Sensitivity to Disturbance at Nest and Roost Sites
Crossbills tolerate close approach to nests; see Bailey et al. (171) and Nethersole-Thompson (149). Roost site disturbance is unlikely to cause much harm because roost sites (dense growth in conifers) are generally plentiful in crossbill habitat.
Shooting and Trapping
Although tame and easily shot (47), and until the mid-1900s in Austria, Germany, and Switzerland trapped, caged, shown, and even sold at market (142), shooting and trapping probably pose limited problems for this species.
Pesticides and Other Contaminants/Toxics
Crossbills are often killed by passing cars while feeding on sand and road salt used by municipalities to treat roads in winter (CWB), and toxicity from sodium chloride or road salt impairs crossbills and directly contributes to their vulnerability (219).
Degradation of Habitat
Any factor that reduces conifer seed production and the retention of seeds in cones will act to degrade habitat for crossbills. These range from the construction and maintenance of roads to shortened logging rotations (220). Shortened logging rotations act to reduce the occurrence of large cone crops because most conifers do not begin producing many cones until they are about 30 yr old, and maximum cone production does not occur until they are over 60 yr old (154). As expected, crossbills are more abundant in mature forests with large trees, and such forests are especially important when cone crops are small and seeds are likely most limiting (221). Red Crossbill has declined with forest fragmentation (222), but these declines were more likely associated with declining forest age (223). Additional impacts include extensive and expanding outbreaks of forest insects killing vast numbers of trees, such as mountain pine beetle (Dendroctonus ponderosae) in the West; southern pine beetle (D. frontalis), hemlock wooly adelgid (Adelges tsugae) (224), and red pine scale (Matsucoccus matsumarae) in the East. Increasing frequency of fire (225) will reduce the canopy seed bank of serotinous pines (226) like Rocky Mountain lodgepole pine, which is relied upon by Type 5, and may even cause conifer forests to shift to shrub and grasslands (227). Forest insect outbreaks and fire frequency, size, and severity are increasing with climate change (228, 225), and climate change alone is predicted to cause massive declines in conifer forest area including key conifers (e.g., 229). Decreases in forest area could cause crossbill population declines proportionately greater than the amount of forest loss because in, for example, lodgepole pine forests in the Rocky Mountains, the density of crossbills increases logarithmically with increasing forest area up to around 70 km2 (211; see also 220). A final concern is the increasing winter and spring temperatures with climate change, which will act to cause earlier and more complete shedding of seeds from cones increasing the window of seed scarcity between when nearly all of the seeds are shed and the new seed crop becomes profitable to crossbills (143). Such an increase in seed scarcity may explain why there has been over a 20-fold increase in Red Crossbills coming to bird feeders in the Mountain West since the early 1990s (143). Analyses of crossbills specialized on scots pine (Pinus sylvestris) in Europe (Parrot Crossbill and Scottish Crossbill), indicates that their absence from ca. 80% of the range of scots pine (47) is the result of high spring temperatures causing an expanded window of seed scarcity, and that future climates will cause a large range reduction in especially the Parrot Crossbill (119).