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Red Crossbill

Loxia curvirostra

Order:
Passeriformes
Family:
Fringillidae
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Pair of Red Crossbills (male: top; female: bottom).

Medium-sized finch with distinctive crossed mandibles. Adult males are variably deep brick red to reddish yellow with uniformly dark brown flight feathers, and a short, deeply notched dark brown tail. Adult females are uniformly olive to grayish with a greenish yellow breast and rump; typically with pale throat. No regular seasonal changes in plumage, although males may replace red feathers with yellow or greenish feathers or the reverse during annual molt.

© Blake Matheson, California, United States, 24 November 2017
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Adult male White-winged Crossbill with two female and a young male Red Crossbill.

Male Red Crossbills are typically plain-winged, unlike the strongly wing-barred White-winged Crossbill. Rare variant Red Crossbills can have narrow wingbars. There is also a Pine Siskin in the back left.

© Roger Beardmore, British Columbia, Canada, 15 August 2018
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Figure 2. Annual cycle of breeding, migration, and molt for Red Crossbill.

Data from literature on North American Red Crossbills. Thick lines show peak activity; thin lines, off-peak.

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Juvenile male Red Crossbill (25 August).

Juveniles are overall streaked, the body feathers with darker centers. Upperwing median, greater, and primary coverts and tertials brownish, tipped buff (narrowly on primary coverts) occasionally with broader tips on median and greater coverts forming indistinct to distinct wing-bars. Sexes are similar. Note Preformative Molt in action with yellow and red feathers molting in on breast, neck, and head; the red feathers indicate that this is a male.

© David Leatherman, Colorado, United States, 25 August 2017
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Female Red Crossbill undergoing Preformative Molt (29 December).

In North America, this molt includes most to all body feathers and upperwing median coverts.

© Ryan Schain, Massachusetts, United States, 29 December 2012
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Formative male Red Crossbill (4 June).

Plumage is variable depending on extent and timing of molt; some individuals can show yellow feathering resembling Definitive Basic female; some may show red feathering similar to Definitive Basic male but not as intensely red; others show intermediate or mixed feather coloration. Some portions of underparts may show streaking.

© Ric Else, Northern Ireland, United Kingdom, 4 June 2018
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Female Red Crossbill (4 November).

This female appears to be in formative plumage by the lack of yellow and molt limits in the tertials and upperwing coverts but, interestingly shows replaced outer primaries (p6-p9) contrasting with retained inner primaries, suggesting an "eccentric" preformative molt pattern, recorded previously in Europe but not previously, though to be expected, in North America (see Preformative Molt). An alternate explanation could be that it is in Definitive Basic Plumage but had suspended molt for breeding at p5, then resumed after breeding with p6. The Preformative and other molts in crossbiils are complex and in need of further study.

© Simon Boivin, Quebec, Canada, 4 November 2017
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Second Basic female Red Crossbill (2 April).

This individual appears in Definitive Basic Plumage, with uniformly broad and dusky brown wing coverts and primaries, except that it has retained the juvenile s4-s6 during an incomplete Second Prebasic Molt.

© Ryan P. O'Donnell, Utah, United States, 2 April 2009
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Male Red Crossbill completing Second Prebasic Molt (12 November).

This male is replacing p8, s5, and the outer rectrices during a prebasic molt. The worn retained juvenile outer primary (p9) and s6 (the last feathers typically replaced during prebasic molts), along with the yellowish rather than red body plumage, indicate this bird to be completing its Second Prebasic Molt.

© Lev Frid - Rockjumper Birding, Ontario, Canada, 12 November 2017
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Female Red Crossbill following suspended Second Prebasic Molt (4 September).

This dull female shows mostly juvenile upperwing feathers but has replaced the inner four primaries (p1-p4) before suspending the Second Prebasic Molt.

© Robert Lambeck, Colorado, United States, 4 September 2016
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Female Red Crossbill completing Definitive Prebasic Molt (23 September).

This female is replacing p8 and s4. The retained outer primary (p9) and s5-s6 are broad and of good quality, indicating basic feathers and that this bird to be completing its Definitive Prebasic Molt. The rather full yellow coloration also indicates an adult among females.

