Pick up a pine cone the next time you go on a stroll in the woods. Consider it from the perspective of a crossbill—a possible food container. The cone is probably open, with the scales folded back and the winged seed gone, perhaps eaten by animals or spread by the wind. Seeds are high on the list of desired foods since they are a dense source of nutrients. Red Crossbills (Loxia curvirostra) have a highly specialised bill shape that enables them to successfully exploit conifer cones for the seeds that comprise their primary diet. But not all pine cones are the same, and neither are Red Crossbills.
Craig Benkman of New Mexico State University discovered that distinct populations of Red Crossbills had “key” conifer species on which they can eat the fastest. One population, for example, feeds most effectively on Ponderosa Pine, while another focuses on Douglas-fir. Each population eats up to twice as rapidly on the cones of its key species as it does on the cones of other species.
Perched atop a cone, Red Crossbills feed by probing between the scales. They use their lengthy tongues to extract the flying seeds after moving their lower mandibles sideways to push the scales outward. While the bird removes the papery “wing,” the seed is trapped in a groove on the roof of the crossbill’s mouth. The ensuing fine debris drift is a telltale indicator of feeding crossbill flocks.
Benkman discovered that the size of the bill as well as the seed-holding groove differs across populations. He was also able to demonstrate a link between these measures and the pace at which a crossbill could take seeds from various types of cones. Captured wild crossbills were served “cones on a stick” and timed while extracting and consuming 10 seeds in a series of studies. The conifer species on which a population can eat the fastest is also the species on which wild populations are most likely to be discovered feeding, as predicted.
How do changes in bill and groove morphology impact the feeding rate of crossbills? When crossbills feed on the papery cones of firs and spruces, a bigger bill provides greater leverage to rip apart enormous, thick scales. The width of the seed-holding groove matches to the size of the seed of the key species, enabling the birds to husk the seed more effectively.
Red Crossbills with the shortest bills eat on spruce and, in particular, hemlock cones all year. Another population, however, focuses on Engelmann Spruce stands in the summer and Ponderosa Pine stands in the winter. Benkman’s research has demonstrated that the birds in this population can consume most quickly on Ponderosa Pine cones, which are their winter meal. The harsher needs of winter serve as the cutting edge of natural selection in this circumstance.
The story of the interaction between Red Crossbills and conifers has a new twist. Kimberly Coffey, Benkman, and Brook Milligan have discovered that the presence of spines on pine cones decreases the pace at which crossbills can eat. Conifers that depend on wind dispersion for seed distribution gain an advantage if they can inhibit seed eating. One such protection is the formation of thick, woody scales that are difficult to remove or break apart.
Another protective mechanism is the formation of sharp points or spines on the cone scales. Ponderosa, lodgepole, and bristlecone pines are examples of pines having spiky cones. The spines do not seem to prevent squirrels from eating, since they peel off the scales at their base, beginning at the bottom of the cone. Spines, on the other hand, seem to be a deterrent to seed-eating birds, particularly Red Crossbills. Crossbills are hindered by avoiding the spines with their feet since they perch on the cones while feeding. The researchers believe that crossbills’ more specialised adaptations to dining on pine cones—stronger legs to adopt diverse perching orientations and bills with broader spreading scales—leave crossbills less impeded by spines than more generic seed-eating birds like chickadees and nuthatches.
Benkman’s feeding efficiency groups match to Jeffery Groth’s Red Crossbill divisions. Groth investigated crossbill morphology and vocalisations while a PhD student at the University of California, Berkeley. He recognised eight “types” of birds in North America based on sounds, wing length, and beak length. The biggest kind, type 6, is found in the highlands of southern Arizona and Mexico, where the birds eat Apache and Ponderosa Pine. Newfoundland is home to Type 8. The next six kinds have a broader range, including parts of North America’s boreal and alpine forests. All six are possible in southwestern Canada and the northern United States.
