I studied two colonies of Yellow-hooded Blackbirds, of different subspecies, in low and high elevation sites in Colombia in 1987 and 1988. Territories of Agelaius icterocephalus icterocephalus at Agua Prieta (Iow elevation) were one-fifth as large as those of A.i. bogotense at La Herrera (high elevation), and included cattails as the main nesting substrate, while those at La Herrera only included bulrush. Nests at La Herrera were larger than those at Agua Prieta. Whereas at Agua Prieta the birds foraged outside the marsh, at La Herrera territories were used both for nesting and feeding. Breeding was seasonal for both populations.