© Warren Lynn, Colorado, United States, 23 September 2007
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Definitive Basic male Red Crossbill (12 November).

Forehead, crown, and nape are brick red. Back feathers are similar, often with darker feather centers producing mottled effect. Scapulars similar but more extensively brown, with red restricted to feather edges. Feathers of upperwing blackish brown (primaries slightly darker); median and greater coverts narrowly edged reddish brown, remiges finely edged reddish brown or paler. The red plumage along with the uniformly broad and dusky wing and tail feathers indicate Definitive Basic Plumage.

© Cameron Eckert, Yukon Territory, Canada, 12 November 2016
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Definitive Basic male Red Crossbill (5 February).

Underparts are red, gradually becoming more grayish to whitish on the lower belly. Tail is deeply notched, feathers blackish brown to warm brown, edged red to reddish brown.

© Etienne Artigau, Quebec, Canada, 5 February 2018
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Definitive Basic male Red Crossbill (26 February).

Overall, red coloration may vary from brick red to orange red, depending on timing of Definitive Prebasic Molt, and in some birds body feathers can be mixed red and yellow. It is tempting to think that these 'patchy' individuals might represent first-year males, but in this species definitive plumage color can vary depending on the timing of molt relative to breeding. Typically, males that molt in fall attain reddish plumage, regardless of age. Those that molt in winter are yellowish.

© Nancy Barrett, Ontario, Canada, 26 February 2017
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Definitive Basic male Red Crossbill (10 June).

Note that the white tips to the greater and median coverts on this bird are thinner and less distinct than found in White-winged Crossbill. These are found on a minority or Red Crossbills.

© Tom Lawler, Oregon, United States, 10 June 2012
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Definitive Basic female Red Crossbill (13 February).

Female coloration in Definitive Basic Plumage varies considerably; in some individuals yellow or olive coloration largely replaced by dark gray, while others may appear much paler and more yellowish; variation likely linked to timing of molt, as in males. Underparts olive yellow to yellowish, becoming paler and whiter on throat and middle of belly; thighs and flanks tinged gray brown; undertail coverts grayish white with brownish gray centers.

© Kalin Ocaña, British Columbia, Canada, 13 February 2017
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Definitive Basic female Red Crossbill (6 March).

Side of head olive; lores, upper portion and rear portion of auriculars dusky brownish gray. Tail and wing feathers as on adult male, but feathers edged olive, not red. Lower back to rump olive to yellowish green, appearing brighter than remaining upperparts. Note the uniformly dusky wing coverts and remiges (without molt limits) indicating Definitive Basic Plumage.

© Michel Bourque, Quebec, Canada, 6 March 2016

Medium-sized finch. Total length 14–20 cm; mass 23.7–42.4 g (females), 23.8–45.4 g (males) (3). Distinctive mandibles are curved and crossed at the tip. Adult male deep brick-red to reddish yellow (some even greenish) with uniformly dark brown flight feathers, and short, deeply notched dark brown tail. Male coloration depends partly on molt timing. Female is uniformly olive to grayish with greenish or greenish yellow breast and rump; typically with pale grayish to whitish throat, which may be more or less mottled (26). Immature male may resemble adult male or adult female, or exhibit plumage intermediate between male and female. Immature of both sexes is generally distinguished from adults by buffy edgings on wing coverts. Underparts of juvenile are heavily streaked dark brown. No regular seasonal changes in plumage, although males may replace red feathers with yellow or greenish feathers or the reverse during annual molt.

Similar Species

Red Crossbill is readily distinguished from all species in North America, except the White-winged Crossbill (Loxia leucoptera) and Cassia Crossbill (L. sinesciuris), by its distinctive crossed mandibles; recently fledged juveniles with little or no mandible crossing might be confused with House Finch (Haemorhous mexicanus). White-winged Crossbill is distinguished from Red Crossbill in all plumages by presence of bold white wing-bars in the former. Adult male White-winged Crossbill is also more orange-pinkish in color, has a more rounded head than Red Crossbill, and black rather than dark brown flight feathers. Rarely, Red Crossbill (immatures usually) also shows white tips on median and greater wing coverts forming wing-bars, but white edge on inner greater coverts is never as broad as on White-winged Crossbill, and white areas of coverts merge diffusely (not abruptly) into dark portion of these feathers (27).

