Female Black Oystercatchers can be identified by prominent, dark, irregular "eye flecks" or pigment spots in their yellow irises, often making the pupil appear larger, irregular, or connected to black spots. These flecks are a reliable, ~94% accurate field method to distinguish females (who have prominent flecks) from males (who have minimal or no spots).
OM-1MarkIIOM 150-400mm F4.5 lens400mmf/4.51/2000s800 ISO+0.5 EV