May Phenology 3: Bird Arrival Dates

by Carl Strang

The pattern in the past has been for migrant bird arrival dates to be more similar between years than is true of flower and insect first appearances. Birds are capable of making small adjustments, but in general they are following the dictates of photoperiod, which observes the same calendar each year. Median arrival dates were thus only 3 days later at Mayslake Forest Preserve in 2013 than in 2012 (16 species), 8.5 days later than in 2011 (20 species), 1 day later than in 2010 (19 species), and 9 days later than in 2009 (19 species).

The eastern wood-pewee matched the median difference in arrival dates for both 2010 and 2009.

The eastern wood-pewee matched the median difference in arrival dates for both 2010 and 2009.

The respective differences for April and earlier were 7, 3, 2.5, and 5 days. The pattern of past years was sustained in magnitude, but it is also true that all median arrival dates were later in 2013 than in the other years. The late development of flowers and emergence of insects probably accounts for this. The birds can adjust their migratory timing that much.

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