The breeding ecology of the Yellow-headed Blackbird (Xanthocephalus xanthocephalus) has been well studied by Willson and Orians (1963) and Willson (1966) and the information presented in this study essentially agrees with their findings. In our study the number of nests, active and inactive, found in 1973 was tenfold greater than that found in 1972 and in 1974 it was fifty percent greater than in 1973. We would like to propose explanations for these different rates of increase in nest building in three consecutive years in the same habitat.