Bird of the Month: Red Crossbill (Loxia curvirostra)
Distinctly curved bills with crossed tips have evolved to make the crossbills specialists in opening the cones of evergreen trees.
Length: 6.25 inches
Wingspan: 11 inches
Weight: 1.3 ounces (36 grams)
AOU code: RECR
By Andy McCormick
There are four recognized species of crossbills in the world (Clement, et al). They share the genus Loxia, Greek for slanting or crosswise.
The Red Crossbill, L. Curvirostra, and the White-winged Crossbill, L. leucoptera, are the only two crossbill species in North America. The Red Crossbill’s species name is literally from the Latin for curved beak.
Adapted for the Evergreen State
The genus’s unique bill functions like sideways pliers, with the mandibles moving in opposite directions.
Once a crossbill has its bill set along a seed cone’s scale, “[it] uses [its] mandibles to bite between cone scales, so that lateral abduction of lower mandible (in the direction of its crossing) opens the cone scale, thereby exposing the seed” (Adkisson).
Using its tongue, the crossbill then extracts the seed.
Photos: Red Crossbills, male (above) and female (below). Photos by Mick Thompson.
Red Crossbills are highly nomadic and they forage in flocks. Because cone crop production varies from year to year and quality can vary locally, crossbills may roam long distances in search of good and abundant seeds.
Flocking is thought to aid the birds in assessing crop quality and to provide protection against predators. Once a flock has found a good source of seed, it will revisit a productive tree repeatedly while ignoring neighboring trees (Adkisson).
The "It" Bird
There is great interest in Red Crossbills among ecologists and evolutionary biologists. Not only has the genus evolved a unique use of the bill to pry open seed cones, but there may be up to ten types or subspecies of Red Crossbill in North America.
Some work has been done to distinguish types of Red Crossbills by their call notes and the varieties of trees they prefer.
In the Pacific Northwest, three crossbill types are the most common. Type 3 birds have a core range in the Pacific Northwest, have a small bill, and prefer hemlock and spruce trees. Type 4 Red Crossbills also have a core range in the Pacific Northwest, have a medium size bill, and prefer Douglas firs. Type 2 birds prefer Ponderosa pines (Adkisson, Paulson).
More research is needed to determine whether these differences are enough to say the three types are different species. Field identification among types is unreliable.
Feed and Breed
Red Crossbills nest in the southern taiga forest across Canada and in the montane western United States. Breeding occurs when seeds are available, which may be in winter or spring. Crossbills can be flexible, since they do not have to wait for the seed cones to open on their own.
The Red Crossbill builds a nest on a horizontal branch, well out on the limb, about 40 feet high. Typically three to four eggs are deposited. The female incubates the eggs for about two weeks, and first flight occurs almost three weeks later (Kaufman).
More Than Just a Funny Beak
The Red Crossbill is a chunky bird, with a short, notched tail and large head and bill. The male is brick red overall, with dark, unmarked wings.
The female is very different, with a yellowish-olive body, dark wings, and a gray throat (Alderfer).
The common flight call of kip-kip or gyp-gyp-gyp is useful for identifying these birds in flight.
See a male Red Crossbill using his acrobatic skill to open closed cone seed pods at the Macauley Library website.
For references, email Andy McCormick: firstname.lastname@example.org.