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Red Crossbill

Loxia curvirostra

Order:
Passeriformes
Family:
Fringillidae
Sections

Breeding

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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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Red Crossbill on nest.

Nests are well concealed in dense cover on side branches next to or away from trunk, 2–20 m high.

© Tom Zavitz, Montana, United States, 14 February 2017
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Red Crossbill nest.

Small conifer twigs form the base and outside of the nest; grasses, Usnea lichen, black tree lichen, conifer needles, fine shreds of bark, hair, fireweed seedpod fibers, and feathers make up inner cup and lining.

© Ryan Brady, Wisconsin, United States, 25 March 2018
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Red Crossbill nest.
© Amy Lawes, Quebec, Canada, 7 April 2018
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Female Red Crossbill feeding young.

When able to fly well, fledglings follow parents closely.

© Lev Frid - Rockjumper Birding, Ontario, Canada, 1 May 2018

Phenology

The Red Crossbill is noted for opportunistic breeding in response to the variable nature of conifer seed crops (47, 17), with many crossbill populations moving considerable distances even between successive breeding attempts (47). Nevertheless, seasonal patterns of both conifer seed availability and the necessary feeding intake rates for reproduction set limits on when breeding occurs (17), and physiological mechanisms appear to cause refractoriness to reproduction in autumn (18, 19). This results in predictable if not regular seasonal patterns of breeding for most populations of crossbills (Figure 2); although because of the irregular production of cone crops, a crossbill might breed 2 or more times in a single year, yet not at all in another year (47).

In both North America and Eurasia, nesting is common in late winter to spring, even into early summer (47, 32). In Europe, this coincides with the opening of most conifer cones, and thus, the period of greatest seed accessibility to crossbills (14); in the mountain pine forests of the Pyrenees, France, nesting often commences in November and especially December when cones begin to open (194). In contrast, crossbills in much of North America from late winter to spring rely on seeds held in cones that opened the previous autumn. Although many seeds will have been shed (thus becoming mostly unavailable to crossbills) or eaten, large numbers of seeds, which are critical for meeting the demands of breeding, can remain in cones in late winter. This is most likely to follow a large cone crop—generally, the larger the seed crop, the higher the pollination rates and the more filled seeds per cone (195)—and weather that has been consistently cool and humid keeping the cone scales from spreading far apart and releasing the seeds.

Many North American populations also nest in mid to late summer with the development of a new seed crop, and the opening of cones, which is particularly beneficial to the young that are less proficient foragers than adults (17); see Santisteban et al. (20) for Cassia Crossbill. Summer nesting (initiation of nesting usually in July) occurs in the smallest Type 3 feeding on western hemlock (19), Type 4 feeding on douglas-fir (196), and the much larger Type 2 feeding on ponderosa pine (CWB). Multiple call types (types 2, 3, 4, and 5) also breed while feeding on the thin-scaled cones of Engelmann spruce and Rocky Mountain douglas-fir (155; C. Porter, personal communication), and multiple call types will breed while feeding on tamarack, white spruce, and red spruce during late summer (17, 141; T. Spahr, personal communication). Because cones of conifers in Eurasia (and in Mexico) do not open in early autumn (except those of Larix; 47, 157), and feeding rates are lower on closed than on opening cones (14), crossbills in northern Europe rarely if ever breed in late summer when feeding on seeds in the relatively hard cones of pine (47, 32). An exception occurs in the Mediterranean Basin, where late summer nesting has been found in multiple locations where crossbills rely on seeds in hard cones of pine (194, 139).

Nesting generally ceases during autumn; e.g., egg dates in North America range from mid-December to early September (186, 197, 171, 18) with fledged young being fed until late November (17, 19). In contrast to spring and early summer, breeding in late autumn, even if feeding intake rates were sufficient for fledging young, is likely disadvantageous because decreasing temperatures and day-length associated with the onset of winter should be particularly challenging to recently independent offspring (17); indeed, nests begun in November–December in the Pyrenees tend to fail (194). In addition, autumn breeding could interfere with molt (17, 19). Although each call type likely differs in the regularity with which they breed, given the seasonal nature in the accessibility of seeds and breeding demands, it makes adaptive sense that day-length influences gonadal cycles, as found for Type 3 crossbills (18, 19). Such gonadal cycles both facilitate the rapid initiation of breeding upon locating suitable conditions, and prevent breeding when conditions, especially for the young, are likely to deteriorate. Careful documentation of nesting would help to further characterize the limits to autumn nesting.

