By Jim Pauff
Grosbeaks are placed in the family Fringillidae—finches—making them cousins to cardinals and house sparrows, deflating their mystique. Their un-finchlike quality is unique to themselves. Go North and watch Evening Grosbeaks fly like butterscotch lollipops around a feeder. Watch a Rose-breasted male in Texas in April singing like a robin, dressed in the black, white, and red of a German diplomat. And there is the uncommon Blue Grosbeak—smaller than the rest with brown sparrow wings. Their summer range goes all the way up to Pennsylvania and New Jersey but there are more of them living in the South. Bird experiments are run on blues due to their uncommon color. Ditto Indigo Buntings, for the same reason. Blue is rare in birds as in flowers. Therefore ornithologists theorize that rare color to be a key factor in their lives. Field observations reveal bluer male grosbeaks hold superior territories. It was logically concluded blue was also key to mate selection. This proved wrong.
Blue Grosbeak plumage emits a large amount of ultra-violet color that human eyes cannot see—which other Blue Grosbeaks react to. If the birds are placed behind glass that stops transmission of ultra-violet both male and females have little interest in them. In an experiment at Auburn University in 2003, captive Blue Grosbeak males were made bluer or drabber by watercolor markers dabbed onto their plumage. They were put into cages next to females and mating interest observed. It was expected bluer males would attract more females. But the females didn’t care how bright or drab males were. They paired up equally. Unknown choice factors were operating. A similar experiment was run in Central America in 2011 with the same results. Blue didn’t equate to handsome for female grosbeaks.
So, why are Blue Grosbeaks blue? It is now postulated that the strength of blue deters territory challenges by other males. Color intensity varies in the males as it is connected to nutrition at the time of molt. Bright apelets on Red-winged Blackbird wings do the same thing. As with the blackbirds, strength of color in Blue Grosbeaks deters fighting where someone could get hurt. Cover a Red-winged Blackbird’s puffy apelets—as scientists have done—and other males immediately take over his holdings. Make a Blue Grosbeak drab and, presumably, other males will bully their way past his land claim.
It is now believed females choose males based on the quality and size of the territory they hold. Females nose around before they commit. They choose males not because they are handsomely blue but because they are land rich. In the words of Oliver Hardy, dumped by his movie girlfriend for a richer man, “Twas ever thus!”