Song and immunological condition in male barn swallows (Hirundo rustica)

Male secondary sexual characters may have evolved as intra- or intersexual signals of male phenotypic or genetic quality. In birds, singing performance may have the function to honestly reveal health and vigor of individual males. Infectious diseases and poor body conditions would therefore be expected to negatively influence singing performance. Since bird pathogens are known to elicit both a humoral and a cell-mediated immune response, it can be predicted that a negative relationship exists between singing performance and activity of the immune system. This prediction was tested for the first time in this correlational study. The relationships between song rate and features and hematological variables (concentration of leukocytes in peripheral blood, ratio of gamma-globulins to total plasma proteins, blood cell sedimentation rate, hematocrit) and body condition were analyzed in a population of barn swallows (Hirundo rustica). Song rate was negatively correlated with lymphocyte concentration and with the ratio of gamma-globulins to plasma proteins, Spectrographic analysis showed that features of song were not significantly correlated with hematological variables or body condition. The level of circulating testosterone was not correlated with song rate nor hematological variables. This study is the first to show a correlation between a bird's singing performance and hematological profile and suggests that song rate of male barn swallows may reflect their health status. Song in this species might thus have evolved because it allows prospecting females to assess aspects of phenotypic and/or genetic quality of potential mates.