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Red Crossbill, percna subspecies
2.1 Threat Classification
Section 1.4 briefly outlines what are believed to be the current or potential threats impacting on percna. Analysis and empirical investigation are required to improve our understanding of the threats. To detail and prioritize research requirements, a Threat Classification Table (Appendix 1) was developed. This table relates threats under broad categories. For each specific threat, information is presented on the following: level of causal certainty (i.e., level of certainty that each threat is impacting or has impacted percna), timing, frequency, extent, severity, and level of concern. Categories used in this table are presented in Appendix 1.1.
2.2 Threats Narrative
Throughout this section, the most important questions for threat clarification research are targeted to identify if and how potential threats may be impacting percna.
2.2.1 Potential Interspecific Relationships
Parchman and Benkman (2002) theorized co-evolution as a key factor in the decline of the Red Crossbill in Newfoundland. Mainland populations of black spruce evolved with seed predation by both red squirrels (Tamiasciurus hudsonicus) and Red Crossbills (along with a variety of other seed predators). Conversely, red squirrels were absent from insular Newfoundland until 40 years ago, and thus black spruce populations there evolved in the presence of seed predation primarily by cone-dependent finches. Parchman and Benkman (2002) further suggested that the absence of squirrels from the island resulted in thinner spruce cone scales compared with those found in mainland populations. The arrival of red squirrels on the island introduced a novel seed predator capable of potentially outcompeting Red Crossbills for food (Parchman and Benkman 2002). Benkman (1989) further suggested that Red Crossbills were more susceptible to this competition than White-winged Crossbills, because the former are more sedentary. Benkman (1993a, 1993c) and Parchman and Benkman (2002) also argued that percna is specialized for feeding on the black spruce cones, which are now also targeted by squirrels in Newfoundland. However, further work is needed to confirm the speculations of Benkman and co-workers. In Europe, Summers and Proctor (1998) identified different tree and cone preferences for red squirrels and crossbills in native scots pine (Pinus sylvestris) forests that likely reduce competition between the two species. It is unknown if conifer seed–eating finches and/or potentially nest-predating Gray Jays (Perisoreus canadensis) (Adkisson 1996) may cause harmful levels of interspecific competition. There is no evidence to suggest that other conifer seed–eating bird populations in Newfoundland have suffered similar declines.
Knowledge gaps regarding interspecific competitors will be filled once habitat associations of Red Crossbills are identified and we know more about the dependency of crossbills on different food sources as well as the extent and quality of the habitat that provides these food sources. The role of interspecific competition in the decline of percna will be evaluated through programs laid out in a subsequent action plan.
2.2.2 Potential Decline, Alteration, and/or Degradation of Habitat via Natural and Anthropogenic Factors
The roles that habitat loss or degradation through natural (fire, insects, tree disease) and anthropogenic disturbances (forest harvesting, urban and agricultural expansion, fire suppression) may have played inpercna declines and the roles they may continue to play in preventing recovery are currently unknown. Knowledge of important habitat as well as seasonal utilization of habitat (section 1.5) is required for proper assessment.
Fire, insect outbreaks, and tree diseases potentially alter the levels and quality of Red Crossbill habitat and food abundance. All three natural disturbances are considered to affect populations with high levels of severity on local and/or widespread scales (Appendix 1).As a natural feature of the boreal forest ecosystem, fire may destroy large areas of forested landscape at one time, thereby reducing habitat and cone availability. The forests require a number of years to regenerate. Additionally, regeneration may result in a post-fire habitat mosaic that is different from the original. In Newfoundland, fire promotes the regeneration of black spruce (COSEWIC 2004). Fire suppression discourages natural regeneration of black spruce and red pine (Fowells 1965).Insects may also impact percna in several ways. Insects can kill trees and thus deprive crossbills of both food and habitat simultaneously. Insects that kill trees have the potential to more negatively impact crossbills over a longer period of time than insects that do not directly kill trees. Insects could also be a threat to crossbills when they feed on the flowers or seeds of conifers, thereby reducing food availability (B. English, pers. comm. 2005). Spruce budworms (Choristoneura fumiferana) in particular feed on conifer flower buds in the spring. Canadian Forest Service research in Newfoundland during the last major spruce budworm outbreak (in the 1970s) showed that no seed was produced, even in areas of light defoliation. Other cone-feeding insects also represent a threat to percna, but these generally exist at consistently low levels (B. English, pers. comm. 2005).