Nest Site

Selection Process

Both sexes play a role, male leading or following female, both perching or squatting in prospective sites, male often singing (171); see Ross (182) and Nethersole-Thompson (149) for Scottish Crossbill and Red Crossbill in Scotland.

Microhabitat

Few data. Nests are generally spaced 50–100 m apart (171, 32); see Nethersole-Thompson (149) for Scottish Crossbill. Nest dispersion and limits of foraging by pairs is probably related to abundance and patchiness of food supply (47, 32).

Site Characteristics

Red Crossbill prefers more open woodlands for nesting perhaps to minimize predation by arboreal mammals (198). Nests are well concealed in dense cover on side branches next to or away from trunk, 2–20 m high (197, 199, 171, 149), in pine, spruce, douglas-fir, or hemlock. Nest location relative to distance from trunk may vary between late winter and summer, with nests closer to the trunk in winter but this requires quantification (T. Hahn, personal communication). Found to nest on northeast (198), south or southeast (171), and south or southwest sides of trees (194). In the study by Summers et al. (198) in Scotland, the northeast side was on the leeward side of the tree.

Nest

Construction Process

The nest is built by the female; male sometimes carries nesting material to nest (171, 132).

Structure and Composition Matter

Small conifer twigs form the base and outside of the nest; grasses, Usnea lichen, black tree lichen, conifer needles, fine shreds of bark, hair, fireweed (Chamerion) seedpod fibers, and feathers make up inner cup and lining (197, 200, 132). Nests built during winter are bulkier than in summer (47).

Dimensions

Outside: 105–127 mm across, 52 mm deep; inner cup: 60 mm wide, 27 mm deep (199).

Microclimate

No information, other than usually well-shaded by trunk or branches.

Maintenance or Reuse of Nests, Alternate Nests

No reports of nest reuse or alternate nests; reuse is doubtful given that suitable nest sites are probably plentiful in most forests and because nest rim is caked by dried feces in last days of nestling life (171, 47).

One nest site was reused in subsequent years (194).

Nonbreeding Nests

No information.

Eggs

Shape

Ovate.

Size

Egg size varies with body size. Average length × breadth varies from 20.4 × 14.8 mm (37 eggs, presumably from L. c. minor in New England) to 21.2 × 15.4 mm (19 eggs from L. c. percna in Newfoundland) to 21.95 × 16.26 mm (14 clutches from L. c. benti in Colorado) (132).

Mass

Estimated mass based on measurements: L. c. curvirostra, 2.95 g; L. c. balearica, 2.75 g; and L. c. poliogyna, 2.76 g (32).

Color

Ground color varies from white to pale green to rose, with cinnamon, reddish brown, or purplish splotches and streaks concentrated on large end (171, 132).

Surface Texture

Smooth.

Eggshell Thickness

No information.

Clutch Size

Usually 3 eggs per clutch (171, 132), uncommonly 2 or 4 eggs, and exceptionally 5 or 6 eggs (47, 149, 32, 194). Seasonal variation in clutch size was not detected by Clouet (194), but mean clutch size tends to increase both with latitude from 3.3 eggs in Algeria, 3.4 eggs in French Pyrenees, and 3.7 to 3.8 eggs in England and the Netherlands (data summarized in Clouet [194]), and with increases in cone crop size (201). No information on number of clutches produced by female in the wild.

Egg-Laying

Usually one egg laid per day until clutch is complete (171, 47, 149, 32, 142).

Incubation

Onset of Broodiness and Incubation in Relation to Laying

Incubation may begin with first egg during cold weather (47), but otherwise with last egg (171, 47). Scottish Crossbill sits on eggs of incomplete clutch at night (149).

Incubation Patch

One large incubation patch, develops only in female. Patch is highly edematous during incubation and when nestlings are small, then regresses to bare and wrinkly as nestlings grow. Bare, dry incubation patches persist after winter/spring nesting until summer when females arrive in new locations to breed. Patches become feathered at the end of summer well into the annual prebasic molt (T. Hahn, personal communication).

Incubation Period

Length of incubation 12–16 d, most often 14 d (197, 171, 132, 149).

