Red-winged Blackbirds feeding on maple keys

Last weekend I came across a large flock of Red-winged Blackbirds feeding on Manitoba Maple keys in Mississauga. These seeds are a favourite food of Evening Grosbeaks but I can't recall seeing any other species of bird feeding on them, although I'm sure it happens very regularly.

As far as I could tell, every blackbird in the flock was a first-fall male, although there were a few Brown-headed Cowbirds and European Starlings mixed in. This segregation is very commonly seen in migrant Red-winged Blackbirds, with most smaller flocks have only a single age and sex class. Indeed, I saw separate flocks of both females and adult males in the area on the same day. As far as I can tell this phenomenon doesn't appear to have been studied extensively. A few possible explanations:

• Less dominant birds (i.e. females and younger birds) avoid flocks with more dominant adult males that they may be very aggressive and hamper their feeding and roosting. This paper shows that older males are dominant at roost sites.
• In many species, more dominant birds will remain farther north in winter to more quickly acquire prime territories in spring, while less dominant birds will winter in warmer and more distant areas. This study suggests that delayed arrival in female Red-winged Blackbirds may be linked to lower reproductive success. Segregation may be simply due to different age and sex classes having different migration routes and timings.

There's still new things to see in even the most common of birds!