Distinguishing between the call types of Red Crossbill and Cassia Crossbill is more difficult, and is based largely on differences in contact calls; more study is needed (see Systematics, and especially the Audio Gallery).

Detailed Description

The Red Crossbill has 9 functional primaries (numbered distally, p1 to p9), 9 secondaries (numbered proximally, s1 to s9, and including 3 tertials, s7–s9 in passerines), and 12 rectrices (numbered distally, r1 to r6, on each side of the tail). Although geographic variation in size and vocalizations is complex (see Systematics: Geographic Variation), that of plumage is slight at best, as far as known, and the following descriptions pertain to all North American populations. Geographic variation in molt strategies may occur in response to variable environmental and migratory constraints, day-length regimes, and breeding seasonality of different populations (see Molts), but further study is needed to determine this.

Plumages

See Molts for molt and plumage terminology. The following is based primarily on detailed plumage descriptions of Dwight (28), Ridgway (29), Tordoff (30), Oberholser (31), Clement et al. (27), and Cramp and Perrins (32); see Svensson (33), Jenni and Winkler (34), and Pyle (35) for age-related criteria. Sexes are similar in Juvenile Plumage, but show variable appearance in subsequent plumages. Definitive appearance usually assumed at Second Basic Plumage, but occasionally at Third Basic Plumage.

Natal Down

Present primarily April–July in North America (but occasionally may occur year-round), in the nest. Hatchlings naked, except for some downy feathers on head and back (T. Hahn, personal communication). Down described by Oberholser [31] as "light drab to mouse gray."

Juvenile (First Basic) Plumage

Present primarily June–October in North American populations, although some populations may breed during at least 10 mo of the year (Figure 2), so juveniles may appear in all months except perhaps January and February. Upperparts gray brown, the feathers tinged pale green or brown and edged paler or buff, but crown feathers often tipped olive, and feathers of back and scapulars edged olive green; rump yellowish to yellow green, the feathers with dark centers creating streaks; uppertail coverts dark brown edged with olive. Tail and wing feathers similar to those of Definitive Basic Plumage female except upperwing median, greater, and primary coverts and tertials tipped buff (narrowly on primary coverts; 30) occasionally with broader tips on median and greater coverts forming indistinct to distinct wing-bars. Sides of head dark olive, the auriculars finely streaked buff. Feathers of lower throat finely edged yellow or olive-green; remainder of underparts dull buffy gray to pale yellow, becoming brown or buffy brown on flanks, and heavily streaked throughout with blackish brown, but less so on belly. Sexes similar (35). Juvenile body feathers (especially undertail coverts) can be more filamentous than in later plumages due to lower barb density.

Formative Plumage

"First Basic" or "Basic I" plumage of Humphrey and Parkes (36) and later authors; see revision by Howell et al. (37). Present primarily September–August in North American populations although also developed and present at other times of year following aseasonal breeding (see Preformative Molt).

Male. Plumage variable depending on extent and timing of molt; some individuals can show yellow feathering resembling Definitive Basic female; some may show red feathering similar to Definitive Basic male but not as intensely red; others show intermediate or mixed feather coloration (see Preformative Molt). Some portions of underparts may show streaking.

Female. Plumage similar to adult female but averages duller and browner (less bright yellow); some streaking on underparts sometimes retained or present in replaced feathers.

