This spring seems like a particularly good one for the Red-winged Blackbirds. They are everywhere in great numbers. In fact, they have become one of the most common birds in North America. This is the first year I have stopped to notice that there is more going on than just big, bright, red-winged males screeching in the cattails…
and smaller-bodied, sparrow-colored females skulking through the marsh plants.
There is another player in the mix, sometimes screeching and displaying, sometimes lurking furtively near a calling adult male.
Second-year male Red-winged Blackbirds are not quite ready for prime time, but they do a lot of practicing of “singing” and posturing like their adult male role models. In general, they seem a little larger than females, but definitely smaller than adult males that chase them around the marsh and off their territory.
Red-winged Blackbirds are polygynous, and a big, loud, gaudy male might attract as many a dozen or more females to his territory. In his case, bigger is definitely better for several reasons. Bigger male Blackbirds win fights, can claim and defend the best territories, can go longer without food, can tolerate cold better, and apparently are more attractive to female blackbirds.
As a result of this sexual selection (by females) for large males, there are marked size differences between the sexes, with males being 50% or more heavier and 20% longer-bodied than females.
But smaller second-year male Red-winged Blackbirds are at a size disadvantage in trying to compete with the seasoned adult males. What is their role in this breeding system? It is thought that by hanging out on the periphery of a territorial male, the younger bird may be able to sneak in and copulate with females — an advantage for him, but not the territory owner. On the other hand, a sub-adult male hanging around a premium territory may discourage other interlopers from invading — an advantage for the territory owner.
Further, by deferring its maturation, the teen-ager has another year to practice its song and display and get bigger. At some point in the future, it might be in a position to challenge that territorial male for rights to maintain a choice territory.