Abstract Leptin is a multi-functional adipokine in vertebrates. The leptin gene and protein are found in many vertebrates; however, the existence of leptin in birds remains controversial. Here we detected leptin-like activity in avian blood using chicken leptin receptor (chLEPR) and green fluorescent protein (GFP)-fused chicken signal transducer and activator of transcription (chSTAT3) co-expressed in CHO-K1 cells (CHO-chLEPR/STAT3). We validated that rat serum specifically induces intranuclear migration of GFP-fused chSTAT3 (GFP-chSTAT3) in CHO-chLEPR/STAT3 cells, but not in CHO-K1 cells expressing GFP-STAT3 (CHO-chSTAT3) before testing the avian blood samples. Blood of chickens (Gallus gallus), wild jungle crows (Corvus macrorhyncos), and carrion crows (Corvus corone) accumulated the GFP signal into nuclei, and frequency varied in each blood sample. Western blotting showed that chicken and crow blood samples specifically phosphorylated GFP-chSTAT3 in the chLEPR-transfected cells. These results indicate that avian blood contains a leptin-like molecule that specifically binds to LEPR, suggesting that the leptin system is conserved across all vertebrate classes.