Red crossbill (Percna) COSEWIC assessment and status report: chapter 8
Biology
Red Crossbills do not follow the breeding phenology typical of many North American passerines. Instead, across the entire L. curvirostra range, they can be found breeding during any month of the year (Tordoff and Dawson 1965; Newton 1972; Godfrey 1986). Like some other birds that breed opportunistically, timing of nesting in Red Crossbills cannot be solely explained by photoperiodic cues (Tordoff and Dawson 1965). Instead, breeding is at least partially influenced by abundant conifer food supply (Griscom 1937; Newton 1972), resulting in Red Crossbills breeding during months that could not sustain nesting attempts by avian species not dependent on conifers. The period during which Red Crossbills breed depends on cone supply in the habitat: breeding can occur over a range of many months in mixed conifer forests that have different species of seeds ripening at different times (Newton 1972). This breeding strategy allows Red Crossbills to adapt to predictable (seasonality) and unpredictable (food availability) environmental contingencies (Hahn 1995).
Evidence of temporal flexibility in reproduction of the percna subspecies is sparse, as few adequate nest records exist. McCabe and McCabe (1933) present information on one percna pair breeding in April. Reeks (1869) considered Red Crossbills to be common early nesters in western Newfoundland. Rooke (1935) considered most crossbills seen during an August/September excursion to Newfoundland to be nesting. Peters and Burleigh (1951) describe Red Crossbills in Newfoundland as “often very early nesters, sometimes nesting in January or February and at other times not until mid-summer.” Information on observations of Red Crossbill breeding chronology in Newfoundland is compiled in Table 2.
Red Crossbills are presumably monogamous and frequently occur in pairs, and pair bonds can persist year round, even during nonbreeding periods (Adkisson 1996). Like other cardueline finches, Red Crossbills nest in loose aggregations, with pairs of crossbills defending only a small territory around their nest and foraging away from the aggregation site, which is necessitated by the irregular and local distribution of food supply (Newton 1972). Red Crossbill nests are usually well concealed and generally located in dense foliage on side branches high in conifer trees (Newton 1972; Adkisson 1996). Nests are constructed on a base of twigs and consist of interwoven grass, moss, lichens and bark, and are lined (more heavily in winter) with lichens, moss, hair and feathers (Newton 1972). Females construct the nest, while males may provide nesting material (Austin 1968). Males feed females regurgitated conifer seeds as a form of courtship feeding (Létourneau 1996).
The female incubates the eggs alone, without leaving the nest, where the male regularly feeds her (Austin 1968). If a second nest is started in quick succession to a first successful nesting attempt, the male takes responsibility for the first brood while the female incubates the second nest (Adkisson 1996). Eggs are incubated for 12 – 16 days (Newton 1972; Godfrey 1986), although Austin (1968) states that incubation may last for 18 days. The female tends to lay one egg per day, leading to different aged (and sized) chicks within a brood (Newton 1972). This asynchronous hatching may be an adaptation to crossbills’ unpredictable food supply that can result in the youngest perishing during times of seed shortage (Newton 1972). Red Crossbills hatchlings are altricial (Létourneau 1996). The period from hatching to fledging lasts typically for 18 – 22 days, although durations ranging from 15 (Snyder 1954) to 25 days have been recorded, a flexibility that may be due to variable food supply (Newton 1972).
There has not been a directed effort to locate Red Crossbill nests in Newfoundland. Only three records of nests are available for Newfoundland, all of which were found in the 1970s (see Table 2). Limited information exists on demographic indicators in populations of Red Crossbills. Age at first breeding is unknown in Red Crossbills, but occasionally wild birds in immature plumage show evidence of breeding (McCabe and McCabe 1933; Newton 1972; Adkisson 1996). An immature bird has been heard singing in Newfoundland (WAM). While Red Crossbills usually have clutches of 3 or 4 eggs, and occasionally 2 - 5 (Austin 1968; Godfrey 1986; Létourneau 1996), little information is known about percna. There are records of a 3-egg and a 4-egg clutches in Newfoundland, as well as a brood of three, 2-3 day old chicks (WAM unpublished files; cf. Peters and Burleigh 1951). The percna subspecies may have more than one brood per season, with short intervals between clutches particularly when seed crops are abundant (although no information on the number of Red Crossbill broods in a season is available, Adkisson (1996) speculates that 2-4 broods a year could be likely, as occurs in White-winged Crossbills). There is no information on reproductive success or the proportion of females rearing at least one brood to independence in the percna subspecies.
Little information is available on the life span of Red Crossbills. Banding data show that the North American longevity record for the species is four years and two months, from a bird banded and recovered in British Columbia (Brewer et al. 2000). Captive birds may live up to 8 years, with females suffering higher annual mortality than males in captivity (Adkisson 1996). There is no specific information on the life span or rates of annual mortality of the percna subspecies.