In both sexes Formative Plumage further distinguished from Definitive Basic Plumage by molt limits between worn juvenile and fresh formative feathers among upperwing and tail (33, 34, 35): some but not all inner upperwing greater coverts replaced (ca. 15% replace none and 30% replace all), contrasting with duller and browner retained outer coverts, often with buff tips when fresher; 1–3 tertials occasionally replaced, contrasting with older retained juvenile tertials and secondaries; primary coverts duller and browner with pale buff edging when fresh, contrasting with newer formative greater coverts; retained juvenile outer primaries and rectrices thinner, more pointed, browner, and relatively more worn, 1–2 central rectrices occasionally replaced and contrastingly fresh (34, 35). These limits could result from either suspended or incomplete Preformative Molts. Occasionally, birds may also show an eccentric Preformative Molt (reported in Europe, but not yet in North America; although see Molts and ML74137351), limits occurring between block of juvenile outer secondaries and inner primaries in center of wing (usually 5–8 remiges) and fresher, replaced formative inner secondaries and outer primaries; these birds may also replace 1–2 outer primary coverts and up to all rectrices (34).

Second Basic Plumage

Present primarily September–August in North American populations. Occasional individuals (up to 6% in North America and up to 50% reported in Europe) may retain up to 6 juvenile middle secondaries (distally from s6) and up to 4 juvenile inner or middle primary coverts during the Second Prebasic Molt. These retained feathers can be identified by being very brown, narrow, and worn, contrasting with replaced basic feathers (34, 35). Older birds may also show a similar pattern following previous suspension and resumption of Definitive Prebasic Molt, but older secondaries and primaries are not as narrow or worn, contrasting less with replaced remiges.

Definitive Basic Plumage

Present primarily September–August in North American populations. See Definitive Prebasic Molt for variation in timing and incidence of suspension.

Male. Most individuals show forehead, crown, and nape brick red; back same color or slightly paler, often with darker feather centers producing mottled effect; scapulars similar but more extensively brown, with red restricted to feather edges; lower back through rump pink to brick red, appearing brighter than back because feathers lack dark centers; uppertail coverts blackish brown, edged and tipped with red. Tail deeply notched, feathers blackish brown to warm brown, edged red to reddish brown. Sides of head brick red; lores, upper portion and rear edge of auriculars dusky gray brown. Feathers of upperwing blackish brown (primaries slightly darker); median and greater coverts narrowly edged reddish brown, remiges finely edged reddish brown or paler. Underparts red, gradually becoming more grayish to whitish on lower belly; undertail coverts grayish white, feathers with brownish gray centers. Red feathers of underparts often with pale whitish shafts producing somewhat striated appearance. Overall, red coloration may vary from brick red to orange red, depending on timing of Definitive Prebasic Molt (see below), and in some birds body feathers can be mixed red and yellow. Previous assumptions that yellow or mixed red-and-yellow males were only found in first year generally in error (3, 38). Rarely, males may have whitish edges on median coverts and/or greater coverts and tertials producing wing-bars resembling those of White-winged Crossbill. However, white edges to inner greater coverts are never as broad as in latter species, and white area merges diffusely (not abruptly) into dark portion of feather (27).

Female. Plumage pattern generally as in male, except red feathers replaced by yellow to olive to gray. Forehead, crown, nape, and back olive, variably tinged green or yellow; feathers with darker centers producing mottled effect, the crown and back generally brighter yellowish or olive; scapulars dark olive gray with paler edges; lower back to rump olive to yellowish green, appearing brighter than remaining upperparts; uppertail coverts dark olive gray with paler edges. Tail and wing feathers as on adult male, but feathers edged olive, not red. Side of head olive; lores, upper portion and rear portion of auriculars dusky brownish gray. Underparts olive yellow to yellowish, becoming paler and whiter on throat and middle of belly; thighs and flanks tinged gray brown; undertail coverts grayish white with brownish gray centers. Female coloration varies considerably; in some individuals yellow or olive coloration largely replaced by dark gray, while others may appear much paler and more yellowish; variation likely linked to timing of molt, as in males.

In both sexes, Definitive Basic Plumage separated from Formative Plumage by having wing and tail feathers uniform in quality and freshness: tertials and inner secondaries uniform in wear; primary coverts duskier, not showing buff edging, not contrasting in feather quality with greater coverts; basic outer primaries and rectrices broader, more truncate, duskier, and relatively fresher (34, 35). Occasional individuals may show 2 generations of basic flight feathers following incomplete or suspended molts; some of these may be in at least their Third Basic Plumage but reliable age-determination of these unlikely due to the complex molts of this species (34, 35).