In Newfoundland, both red and eastern white pine stands may have been important habitat for Red Crossbills, but both tree species are currently impacted by introduced fungal diseases. White pine blister rust (Cronartium ribicola) affects eastern white pines, whereas red pines are affected by scleroderris canker (Gremmeniella abietina) (COSEWIC 2004). Eastern white pine, in particular, has undergone a precipitous decline in Newfoundland, whereas red pine has been undergoing a slow but steady decline for thousands of years (B. English, pers. comm. 2005 ). Tree loss from insect and fungal damage is higher than loss from either harvesting or fire (COSEWIC 2004).
The impact of forest harvesting on percna is uncertain. Current forestry practices have shortened rotation ages for conifer stands in Newfoundland (Thompson et al. 1999, 2003). While the extent of late successional forest (81+ years) may be somewhat reduced, the overall availability of forest stands of cone-bearing age (i.e., 40+ years of age) across the island remains substantial. Approximately 58% of the productive softwood forest on the island (1.6 million hectares) is age class 3 or higher (i.e., 40+ years; see Appendix 2), and forest management policy in the province (Government of Newfoundland & Labrador 2003) sets a target of at least 15% forest cover in age class 5 or older (i.e., 81+ years). Further, with a typical minimum age at harvest of 65 years (and more frequent harvest at 81+ years), stands of cone-bearing age would be available for at least 30, and more commonly 50, years following harvest and regeneration.
With regard to habitat quality, fragmentation has been suggested to negatively influence crossbills (Helle 1985). Although forestry practices in general may tend to create a fragmented landscape, it is not clear that this threat is significant for the percna subspecies, given the naturally fragmented nature of forest in Newfoundland. Currently, red and eastern white pine exist at low densities in locations scattered across the island, with some notable high concentrations. The sharp decline of eastern white pine and longer-term decline of red pine (B. English, pers. comm. 2005) may indicate a shift in composition of the available food supply and a lowering of habitat quality. It is possible that the long-term decline in red pine and the more recent decline in eastern white pine resulting from 19th- and early-20th-century harvesting and introduced disease, in combination with other threats, may have caused the decline of percna. Red Crossbill declines have been associated with declines in these two pine species on the northeastern North American mainland (Dickerman 1987; Erskine 1992).
2.2.3 Potential Allee and Cumulative Effects
Current estimates suggest that percna populations in Newfoundland are small (500–1500 individuals; COSEWIC 2004). When there is a positive relationship between population density and the per capita growth rate, population growth slows when densities are low (Courchamp et al. 1999). Although these relationships are unconfirmed in percna, low population numbers combined with Red Crossbill behaviour, particularly with respect to foraging, suggest that Allee effects will have to be considered as a potential limiting factor. Red Crossbills often forage in groups, and Smith et al. (1999) found evidence to suggest that feeding performance of Red Crossbills is affected by “public information” among flock mates. During this study, the variance in both the number of cones sampled and the time spent on empty patches (areas with no seeds) decreased when crossbills foraged with two flock mates compared with when foraging was solitary (Smith et al. 1999). Thus, if percna populations become small enough to inhibit effective foraging behaviour, this may further reduce survival rates. The cumulative impact of all the possible threats is a legitimate issue for Red Crossbill recovery. Having a more complete understanding of these threats is critical in determining how to approach recovery efforts and research requirements.
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