| TYPE | KEY SPECIES | RANGE |
| 1 | Sitka spruce, hemlocks | Canada – boreal and mountain forests, U.S. – Appalachians and northwest |
| 2 | ponderosa pine and Engelmann spruce | Canada – boreal and mountain forests, U.S. – Appalachians and west |
| 3 | western hemlock, spruce, larch | Canada – boreal and mountain forests, U.S. – northern Appalachians, northwest and Alaska |
| 4 | Douglas-fir | Canada – boreal and mountain forests, U.S. – northern Appalachians, northwest |
| 5 | lodgepole pine | Canada – western, U.S. – western |
| 6 | Apache pine and ponderosa pine | Arizona and Mexico |
| 7 | lodgepole pine and Engelmann spruce | Canada – western, U.S. – western |
| 8 | black spruce | Newfoundland |
While many of the kinds’ ranges overlap or coincide, flocks tend to consist of just one variety, as shown by their vocalisations. Mating between kinds seems to be an anomaly rather than the norm. Furthermore, Groth discovered modest but discernible changes in their DNA, suggesting that the kinds diverged within the previous 100,000 years. Groth suggests that the various Red Crossbill kinds are, in reality, different species based on vocalisations, behaviour, and genetic differences.
Conservation
The needs of each group of Red Crossbills, whether subspecies or different species, must be evaluated in order to establish successful conservation measures. While Red Crossbill numbers in North America look to be in good condition, with Breeding Bird Survey data suggesting a rise over the previous 35 years, there are a few reasons to be concerned. For starters, many of the inhabitants are migratory, which makes monitoring difficult. Second, since the various categories are not tracked separately, a fall in one type may be obscured by a rise in another. Third, we may learn from Europe, where the replacement of natural forest with areas highly managed for lumber extraction has resulted in a significant drop in Red Crossbill numbers.
Red Crossbills forage across broad regions in search of dense cone crops. According to Robert Askins’ book, Restoring North America’s Birds, populations that rely on uneven cone crops need a big enough range such that some area of it bears a strong crop in a given year. Many conifers, particularly shade-tolerant varieties, generate a very consistent supply of cones each year. True firs, spruces, and hemlocks are examples of consistent suppliers in the Northwest. Lodgepole and ponderosa pines, as well as western larch, are examples of conifers that produce crops at lengthier intervals. As a result, Type 2 birds, whose primary food source is ponderosa pine, may be far more migratory than Type 3 birds, who rely on western hemlock.
The loss of Red Crossbill populations in managed timberlands in Europe seems to be mostly due to the ageing of trees in a managed forest. Trees in these woods are often cut before they reach their peak cone-producing age.
Red Crossbills nest in dense cone crop regions wherever they present in a given year. Red Crossbills are able to reproduce throughout most of the year as a result of their ability to adjust to the sporadic occurrence of large harvests. As a result, the availability of concentrated crops affects their reproductive performance.
Another example is the Newfoundland crossbill. Following the introduction of red squirrels to the island in the 1960s, populations of this very sedentary subspecies have plummeted. The squirrels proliferated quickly, becoming the dominant eater of black spruce cones, which are the backbone of the Newfoundland crossbill’s diet. It’s ironic that the squirrels were brought to provide food for an endangered marten subspecies (Martes americana atrata).
Crossbill conservation obviously revolves on the amount and quality of conifer forests. However, determining possible difficulties and developing preventative interventions will need filling up the gaps in our knowledge of the different kinds’ life histories.
Identification
Although knowledge of dietary preferences and ranges may help differentiate distinct varieties of Red Crossbills in the field, vocalisations are the sole accurate trait short of catching and measuring the birds. The calls are tough to detect for the typical birder, so recording them may be the best approach. To identify the kind, compare the recordings to those on Groth’s website. Interested birders who learn to discriminate between various species may be valuable contributors to the development of conservation strategies.
Lisa Hardy is an avid birder and geologist. She has lived in northern Idaho for the last ten years and can be reached at [email protected].
