Main Foods Taken
Food Capture and Consumption; Foraging Behavior
Red Crossbill usually forages for seeds in cones still attached to branches, although in spring and summer especially may forage on fallen cones (149, CWB). Closed cones of spruce, hemlock, and douglas-fir are usually removed from the branches, whereas closed cones of most pines and open cones of all conifers are generally left attached to the branch (132, CWB); notable exceptions are the closed cones of lodgepole pine in the Sierra Nevada (150; CWB) and of most pines in Europe, which are usually removed from the branches before foraging (CWB).
To extract seeds, crossbills generally hold the cone with the foot that is on the side opposite to which the lower mandible crosses (151), and use their mandibles to separate the cone scales, thereby exposing the seeds at the base (152, 47, 151, 131). Knox (151) found that the leg that normally holds the cone is longer, whereas Groth (58) did not find such a pattern. It is worth noting that Knox (151) measured Red Crossbills in northern Europe, which consistently remove closed cones (this represents most of the year in Europe) to forage on them and need to use their feet to secure them (47); in contrast, Groth (58) measured Red Crossbills in North America, which generally remove only the smallest cones (e.g., spruce), and then only when they are closed (which represents only a few months). See video for examples of similar foraging adaptions in the White-winged Crossbill.
On closed cones, crossbills bite between the overlapping cone scales to form a gap (see Benkman and Lindholm  for experimental confirmation of the importance of pointed crossed mandibles for accessing seeds in closed cones). Crossbills have crossed mandibles presumably because slender tips are needed to slide efficiently between closed scales and strongly decurved upper and recurved lower mandibles can withstand (and exert) strong forces at the tip of the mandibles (compression forces). If the mandibles were straight and pointed, the shearing forces could cause the slender mandibles to break near the tip. Once a gap is formed, crossbills abduct the lower mandible (to the side to which it is crossed) to spread the scales apart. Crossbills usually orient themselves so that their lower mandible points toward the scale that is on the distal side of the gap (distal relative to the base of the cone). The upper mandible is pushed against the basal scale, widening the gap. The head is often twisted to the side as resistant scales are pried apart (47). If this does not expose a seed, the mandibles are reached farther into the gap, sometimes in a biting motion, and then laterally abducted. This process is repeated until a seed can be lifted free by the tongue; often seeds in green closed cones need to be dislodged with a hooking motion of the upper mandible (14).
The woody seed coat is removed before the kernel is swallowed. The seed coat is cracked and removed (i.e., husked) by the lower mandible while the tongue helps secure the seed against a lateral groove in the horny palate (47); the seed coats of hemlock are rubbery, and presumably torn to remove the seed coat (CWB). Empty seeds are dropped almost instantaneously; probably less than 1 of every 1,000 full seeds is dropped accidentally (CWB). Mechanical considerations and observations of foraging crossbills reveal that the seed is secured in the lateral groove on the side opposite to which the lower mandible crosses (4). Foraging experiments used to test whether 4 different call types have average groove widths optimal for husking seeds of the conifers on which they were hypothesized to be specialized (based on whether they tend to produce fairly regular seed crops and hold seeds in cones more consistently through winter and into spring [most conifers in North America shed nearly all their seeds by winter; 154]: ponderosa pine for Type 2; western hemlock for Type 3; douglas-fir for Type 4; and Rocky Mountain lodgepole pine for Type 5) found strong support for morphological adaptation to their key conifers (4). Using the linear relationship between optimal groove width and the cube-root of seed mass, Benkman (4) predicted the optimal groove width for foraging on Sitka spruce, the one other hypothesized key conifer in the Pacific Northwest. Although he and one of his graduate students failed to find such a call type common in the Sitka spruce forests in Alaska in the 1990s, years later Irwin (6) discovered such a call type, Type 10, in northern California. Moreover, Irwin measured its husking groove widths and found a close match to that predicted by Benkman.
Bill structure is also critical for efficient foraging on cones with bill depth closely related to foraging performance on a given conifer (estimated ability to meet energy demands; the strong positive allometry between body mass and bill depth means that larger billed crossbills have greater energy requirements) (4). Consequently, bill depth increases with increases in the scale thickness and hardness of the conifers each call type tends to specialize on (see Habitat: Habitat Associations of Call Types). The close match between the average bill depths and husking groove widths and the predicted optima are not consistent with bill structure representing a compromise between foraging on multiple species of conifers (at least for these 5 call types; 4, 8, 6). This conclusion is further supported by experiments that revealed that Type 5 has an average bill depth that matches the optimum for extracting seeds from Rocky Mountain lodgepole pine cones but is much larger than the optimum for foraging on Engelmann spruce; Type 5 commonly utilizes Engelmann spruce including when nesting but it lacks key conifer characteristics (7). With that said, bill depth within call type can vary regionally, suggesting that variation in bill size may be linked to local rich patch resources in different regions (155); this is an area of potential future research.
Rapid assessment of which trees to forage on is also critical to feeding performance. Crossbills preferentially forage on the conifer species providing the highest feeding intake rates, and switch from one conifer species to the next as the relative profitabilities of the conifers shift (14). Crossbills are also selective of which trees they forage on within a species, and even vary in their use of local forest patches based on subtle differences in cone structure that impede crossbills from accessing seeds (156). Trees differ in the thickness of cone scales, seed kernel masses, number of seeds per cone, and cone maturity, all of which affect the accessibility of seeds and kernel intake rates (131, 157). Because crossbills consistently avoid foraging on trees with relatively large cone masses and thicker cone scales, trees have evolved such traits as defenses, especially where seed predation and selection by crossbills is greater (157, 158), and some crossbills are engaged in coevolutionary arms races with conifers especially where tree squirrels (Tamiasciurus and Sciurus), strong competitors for the seeds, are absent (e.g., Cassia Crossbill, Type 8, and L. c. balearica; 57, 8, 48, 159, 157).
In addition to using their own personal information when foraging, Red Crossbills discern successful foragers and rapidly join them (“local enhancement;” 160). Moreover, flocks can assess low quality trees (i.e., low relative seed intake rates) more rapidly than solitary individuals by apparently watching the feeding rates of flock members and by listening to vocalizations (21). Crossbills usually remain silent while foraging in flocks, but produce contact calls before flying away (132, 47). It is conceivable and consistent with casual observations (CWB) that individuals call when their foraging rates are low, but remain silent when rates are high. If only a few birds call, implying a generally good tree, they then resume foraging, but if many call, often increasing to a crescendo, implying a generally poor tree, then the flock flies off. This would provide a mechanism for crossbills to rapidly assess tree quality when the foraging behavior of flock mates cannot be seen, and maintain flock cohesion. Because such assessment of resources only works well when flock members have similar feeding abilities (i.e., similar bill structures), this should favor assortative flocking by bill size, and, because contact calls represent a “marker trait” that identifies bill structure, assortative flocking by call type. As expected, the propensity for individuals of different call types to land in response to playbacks of different call types decreases with increasing dissimilarity in bill depth (22). The benefits of assortative flocking could have been the main reason for the rapid evolution of distinct contact calls associated with different bill structures (i.e., call types). Moreover, reproductive isolation between call types could largely be the byproduct of assortative flocking where mates are chosen in flocks (22).
Crossbills often leave seeds in cones (32; CWB), which has been called “wasteful” (161). However, using the marginal value theorem as a guide for cone/patch leaving, Benkman (162) showed that White-winged Crossbill maximizes its feeding intake rate by leaving some seeds in the cones of tamarack (Larix laricina). This is likely to apply to Red Crossbill, as solitary Type 2 Red Crossbills in captivity were found to depart food patches at close to an optimal time (21).