The survival of Red Crossbills is most closely linked to abundant conifer seed resources. This dependence on conifer crops occurs across large areas, as crossbills move nomadically in search of food. Low rates of food availability lead to irruptions out of typical ranges that could lead to increased levels of mortality if sufficient food resources are not located. Juveniles may irrupt in greater proportion than adult birds when cone crops are moderate and when only the most vulnerable birds must seek other food sources; the proportion of adult birds irrupting is greater when cone crops are poor (Newton 1972). There is no specific information on age-related differences in irruptive movements and survival for percna.
Although Red Crossbills sometimes nest in months with very cold temperatures, the early survival rate of offspring might not be greatly affected by low temperatures. This is because the female Red Crossbill virtually broods the nestlings continuously (Newton 1972) for the first few days after which she helps the male feed the young (R.W. Summers, pers. comm. 2003). Brooding might also increase annual mortality among females. Young Red Crossbills depend on post-fledging parental care, because newly fledged young have not yet developed crossed mandibles essential for accessing seeds from closed cones. They survive on parental feeding for one to two months or longer (Newton 1972). Once young Red Crossbills are feeding independently, they quite likely suffer higher mortality during their first winter than adults due to inefficient foraging (Adkisson 1996).
Recruitment to the breeding population of Red Crossbills is likely highly dependent on food abundance. During years of poor cone availability in Europe, only a fraction of the birds in a population nest (Newton 1972), and it is speculated that this reduced reproductive output might not balance high mortality rates suffered from food shortages. This could also hold for the percna subspecies although there is no information on recruitment for this taxon. Red Crossbills have relatively high reproductive potential owing to the possibilities of laying multiple clutches in a year and of juveniles breeding, particularly during periods with abundant conifer seeds. Reproductive potential could be high enough to promote recovery from losses stemming from emigration or starvation in poor cone years (Newton 1972), given that a sufficient breeding population remains and conifer seed availability increases. Specific information on the ability of the percna subspecies, as well as other Red Crossbill taxa, to recover from years of poor or no recruitment is not available.
Red Crossbills, like other cardueline finches, do not undertake regular seasonal long-distance migrations. Instead, they exhibit nomadic and irruptive movements that reflect the productivity of the cone-bearing trees on which they depend (Newton 1972). These movements are evident on a continental scale (Bock and Lepthien 1976) as well as on regional ones. In North America, Red Crossbill irruptions usually occur in autumn when cones ripen, but movements may also be undertaken in the spring or early summer (Adkisson 1996). On a North American scale, Red Crossbills exhibit more extensive irruptive movements than White-winged Crossbills (Bock and Lepthien 1976). In Europe, irruptive birds may return to their core boreal ranges in the year following their movement (Newton 1972). Annual fluctuations in conifer seed abundance are the ultimate driving force behind the nomadic movement of Red Crossbills. Red Crossbills could be stimulated to move by food shortage or possibly based on social assessments of cone and seed abundance in an area (Benkman 1990). An additional proximate factor that could influence crossbill movements is overcrowding within the birds’ regular range, which might also stimulate irruptions (Newton 1972).
Movement of the percna subspecies is not well known. Birds appear irregularly across Newfoundland, likely as a result of nomadic movements. There is some speculation that Red Crossbills move off Newfoundland during conifer seed shortages on the island (see discussion in the section Newfoundland Range – percna), but only limited data exist to support or refute this hypothesis. Other bird species with subspecific designations are sedentary on insular Newfoundland (Peters and Burleigh 1951). Other large-billed island species of crossbills, the Scottish Crossbill (L. scotica) and the Parrot Crossbill (L. pytyopsittacus), are also sedentary (Marquiss and Rae 2002; Summers et al. 2002), thus a pattern of limited movements would not be a wholly unexpected strategy in the Red Crossbill percna subspecies. Red Crossbills are occasionally seen on the French islands of Saint Pierre et Miquelon (R. Etcheberry, pers. comm., 2003) that lie off the Burin Peninsula on Newfoundland’s south coast. These sightings probably do not represent a stable population, as there is no evidence that Red Crossbills breed on the islands. Red Squirrels have been unsuccessfully introduced to the island (R. Etcheberry, pers. comm., 2003), and their demise may be due to the small number of conifer trees there that likely also impedes crossbill breeding.
Crossbills forage by placing the tips of their mandibles between the scales of a cone and creating a small gap between the tightly closed scales. The birds then abduct their jaw, and sometimes twist their head (Newton 1972), which creates a larger gap between the scales and exposes the seed held within (Benkman 1987a). Crossbills use the spoon-shaped tip of their tongue to bring the seed to their bill, where the seed is husked, and the seed is consumed (Benkman 1987a). The crossed mandibles of crossbills make them unique in their ability to forage on closed conifer cones. Experienced crossbills with bills that have been experimentally “uncrossed” by trimming cannot forage on closed cones (Benkman 1988). Thus, crossbills are able to exploit the seeds in tightly held cones before they open and become accessible to other conifer seed eating finches such as Pine Siskins (Carduelis pinus) and Pine Grosbeaks (Pinicola enucleator). Crossbills cannot easily pick up seeds on the ground and must consume seeds from cones (Newton 1972).
