I just wanted to add some information to my comments about variation in 
individuals. I looked at data from Common Terns trapped on Great Gull Island in 
the summer of 2017. We have weights on 918 individuals. The weights ranged from 
99.1 grams to 158.9 grams. That means at the extremes, the largest birds were 
60% more massive than the smallest birds. That kind of difference is likely to 
be noticeable in the field. And that difference is not between different 
subspecies or populations, but within a single colony.

I should point out that these 918 birds were nearly all trapped on nests, so 
they are healthy birds. Also the birds at the top and bottom of that range were 
not extreme outliers. At both extremes, there were numbers of individuals 
within 10 grams of those lowest and highest weights. For the statistically 
minded, the average weight was 126.0 grams and the standard deviation was 9.45.

Joe DiCostanzo

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