January 2021 Bird of the Month: Red Crossbill

By Gabriel Foley January 29, 2021
Red Crossbill (Northeastern or type 12) Loxia curvirostra (type 12)

The Red Crossbill is best treated as a historical breeder in Maryland and DC. Two accounts published in the Auk in the mid-1880s provide evidence of regional breeding. One, from DC, reported a recently fledged young. The second, from Prince George’s County, stated that a collected female “showed unmistakeable evidence of having recently incubated”. Perhaps more surprisingly, it also provided a second-hand report of multiple nests found in Baltimore City. Stewart and Robbins report in The Birds of Maryland that Red Crossbills were seen summering in Dorchester County from 1932–33, but no further evidence of breeding was reported.

Red Crossbills have a fascinating life history shaped by their dependence on a specific food source that regularly fluctuates in abundance. Local food shortages instigate large-scale, irruptive movements in search of sufficient food. If these new food sources are plentiful enough, the wandering crossbills may attempt to nest. This means that, while there hasn’t been evidence of Red Crossbills breeding in Maryland or DC for 135 years, an irruption coinciding with a substantial local cone crop could elicit impromptu breeding. But Red Crossbills don’t carry food in their bills, and females remain on the nest throughout the entire incubation period, so detecting breeding is tricky. Observers who are fortunate enough to see a crossbill in Maryland should be on the lookout for indications of breeding.

Red Crossbills occur on four continents; in North America, they are most abundant in western coniferous forests, south of the White-winged Crossbill’s range. They exhibit substantial variation in size, color, and bill shape, but in North America there is considerable overlap in the breeding range of each different size and shape. Since geographic isolation is important for subspecies recognition, the variation is instead sorted into ten Types. Variation in color doesn’t associate well with different Types, but size, bill shape, song, and call type all differ with each of the ten Types. Their call is the most useful identifier of Type, and calls should always be recorded when possible.

The unique bills that provide crossbills with their name are well-suited to extracting seeds from coniferous cones. Bill size—specifically, bill depth—affects foraging efficiency on different species of cones and this has led to the formation of the different Types. As they’re foraging, Red Crossbills must make decisions about which trees will provide easiest access to the most seeds. A flock can make better, faster decisions about this than a lone individual can, so crossbills tend to forage in flocks. The flock uses calls to communicate with flock members; the flock will be silent while eating, then call rapidly before flying off. But using the flock to identify good food sites works best if all the flock members have similarly sized bills, since bill depth influences foraging efficiency. A different call associated with each different bill depth helps an individual know what sort of bills a flock has, and as a result what sort of trees they will forage on. They’ll preferentially select flocks that sound more like themselves. And since Red Crossbills form pairs from within their foraging flock, this selection pressure is strengthened and leads to the formation of multiple distinct Types that are distinguished by call and bill depth.

There are ten Types in North America: types 1, 2, 3, and 10 will all nest in the northeastern US. Nesting Red Crossbills are not as seasonally restricted as many species are. They avoid nesting during fall months, but otherwise can nest year-round. Breeding is most frequent during early winter, when seeds are plentiful and accessible, or in late summer, when cones are developing.

Crossbills feed their young a slurry of seeds carried in their crop. The dark color of the viscous liquid has led some observers to postulate that the seeds are supplemented with insects, even in winter. However, others have noted crossbills’ propensity to gather soil and grit which would similarly discolor the partially digested seeds, and caution against attributing the coloration solely to insect protein.

Females build the nest in a conifer tree out of small conifer branches, then line it with conifer needles, lichens, fine grasses, or feathers. Several pairs will often nest semi-colonially. Their nests are spaced 50–100 yards apart and they do not maintain real territories. She lays 2–4 pale, darkly blotched eggs and depending on temperature, will begin incubating after laying either the first or the last egg. After beginning incubation, she remains on the nest continuously for 12–16 days, again depending on the ambient temperature. After the chicks hatch, the female continues her vigilant brooding for another 5 days. Throughout, the male accommodates her adhesive behavior by bringing her and the chicks food at the nest. Eventually, both sexes help feed the young and will forage up to half a kilometer away. Depending on temperature and food supply, the young will fledge in 15–25 days.

After fledging, the young birds somewhat resemble a female House Finch, with a finely streaked breast and brown back. Their parents will continue to look after the fledglings for another month, although if conditions are good for another brood, the female will begin incubating immediately and the male will look after the fledglings on his own.

Red Crossbills nesting in Maryland or DC is highly unlikely. Nevertheless, their peripatetic habits, nearly year-round nesting phenology, and discreet breeding behavior makes it worth being aware of the possibility.

References: Benkman, C.W. and M.A. Young (2020). Red Crossbill (Loxia curvirostra), version 1.0. In Birds of the World (S.M. Billerman, B.K. Keeney, P.G. Rodewald, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.redcro.01