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COSEWIC Assessment and Status Report on the Common Nighthawk in Canada

Limiting Factors and Threats

While there are no specific studies on the subject, the decline of the Common Nighthawk may be partly related to a general decline in insect populations in both the breeding and wintering grounds, due to large-scale insecticide use since the mid-1900s (Cane and Tepedino, 2001; Conrad et al. 2004). For example, it has been assumed that mosquito control programs in most urban areas in North America are probably responsible for the decrease in several species of aerial insectivores such as the Common Nighthawk (Poulin et al. 1996). Similarly, the decline in the Eurasian Nightjar (Caprimulgus europaeus) is believed to be due in part to the decrease in insect populations from large-scale pesticide spraying programs in Europe (Conrad et al. 2004; UK Forestry Commission, 2006; UK Biodiversity Action Plan, 2006).

Data from the United States suggest that factors such as the alteration and loss of habitat may also affect Common Nighthawk populations in Canada (Poulin et al. 1996). For example, fire suppression, changes in harvesting practices that reduce the number of open areas in forested regions and an increase in intensive agriculture are all believed to be responsible for the decline in several open-habitat species such as the Common Nighthawk (Askins, 1993; Degraff and Yamasaki, 2003; R.M. Brigham pers. comm. 2007). In the prairie provinces, the loss and alteration of native prairies from cultivation, fire suppression, and cattle grazing are believed to be the primary factors in the decline of the species since 1900 (Jones and Bock, 2002, Northern Prairie Wildlife Research Centre, 2005). In addition, in urban areas, the gradual replacement of gravel-covered roofs with tar-covered roofs is believed to be a primary cause of habitat loss (Poulin et al. 1996). It is also thought that the recent colonization of large roofs by Ring-billed Gulls (Larus delawarensis) in some major cities (such as Montreal) reduces the amount of habitat available to the Common Nighthawk (J. Gauthier, pers. comm. 2005). In northern Canada, where some unknown portion of the population occurs, Common Nighthawk populations may be little affected by the alteration and loss of habitat.

The increase in terrestrial predators, such as domestic cats, striped skunks and raccoons (Procyon lotor), as well as avian predators, such as American Crows and Common Ravens may play a role in the decline of the species, especially in urban areas (Poulin et al. 1996; R.M. Brigham, pers. comm. 2005).

Collisions with motor vehicles have been reported as a mortality factor for several Common Nighthawk populations in North America. Populations that use dirt roads in managed forests as roost or nest sites are affected by increased vehicle traffic (including ATVs), which collide with adults or destroy nests (Bender and Brigham, 1995; Poulin et al. 1996; J. Gauthier, pers. comm. 2005). Nighthawks can also collide with aircraft. Indeed, relatively high mortality rates have been reported during fall migration at some sites (Cumming et al. 2003).

Extreme climatic fluctuations in the spring could also affect adult survival and breeding success, although this has not been documented. Given that the Common Nighthawk does not enter into torpor as frequently as other goatsuckers (Fletcher et al. 1993), the species is more susceptible to prolonged periods of cold weather in the spring. Moreover, the increased frequency of tropical storms in the Gulf of Mexico may negatively affect nightjars during their autumn migration (e.g. Chimney Swifts, Chaetura pelagica, J. Gauthier, pers. comm. 2005). Further studies are necessary to assess the effect of climate change on the ecology of the Common Nighthawk.