Red crossbill (Percna) COSEWIC assessment and status report: chapter 10
Limiting Factors and Threats
The boreal forest landscape that presently exists in Newfoundland is very different from that of previous centuries. Many anthropogenic (forestry activities, human caused fires) and natural (infestations, fire cycle, forest regeneration patterns) factors have combined to reduce the cone crops that Red Crossbills depend on. Red Crossbills are very vulnerable to habitat loss because their nomadic behaviour concentrates them in regions of good cone crops (as compared to generalist songbird species that are more evenly distributed in a habitat; Benkman 1993a). A reduction of cone crops due to forest change, whether it is of possible “key conifers” such as pines or Black Spruce, or alternate conifer species like Balsam Fir and White Spruce, negatively impacts Red Crossbill activity and possibly lowers recruitment to the population. It is not known whether Red Crossbills escape conifer seed limitations by moving to the mainland (and, if this does happen, whether they successfully breed on the mainland, or whether they return to Newfoundland to breed in subsequent years). For further details on habitat loss in Newfoundland, see the sections on Habitat Requirements and Trends.
Red Squirrels were introduced to Newfoundland in small numbers starting in 1963 (Minty 1976; Payne 1976; Goudie 1978) and are now abundant. They may be presently competitively excluding Red Crossbills by out competing them for food resources in the form of conifer seeds (Benkman 1989, 1993c; Pimm 1990). This could be particularly true of supplies of Black Spruce seeds that are normally held within cones and could be an important food for Red Crossbills when other conifer seeds are in short supply. Red Squirrels are abundant in Black Spruce forests in Newfoundland; in a study in 1998/1999 they were found to be close to four times more abundant in Black Spruce stands than Balsam Fir stands (Wren 2001). The impact of squirrels as seed predators in Newfoundland forests is largest during years of seed shortages. West (1989) found that during a good cone crop year, Red Squirrels harvested less than 1% of cones from Black Spruce trees by October. In small cone crop years, Red Squirrels removed 64 – 96% of cones in a Black Spruce plantation by October. Red Squirrels could also contribute to seed limitations in species other than Black Spruce. One study of Red Squirrel cone predation in Newfoundland White Pine stands estimated an average cone loss of 84% over a three-year period from 1998 – 2000 (with a range of values from 32 to 100%); however, cone loss could not be specifically attributed to squirrels (English 1998). While the impacts of Red Squirrel competition on Red Crossbills in Newfoundland are speculative and not quantified, studies in other locations have indicated that conifer trees that have evolved in the absence of squirrels do not have the same squirrel-related defences as do conifers that evolve with squirrels, allowing the co-evolution of Red Squirrels and Lodgepole Pine in areas around the Rocky Mountains (Benkman et al. 2001). Thus, it is possible that Red Squirrels could be limiting the food supply and/or hindering the recovery of an important conifer for Red Crossbills in Newfoundland.
The extent to which Red Crossbills can escape competition from Red Squirrels by switching to other food resources is unknown, as are the ultimate population level impacts of competition with Red Squirrels. In Scotland, Summers and Proctor (1999) have shown that crossbills and squirrels show preferences for different shapes and density of pines. Squirrels prefer more dense stands, possibly because they can avoid crossing ground to access other trees, whereas the crossbills more often used the older more open pine stands. Summers and Proctor (1999) suggest that these behavioural patterns likely reduce competition between squirrels and crossbills and that the competitively inferior crossbills may avoid competitive exclusion during periods of low cone abundance by moving to other sites (see also Summers’ (2002) consideration of the different sizes of cones utilized by crossbills and squirrels).
Predation is an additional source of mortality for Red Crossbills although the level of this threat is unknown. Gray Jays (Perisoreus canadensis) are predators of Red Crossbill nests (Adkisson 1996), and, along with Red Squirrels are the major predators of bird nests in Balsam Fir forests in Newfoundland (Lewis 1999). Although Red Squirrels are a recently introduced species to Newfoundland that may be exerting considerable predation pressure on Red Crossbills, the impacts of Red Squirrel egg and nestling predation on percna (or on other Red Crossbill subspecies in other parts of North America), have not been quantified. Some Red Crossbills may be killed on roadways where they are attracted to salt and grit. For example, a recent Red Crossbill specimen in Newfoundland was killed in a vehicle collision in Terra Nova National Park (see photographs of the prepared skin, Figure 2), but the importance of vehicle collisions to the overall level of mortality of Red Crossbills is unknown.
The potential for growth of the Red Crossbill population under current conditions is likely extremely compromised by low numbers and consequent reduced population viability. Population growth potential is also contingent on years of good cone crops within Newfoundland’s boreal forests. Successive years of heavy cone crops cannot guarantee population growth, however, unless breeding pairs are naturally situated to take advantage of conifer seed abundance.
The potential for growth of the Red Crossbill population could be greater under managed conditions. In Britain, recent afforestation through the creation of plantations has greatly increased the diversity and abundance of conifer seeds (Marquiss and Rae 1994). This has contributed to an independent breeding population of Common Crossbills in Britain, where the population used to depend on immigration from the European mainland (Newton 1972; Avery and Leslie 1990). A managed habitat beneficial to crossbills must contain old-growth or at least mature conifers, because they offer the best yield of cones (Newton 1972; Benkman 1993b; Holimon et al. 1998). Enhanced pine regeneration and production could likely hold some potential for population growth among Red Crossbills in Newfoundland. Large protected expanses of old growth forest that also include pines, e.g. the proposed Little Grand Lake Ecological and Wilderness Reserve, could also be beneficial to crossbills on insular Newfoundland in the long-term.
Management for population growth of the Red Crossbill might promote conifer seed availability, and this might be achieved on three fronts. First, stand rotation ages in Newfoundland should be increased to ensure higher cone yields. Second, relict Red and White Pine stands in Newfoundland should be protected as potential key conifer source for Red Crossbills. Third, a habitat managed to promote Red Crossbill population growth must take into account the competitive limitations of Red Squirrels on conifer seed availability, and possible effects of Red Squirrel predation on percna. Another possible management step could be intervention to ensure that squirrel-free islands like Anticosti and the Magdalen Islands where Red Crossbills breed (Létourneau 1996), and which may be refuges for percna (although percna has not been confirmed for these islands) remain squirrel free. Many smaller coastal islands of Newfoundland which have been suggested as percna refuges by Benkman (1993c) would likely not be effective refugia. Many of these islands already have Red Squirrels on them, are not well forested, and only support transient populations of Red Crossbills (WAM unpublished files). For instance, Baccalieu Island which has substantial old growth forests and no squirrels has few Red Crossbill occurrences (Wells and Montevecchi 1984), though it should still be kept free of introduced Red Squirrels.
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