Olive-sided Flycatchers declines are widespread. In 2008 the Committee on the Status of Endangered Widllife in Canada (COSEWIC) looked at Olive-sided Flycatcher and found that Breeding Bird Survey (BBS) data indicate significant and widespread declines in Olive-sided Flycatcher populations throughout North America. Canadian populations experienced a 4.0% annual decline for the period 1968-2006, 3.3% annual decline for the period 1996-2006, total decline over that decade of 29%.
The cause of the decline is not clear. Declines have been seen in many birds that make their living taking insects in flight including the Olive-sided Flycatcher. Suggestions range from loss of wintering habitat, to shifts in timing of insect emergences, the use of pesticides and poor reproductive success in logged over habitats - but so far, the research hasn't been done to determine if it is one or more of these factors working in concert against the flycatchers.
This year we do have a pair using the property once again and we have been hearing "Quick! Three Beers" since the end of May. We didn't take it for granted this year.