For much of recent human history, if you were left-handed, you were dealt an unlucky hand. Many cultures to this day consider left-handers sinister, unclean, and backwards. Schoolchildren were punished for being left-handed, and if they made it into the workforce keeping their left hand dominance, the tools they used–in any trade–were likely designed with right-handed use in mind.
In the bird world, dominance on one side of the form also manifests itself. Parrots in Australia have been found to be left eye dominant. In flocking birds, some research suggests that around half of the birds will dodge an obstacle to the left, while the other half will dodge the obstacle to the right, which may help the overall flock navigate its environment. Crossbills have a beak that, of course, crosses over itself. In Red Crossbills, there is an even distribution between lower mandibles crossing right or left. However, in White-winged Crossbills, the lower mandible crosses to the right three times more often than to the left.
White-winged Crossbill. Photo by Aaron Maizlish
These crossbills feed almost exclusively on cone-bearing trees, and will eat over 3,000 seeds each day. The unique design of their bill allows them to successfully pry that seed out of the cone. White spruce, black spruce, and tamarack are the favorite species of the White-winged Crossbill, which has a smaller beak and overall body dimensions than the Red Crossbill; thus, on average, the White-winged Crossbill eats the seeds of smaller species whereas the Red Crossbill goes for larger cones of various pine species.
In Red Crossbills it is thought that the 1:1 beak direction is a response to food availability: if a right-billed bird has foraged along a pine cone, the next left-billed bird that comes along might access seed that the first bird wasn’t able to access. But what explains the apparent “right-billedness” of White-winged Crossbills? There are many theories ranging from genetics to the skewed axis of cone spirals in spruce trees, but it remains somewhat mysterious. In human ancestors, ancient remains of teeth show tools strikes to the teeth that demonstrate right-hand dominance millions of years ago. The working hypothesis is that brain development and lateralization affect the hemispheres of your brain, though perhaps it’s a stretch to suggest that the brains of White-winged Crossbills develop differently.
White-winged Crossbills have more contrarian behaviors in addition to their bills. They will breed at any time of the year, during the short daylight hours in the dead of winter. Their breeding schedule is almost entirely dependent on food availability. Thus, with good crops of spruce or tamarack, White-winged Crossbills could be nesting in Wisconsin at almost any time.
White-winged Crossbill with mandibles crossing to the right. Photo by Andy Reago & Chrissy McClarren
Though mostly restricted to northern Wisconsin where abundant native conifer trees provide food, the White-winged Crossbills will breed in conifer plantings in the southern Kettle Moraine, and other localized spots of high conifer density in southern Wisconsin.
On our mid-winter snowshoe last weekend, we saw a flock of White-winged Crossbills in the Laas Tamaracks just off of Highway 89. Apparently it was a good crop year for the tamaracks; this fall Roger collected a heaping 5-gallon bucket of tamarack cones, and it appears the crossbills are still subsisting on the fruits of that crop. You might be able to find White-winged Crossbills in any conifer-dense area in the sanctuary, and perhaps you can form your own hypothesis on the bill dominance of these birds.
Written by Drew Harry, Faville Grove Sanctuary land steward
Cover photo by Kurayba FCC
