A Blue Grosbeak Mystery
On a warm, cloudy morning, at the far north end of the old field, a Blue Grosbeak was singing from the top of a chinaberry tree. The big silver, conical beak gleamed, the slightly peaked head was deep blue, and in pauses between songs, the Grosbeak switched its long tail energetically. But its plumage showed a lot of brown all over, with patches and shadings of blue, so it looked like a sub-adult Blue Grosbeak – a male from last season that has not yet acquired its full mature plumage – dark, rich blue all over, with striking rust-brown wingbars.
I didn’t have the best view, though, and the Grosbeak flew after only a minute or two, so I’m not sure. Sunlight can do funny things to colors, and often a Blue Grosbeak looks more black or gray than blue from some angles. But – this one looked decidedly brown. It’s puzzling because last summer the only Blue Grosbeak singing in the field all season appeared to be a sub-adult, and I wonder if that’s going to be the case again this year.
It’s a small mystery, and I know I’ll never know the full story because I’ll only see the bird – if it stays around – once a day or so, at most. Maybe if I’m lucky, I’ll see a female, too, and on several occasions in past years I’ve enjoyed catching glimpses of interesting Blue Grosbeak behavior. But there’s so much more of its life that I’ll never see. In general, there’s a lot that is not known about the life and behavior of Blue Grosbeaks, and I think we’re lucky to have them here. It always has seemed to me a signature species of this old field in summer.
The species account in Birds of North America notes: “Although widespread throughout its breeding range, [the Blue Grosbeak] is generally scarce and virtually all aspects of its biology are poorly known, perhaps owing in part to its low numbers. . . . we still lack detailed information about Blue Grosbeak nesting ecology, courtship behavior, and song structure and learning, among other things.”
The account goes on to note: “Nesting in shrubs or tangles of vines along forest edge or roadsides, Blue Grosbeaks commonly produce two broods per year. Limited data suggest that it is quite heavily parasitized by the Brown-headed Cowbird.”*
Since I frequently see and hear Brown-headed Cowbirds in and around this old field, that seems a concern. More and more traffic on an already-busy highway just over the hill, and the replacement of fields, groves and wooded areas with subdivisions nearby have changed this habitat over the past decade or two, and each spring I watch and wonder if a Blue Grosbeak will return to nest here again.
So far the field itself remains large and wild, with a dense stand of full-grown pines as well as more open areas of thickets, vines and shrubs, and a power cut through the middle. So I’m hopeful it can remain good enough habitat for the Grosbeaks – as well as Indigo Bunting, White-eyed Vireo, Gray Catbird and several other species.
*Peter E. Lowther and James L. Ingold. 2011. Blue Grosbeak (Passerina caerulea), The Birds of North America Online (A. Poole, Ed.)