Avian Insulin-like Growth Factor 1 (IGF-1) is a peptide growth factor that plays a central role in regulating growth, development, tissue maintenance, and metabolic processes in birds including chicken (Gallus gallus), duck (Anas platyrhynchos), turkey (Meleagris gallopavo), and zebra finch (Taeniopygia guttata). The IGF-1 protein is identical at the amino acid level among these species, reflecting strong evolutionary conservation and its essential biological functions in avian physiology. IGF-1 is produced primarily in the liver in response to growth hormone stimulation and circulates in the bloodstream bound to insulin-like growth factor binding proteins (IGFBPs), which regulate its stability, transport, and tissue availability. It is also expressed locally in multiple tissues including muscle, bone, and reproductive organs, where it functions through autocrine and paracrine signaling. Avian IGF-1 binds to the insulin-like growth factor 1 receptor (IGF1R), a transmembrane tyrosine kinase receptor that activates intracellular signaling pathways such as PI3K–AKT and MAPK, regulating cell proliferation, differentiation, survival, and protein synthesis. In these bird species, IGF-1 plays a critical role in somatic growth, skeletal development, and muscle accretion, and also contributes to regulation of metabolism, feather development, and reproductive physiology. Due to these functions, IGF-1 is widely studied in avian biology and agricultural research, where circulating IGF-1 levels are commonly used as a biomarker of growth rate, nutritional status, and endocrine regulation of growth in poultry and passerine models.