Patterns of New World partial bird migration at north vs. south temperate latitudes

Abstract

Determinants of partial bird migration remain poorly understood at different latitudes. We describe partial migration of passerine birds across temperate latitudes in the New World. Due to lower seasonality near the coast of North America and in South America, the proportion of partial migrants at high latitudes in North America should be lower when excluding coastal records. We detected a 10% decrease in the proportion of partial migrant species at high latitudes when excluding those whose only winter records at those latitudes are coastal. We also expected a smaller proportion of partial migrants inland in North America vs. South America. However, at high latitudes, (i.e., >39¢ª) we found a similar proportion but a different taxonomic makeup of partial migrants between continents. Specifically for the Tyrannidae (New World flycatchers), we evaluated the latitude at which species overwinter in North vs. South America. The mean latitude at which North American partial migrant flycatchers overwinter when excluding those near the coast is 30.4¢ª N (2.8 SD), which is lower, but not significantly so, than of wintering flycatchers when considering winter records at all distances from the coast (32.3¢ª N, 3.2 SD). Furthermore, the mean latitude at which New World partial migrant flycatchers overwinter in temperate South America (30.3¢ª S; 5.6 SD) vs. inland North America is not significantly different. These results suggest that partial migrant bird species in the New World may be under different constraints, depending on factors such as habitat occupancy and distance of a population from the coast.