In a previous posting, we learned how to identify the difference between male and female Acorn Woodpeckers simply by the distribution of the red feathers on their heads. When it comes to the beautiful Black Oystercatcher (Haematopus bachmani), the answer lies in their eyes!
Blending into the oceanscape as they forage among dark mussel beds and rocks; these striking shorebirds are often not seen until they are heard by their distinctive call.
You may then get a glimpse of their bright red beaks as they take flight. With the male and female looking almost identical, it is virtually impossible to tell the difference unless you get a closer look at their amazing, yellow eyes. As seen in the photograph below, by taking a closer look, you will notice a dark speck right next to the black pupil of the bird in the foreground. This is the female.
An excerpt from The Journal of Ornithology (J. Field Ornithol. 79(2):215–223, 2008) indicates:
"Sexing oystercatchers in the field is difficult because males and females have identical plumage and are similar in size. Although Black Oystercatchers are sexually dimorphic, using morphology to determine sex requires either capturing both pair members for comparison or using discriminant analyses to assign sex probabilistically based on morphometric traits."
Studies revealed: "All adult Black Oystercatchers have bright yellow eyes, but some of them have dark specks, or eye flecks, in their irides. Using an eye-fleck model where all females have full eye flecks and males have either slight eye flecks or no eye flecks, we correctly assigned the sex of 117 of 125 (94%) oystercatchers." Pretty accurate I would say!
For researchers this is important stuff, as it greatly increases the odds of obtaining the correct results in a less intrusive manner. Good for them and good for the birds!