Molts

Molt and plumage terminology follows Humphrey and Parkes (36), as modified by Howell et al. (37, 39). Red Crossbill exhibits a Complex Basic Strategy (cf. 37, 38), including complete prebasic molts and a partial-to-incomplete preformative molt but no prealternate molts (40, 28, 30, 31, 33, 32, 34, 35, 41; Figure 2). Because this species can breed throughout most of the year, its molts and plumages vary more than those of other North American passerines.

Prejuvenile (First Prebasic) Molt

Complete, primarily May–July in North America (although occasionally may occur year-round), in the nest. First feathers erupt at 7 d. Otherwise, little or no information on timing or sequence of pennaceous feather irruption and development. Duration of Prejuvenile Molt among individuals probably ca. 10–15 d to be completed or near-completed by fledging at 15–25 d.

Preformative Molt

"First Prebasic" or "Prebasic I" Molt of Humphrey and Parkes (36) and some later authors; see revision by Howell et al. (37). Partial to incomplete, primarily April–December in North America (Figure 2; see below). In North America, this molt includes most to all body feathers and upperwing median coverts, no (in ca. 15% of individuals) to all 10 (in ca. 30% of individuals) inner greater coverts, and occasionally (in ca. 22% of individuals) 1–3 tertials, and (in ca. 13% of individuals) 1–2 central rectrices, but usually no other secondaries or rectrices and no primaries or primary coverts (35, 41).

Preformative Molt has been more thoroughly studied in the European Red Crossbill (33, 32, 34). In Europe, this molt can take place during 2 different times of year, separated by molt suspension: juveniles hatched during summer undergo Preformative Molt in August–October (at the same time adults undergo Definitive Prebasic Molt) while those hatched earlier (from late winter and early spring) can experience 2 phases of this molt, the first phase beginning 100–110 d after hatching and the second phase occurring during the main molt period in August–October. Birds that undergo both phases of this molt may during the first phase replace some to (exceptionally) all body feathers and sometimes a few upperwing lesser and median coverts, and 1–3 inner greater coverts. Feathers acquired during this phase are usually if not always yellowish rather than red in males. The second phase of this molt in August–October appears to be seasonally fixed and progresses faster and with higher intensity than first phase. During this second phase, males usually acquire red rather than yellowish feathers; thus males that have undergone earlier phase of this molt replace remaining Juvenile body feathers with red feathers and can show mixed red and yellow plumage. Birds that have undergone the first phase of this molt sometimes can have a more complete molt during the second phase, replacing up to 5–6 inner secondaries and 5–7 outer primaries in eccentric sequence, as well as most to all rectrices. This replacement of inner secondaries, outer primaries, and rectrices has not been noted in North America (T. Hahn, personal communication). Summer-hatched birds molt only during the second phase and replace only body feathers, including variable numbers of wing coverts (but no remiges or rectrices), as indicated above. Although a few aseasonally hatched juveniles have been reported to undergo a complete Preformative Molt (42), most retain their juvenile remiges and rectrices for 1.5 yr until the Second Prebasic molt. Preliminary data from North American birds (T. Hahn, personal communication, and see ML74137351) suggests that some of these molt patterns in juveniles may be similar to those of European birds, but confirmation is needed (35).

Second Prebasic Molt

Incomplete to complete, primarily July–November in North America (Figure 2). This molt is probably complete in a large majority of birds but up to 6% of individuals in North America can retain up to 6 secondaries (distal from s6) and up to 4 inner or middle primary coverts (35, 41). Up to 50% of birds in Europe reported to have incomplete molts (34).

