Eco-endo-immunology across avian life history stages

• The immune–endocrine system varies dramatically across life history stages.
• Environmental influences suggest plasticity in the immuno–endocrine system.
• Complex relationships exist between disease ecology and endocrine–immune responses.

Broadly distributed songbirds, particularly those that migrate, encounter a wide range of pathogens. Both pathogen exposure and energy available for immune responses are expected to be affected by environmental variation in climate, habitat quality, and social interactions as well as hormonal mechanisms. Comparisons of Aves in the field have begun to build the framework for understanding how such environmental variation interacts with disease environments as reflected in endocrine and immune responses. In this review, the roles of hormones and immune function across the various life history stages that make up the avian annual cycle are considered with an emphasis on free-living songbirds and the various hormones known to mediate the innate and acquired immune systems including melatonin, prolactin, growth hormone, and several neuroendocrine hormones. Finally, hormone–immune interactions are considered within the framework of disease ecology.