The Red Crossbill represents a rich complex, long recognized as multiple subspecies (1) and now largely treated as “call types” or ecotypes, with each type characterized by distinct contact calls and other vocalizations, and each with a range of morphological variation similar to other songbirds (2, 3). Crossbills are highly specialized for feeding on seeds in conifer cones, and each call type appears to be associated with, if not adapted to, particular conifer species (4, 5, 3, 6, 7, 8) in a core zone of occurrence (9); however, the strength of such evidence varies among call types much like the extent to which they are genetically divergent from each other (10). Indeed, one call type was recently given species status (Type 9, now Cassia Crossbill [L. sinesciuris]) because of its genetic distinctiveness (10) and strong and consistent reproductive isolation (11, 12). Now 10 call types remain in the North American complex, including Type 11 (L. c. mesamericana) which is resident in highlands from southern Mexico to northern Nicaragua (13).
Call types differ in bill depth and palate groove structure, and the average bill depths and groove widths of each of 5 call types (types 2–5, 10) approximate the predicted optima for extracting and husking seeds, respectively, of their hypothesized key conifers (4, 8, 6). Although many of the call types appear morphologically adapted for feeding on those conifers that hold their seeds fairly reliably through winter into spring (i.e., their key conifers; 4), almost all call types feed on seeds of multiple conifer species (3), likely switching with the seasonal shifts in availability to those conifers that provide the highest energy intake rates (14). Indeed, a given call type will at times forage more on seeds in easily accessible alternative conifers than on their key conifer (CWB, MAY; T. Hahn, C. Porter, J. Cornelius, personal communication). For example, Type 5 probably feeds more on large seed crops of Engelmann spruce (Picea engelmannii) than on its apparent key conifer Rocky Mountain lodgepole pine (Pinus contorta latifolia). However, the bill structure of Type 5 matches the optimum for foraging on lodgepole pine and deviates greatly from the optimum for foraging on Engelmann spruce (7), presumably because of the premium on efficient foraging when seed is scarce and only lodgepole pine seed is available.
The Red Crossbill generally breeds south of the northern spruce–fir–larch (Picea–Abies–Larix) forests occupied by the White-winged Crossbill (L. leucoptera), occurring commonly from the moist coastal forests in Alaska (mostly Type 3) and coastal ranges in the Pacific Northwest (types 3, 4, and 10), inland in the Cascade–Sierra Nevada and Rocky Mountains (types 2, 4, 5, and less commonly Type 3) with extensions through the Sierra Madre Occidental of Mexico (Type 6) south to Nicaragua (Type 11), east to the Great Lakes (types 2, 4, and 10; some years many Type 3), the Northeast (types 1, 2, and 10; some years many Type 3), and Newfoundland (Type 8), and south in the Appalachians to Alabama (types 1 and 2). Type 8 is largely restricted to Newfoundland (15). Very little is known about Type 7, which was first described from the Cascades and northern Rocky Mountain region, but recent evidence suggests that "eastern Type 10" might be Type 7, and is the most common call type in the Northeast (13).
The nomadic movements characteristic of most of these call types are driven by the variable nature of conifer cone production over most of North America. When crossbill populations are large and cone crops are small, crossbills are especially prone to erupt from their usual ranges or core zones of occurrence (16). At such times, it is not unusual to find Red Crossbills of various types far from their regular haunts, sometimes feeding on atypical food sources.
Most nesting in North America occurs during 2 peaks: July–September when seed crops are developing and maturing, and January–April where ample seed remains in the cones from the previous year’s seed crop. The prevalence of late summer nesting in North America is probably due to the fact that cones of most North American conifers used by crossbills open and seeds become most accessible in late summer and autumn, unlike in Eurasia where many conifer species’ cones open in spring. Breeding occurs when feeding rates are sufficient to meet the demands of nesting (17), although nesting regularly ceases in some and perhaps all populations in autumn, apparently even when intake rates might be sufficient for continued breeding (18, 19). Presumably autumn nesting is disfavored by both a need to complete molt in a timely manner and by the fact that young fledged later in fall may have reduced chances of survival (17); see Santisteban et al. (20) for Cassia Crossbill.
Only further study will reveal if any of the remaining call types deserve species designation. Contact calls appear to play a major role in maintaining some level of reproductive isolation between call types, by influencing flock composition (assortative by contact calls), and thereby leading to assortative mating because mates are chosen within flocks (21, 22), and because receptive females are more likely to associate with males giving calls of their own call type (23, 24). It should not be surprising, however, if only one or a few call types reach the level of species. Different call types vary in the extent of geographic isolation. Moreover, ecological and morphological divergence does not necessarily lead to strong reproductive isolation (25), and the extent of assortative flocking and pairing might vary depending on environmental conditions (C. Porter and CWB, unpublished data; MAY and T. Spahr, unpublished data).