Canada warbler populations have declined over the past 40 years but the factors responsible for this decline are unknown. This occurs for two reasons. First, Canada warblers are long-distance migrants so diagnosing which portion of the annual cycle limits population abundance, and hence likely responsible for the decline, represents a significant challenge because it requires that we follow individuals across more than a dozen countries. Second, Canada warblers are secretive and difficult to study in the field; the pessimist says this means that most attributes of their ecology are poorly known while the optimist says that almost every discovery is new to science.