Parental Behavior

Only female incubates. Female will incubate nearly continuously during cold weather, which is facilitated by regular feedings by male.

Hardiness of Eggs Against Temperature Stress; Effect of Egg Neglect

No information from North America; numerous accounts of eggs broken by freezing in Europe. In Scottish Crossbill, few eggs are broken by freezing, because they are “remarkably resistant to cold” (149).

Hatching

All eggs hatched within 10 h in a study in Colorado (171). In Scottish Crossbill, the full clutch hatched within 24 h at 4 nests and within 48 h at 8 nests; in one extreme case, hatching occurred over 3.5 d (Scotland; 149). Eggs hatch asynchronously during colder seasons (132, 47).

Young Birds

Condition at Hatching

Eyes closed. Hatchlings naked, except for some downy feathers on head and back (142; T. Hahn, personal communication). Down described by Oberholser (31) as "light drab to mouse gray."

Growth and Development

Able to hold up head at 3 d, eyes open at 5 d, first feathers at 7 d (197, 132, 47). Few growth data. Bills uncrossed at fledging; crossing may be complete by age 30 d (47, 142). Wing and humerus reach full length in a few weeks; full bill length is achieved later (3). Also, skull pneumatization and plumage development only roughly correspond; former probably complete in about 6 mo, while molt is seasonal (see Appearance).

Parental Care

Brooding

Only the female broods nestlings; brooding is nearly continuous for about 5 d after hatching (171). Nestlings (1–14 d old) survive repeated torpidity during female’s absence from the nest (149, 32).

Feeding

Parents regurgitate kernels and fluids from the crop that provide both food and moisture. This slurry has been noted to be “dark and viscid” early in nestling life, suggesting incorporation of insect matter, even in winter (171). Whole kernels found in crops of 5-d-old young in eastern Cascades of Washington (T. Hahn, personal communication); given that crossbills generally swallow seed kernels whole, and then regurgitate them from the crop to the young, whole kernels likely comprise most if not all the food of nestlings and fledglings. Observations of parrots regurgitating soil to fledglings (202) in combination with observations of regular soil feeding by nesting Cassia Crossbills including by adults with dependent fledglings (CWB), indicate that observers need to be cautious before attributing dark material to insects rather than soil. Scottish Crossbill has been seen providing moisture by eating snow and regurgitating into mouths of young (149). In Europe, young fed at intervals of 30–90 min, shorter intervals with very young nestlings (149).

Nest Sanitation

Female consumes all fecal sacs for about 1 wk in Red Crossbill (47) and other crossbill species (149); after 1 wk males also eat feces. In last days of nestling life, nest rim is caked by dried feces (171, 47).

Parental Carrying of Young

Not known for this species.

Cooperative Breeding

Not known for this species.

Brood Parasitism

No cases of intraspecific or interspecific brood parasitism have been reported.

Fledgling Stage

Fledge at age 15–25 d, with extreme of 35 d (47, 149); variation is probably due to fluctuations in food supply and ambient temperature. Fledge at about 75% of adult weight (Ternovskij [203] in Newton [47]).

Departure from the Nest

Parent Scottish Crossbill sometimes restricts food to induce fledging; also much loud calling (tooping and imitations of fledgling Chitoo Calls) may help induce fledging (149). No information on fledgling process for Red Crossbill, but in Scottish Crossbill, fledging may be gradual (climbing about on branches near nest) or sudden (flying from nest) (149). Young Red Crossbill and Scottish Crossbill return to nest to roost for some days after fledging (171, 149).

Association with Parents or Other Young

When able to fly well, fledglings follow parents closely. Male feeds young alone if female lays second clutch (171; see also 65). For Scottish Crossbill, see 149).

Ability to Get Around, Feed, and Care for Self

For several days after fledging, young wait in trees to be fed. Fairly mobile one wk after leaving nest. Foraging efficiency gradually increases as bill-crossing proceeds, with parents feeding young as long as 33 d after fledging (171). Young reported to extract and shell seeds at age 45 d (Ternovskij [203] in Newton [47]).

Immature Stage

Immatures may tend to associate with other immatures (siblings?) after becoming independent of their parents, but such assortment does not appear to last as immatures commonly occur with adults, and likely show similar if not greater propensities for movements as adults (see Newton [47]). Birds in immature plumage have been found in breeding condition (186, 187, CWB; T. Hahn, personal communication).

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