development. The effects of habitat quality and parasite infections on nestling rearing environment were then assessed through this measurement of stress response. Our results showed that newly acquired ecto- and hemoparasite infections were associated with forest habitat structural characteristics, higher prevalence of fleas and blood parasites in more mature forests, and higher prevalence of blowflies in degraded forests. While habitat characteristics did not explain variation in stress protein levels, infestation by blowfly larvae of the genus Protocalliphora and hematozoa infection by Leucocytozoon were significantly associated with higher levels of HSP60. Thus, upregulation of the expression of certain stress proteins seems to be a common physiological mechanism to alleviate the negative impact of parasite infections in growing birds. Habitat characteristics may thus indirectly determine growth conditions for forest birds mediated through their association with one of the most important selection pressures for offspring development, parasite infections.