The present invention provides compositions and methods relating to avian leptin receptor binding domains. The present invention demonstrates that these compositions are useful for detecting the presence of leptin in a sample and distinguishing free leptin from bound leptin in multiple species. Methods and kits are presented for determining free leptin in a sample from an individual by assaying a sample for the binding of leptin to an avian leptin receptor binding domain, and detecting the bound leptin by using a labeled anti-leptin antibody.