Definitive Prebasic Molt

Incomplete to complete, primarily July–November in North America (Figure 2). Primaries (and corresponding primary coverts) are replaced distally (p1 to p9), secondaries are replaced proximally from s1 and proximally and distally from the central tertial (s8), as typical of passerines, and rectrices are probably replaced distally (r1 to r6) on each side of tail, with some variation in sequence possible. In European birds (33, 32, 34), the Definitive Prebasic Molt occurs mainly during August–October, but as with the Preformative Molt, it may occur in 2 phases, the first sometime before July (winter–spring) and the second phase occurring August–October. About 50% of birds may begin primary molt during winter–spring but suspend it sometime before July, with primary molt completed during autumn (August–October) in these individuals. Remaining birds undergo primary molt entirely during autumn without molt suspension. Of birds that suspend primary molt, 72% replace only 1–3 feathers before molt is suspended, while remaining birds replace more primaries (34). Primary coverts are renewed along with corresponding primaries. First phase of 2-phase molt may include one to all upperwing greater coverts, tertials, and/or central rectrices, and part of body feathers, along with some inner primaries. Remaining feathers are replaced during second phase, after July. Rectrix molt during second phase begins with shedding of p2–p6 and ends before all primaries are grown. Molt of secondaries may be suspended and sometimes arrested (with 1–6 feathers distal to s6) retained during incomplete molt.

Preliminary data from North American birds (T. Hahn, personal communication) suggests that Definitive Prebasic Molt patterns are similar to those of European birds. Adult males that complete breeding and molt before July could be predominantly yellow, with yellow feathers observed replacing red plumage (T. Hahn, personal communication). Males acquire red feathers if molt occurs after July. Definitive Basic feathers replaced March–May by males in Kansas were grayish or yellowish green, perhaps because of dietary deficiency or hormonal condition (30, 43). See Cornelius et al. (44) for information documenting that hormonal stress suppression is down regulated during molt, suggesting a resource-based trade-off between the 2 processes.

Bare Parts

Bill and Gape

Bill distinctive, with mandibles curved and crossing at tip. Lower mandible crosses to right as often as to left (45); see Benkman (46) for experiment testing hypothesis of frequency-dependent selection favoring 1:1 ratio). Color black, usually with gray near tomia. No seasonal changes in color of bill or bare parts.

Iris

Iris dark brown to nearly black.

Legs and Feet

Legs, toes, and claws black.

Linear Measurements

Bill Size and Shape

See Appendix 1. Red Crossbill shows the most variation in bill size and shape of all described crossbills (1, 47, 3, 32), as a result of multiple call types adapting to forage on cones of different conifers (4, 8, 5, 48, 6; see Diet and Foraging). Within call types, bill length appears to vary more than width or depth (3), probably owing to variable wear associated with variation in the accessibility of conifer seeds (3). Bill depth is highly heritable (narrow-sense heritability for Scottish Crossbills [L. scotica]: 0.58–0.71) (49). Males average 1–6% larger in bill measurements than females (3). From largest to smallest, bill depth follows as: Type 6 > Type 8 > Type 11 > Type 9 (Cassia Crossbill) > Type 2 > Type 5 > Type 7 > Type 4 = Type 1 > Type 10 > Type 3.

Wing Length

Wing length ranges from 81.1 mm in Type 10 females to 98.4 mm in Type 6 males, a 17.6% difference from smallest to largest birds (3, 6). The relatively sedentary island endemic Type 8 has a shorter wing length than the smaller-billed and highly migratory Type 2 (MAY et al., unpublished data); however, Type 10 has a shorter wing length than Type 3 despite being larger otherwise.

Tail Length

Mean tail length ranges from 51.6 mm in Type 3 females to 57.9 mm in Type 6 males (3).

Tarsus Length

Mean tarsus length ranges from 18.5 mm in Type 3 females to 21.4 mm in Type 6 males (3).

Mass

See Appendix 1. Males range from 3–5% larger in mass than females in all forms (see Groth [3] for all measurements by call type). Nomadic crossbills have elevated levels of body fat during late spring and early summer, consistent with a need to fuel long movements at this time (47, 50, 51), and during winter, consistent with a need for fuel to survive the long and cold nights (51).

Recommended Citation

Benkman, C. W. and M. A. Young (2019). Red Crossbill (Loxia curvirostra), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.redcro.02