Food availability is the most significant limiting factor for crossbill abundance. Fluctuations in conifer seed abundance are the primary driving force in the nomadic movements of crossbills (see discussion in Movements section). Red Crossbills in Newfoundland have been observed feeding on all conifer species. Although Red Crossbills are morphologically adapted for foraging on conifer seeds, they also consume non-conifer seeds. Describing Red Crossbills in Newfoundland, Peters and Burleigh (1951) write “they feed on seeds of fir, tamarack, maple and other trees and sometimes upon the buds of deciduous trees. They also eat weed seeds and even some wild fruits. In the summer they consume a number of insects and larvae.”
Key Conifer Species
There is a robust relationship between the size of a crossbill’s bill and the size of cones that it forages on. Larger-billed crossbills forage on large, hard cones like pine, and most smaller-billed crossbills forage on smaller, softer cones like spruce or larch (Lack 1944). Crossbills forage on different types of conifer trees in different seasons, according to which species have seeds available (dependent on both the reproductive phenology of the tree species, as well climatic conditions that influence the opening of conifer cones). Benkman (1993a) identified “key conifers” that provide reliable seed resources for the crossbill types that specialize on them. Crossbill foraging on these key conifers is optimized by the bill size and palate structure that closely match the cone size and toughness (Benkman 1993a). This specialization becomes most important during (and, in fact, arises as a result of) periods of food limitations, which are most severe during late winter. Key conifers for crossbills hold seeds within their cones through the winter (Benkman 1993a).
The percna subspecies is large-billed, yet Benkman (1989) proposes that the key conifer species for percna is the small-coned Black Spruce. This is because there are rarely years of very poor seed crop within this species, and the conifer’s fire adaptation means that some seeds are held throughout the year without being shed (semi-serotinous). An alternate explanation is that, like other large-billed island crossbills (such as L. megaplaga on Hispaniola, L. c. corsicana on Corsica, L. c. guillemardi on Cyprus, and L. scotica on Great Britain), the percna subspecies is adapted for foraging on Red or White Pines that were once much more common than now (Mannion 1977; Roberts and Mallik 1994; Rajora et al.1998). Red Crossbills have recently been observed foraging in a small stand of Red Pines in Gambo in central Newfoundland (Whitaker et al. 1996; Lewis 1997) and foraging in non-native cultivated Austrian Pine trees (Pinus nigra; interestingly, a key conifer for the large-billed island endemic Red Crossbill on Cyprus, L. c. guillemardi; Benkman 1989) on the campus of Memorial University of Newfoundland (discussed in the archives of Nf.birds 2003).
Based on current information , it is impossible to determine whether Black Spruce or Red or White Pine represent the key conifer species that explains the large bill of the percna subspecies. What is more certain is that currently, given the restricted range of Newfoundland pine, Black Spruce seed resources must be important for Red Crossbills. It should be recognized that other conifer species can also provide seed resources for Red Crossbills. Balsam Fir is a significant forest resource in Newfoundland, comprising almost 50% of the forest (Page et al. 1974). Many cardueline finches in Newfoundland feed on these seeds that can persist in cones during the winter (Wren 2001). White Spruce could also be of secondary importance to Red Crossbills in Newfoundland that have been observed feeding on both Balsam Fir and White Spruce (WAM unpublished files).
Red Crossbills might compete with other cardueline finches for conifer seeds. Those that are common in Newfoundland (as classified in Mactavish et al. 2003) are the Pine Grosbeak, Purple Finch (Carpodacus purpureus), White-winged Crossbill, Common Redpoll (Carduelis flammea) and Pine Siskin. While the non-Loxia finches are not as highly dependent on conifer seeds for food as crossbills are, they all forage on conifer seeds when they are abundant. Crossbills retain an advantage over other finches when foraging on closed or partially closed cones, but other finches like Pine Siskins have been shown to be more efficient than crossbills when foraging on opened cones (Benkman and Lindholm 1991).
White-winged Crossbills are the most significant avian competitors with Red Crossbills, owing to their shared bill morphology. Competition between the two crossbills species is likely more significant when Red Crossbill taxa with small bills are considered (Benkman 1987b). Thus, the relatively large-billed percna subspecies is not nominally expected to be out-competed by the smaller billed White-winged Crossbill for conifer seed resources, but in times of severe seed restrictions, White-winged Crossbills are probably more efficient foragers on smaller, open cones (such as Tamarack and possibly spruce).
The Red Crossbill percna subspecies’ morphological specialization and insularity strongly limits its adaptability to widespread conifer seed shortages. Even if Red Crossbills do irrupt to the mainland to escape Newfoundland conifer seed limitations, there is no evidence that they breed there successfully or return to Newfoundland to breed (European Red Crossbills may return to their original range in the years following irruptive movements;Newton 1972). Breeding site and home range fidelity is not considered to exist in Red Crossbills (Adkisson 1996). Red Crossbills defend only small territories around nest sites and do not defend a feeding territory, due to the variable timing and location of conifer seeds (Newton 1972). Like other cardueline finches, crossbills usually forage in groups or flocks throughout the year. Flocking behaviour allows crossbills to use public information to make more efficient foraging decisions (Smith et al. 1999) and provides the added benefit of increased predator vigilance (Newton 1972).
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