Here is a note about oystercatcher sex. You may have noticed that we are referring to some of the oystercatchers in the study as “he” and some as “she.” However, the truth is we don’t really know if an individual is a male or a female unless we have seen it mating. The plumage of the male and the female are identical Since we haven’t caught any of our tracked birds in the act, we have to arbitrarily decide who to call “he” and who to call “she.”
Obviously, the oystercatchers know the difference!
Humans, meanwhile, can make guesses based on size. Although the sexes overlap in size, females tend to average a little larger than males (about 630 grams on average versus 580 grams). Another way of possibly determining the sex of an American Oystercatcher is to look in its eyes. You may notice that some oystercatchers have an extra black dot in their pupil. Some are more pronounced than others. These have come to be called eye flecks. In the closely related Black Oystercatcher, a DNA analysis showed that females tended to have larger, more defined flecks than males, which had smaller flecks or no flecks. Though like size this is not conclusive, it may be an indicator that a bird is a male or female. Which means that Arnie, who has a pretty well-formed eye fleck, may actually be an…Arnette?