Effects of Human Activity
On balance, the effects of humans on Cliff Swallows are probably positive. Breeding habitat has been enhanced by the widespread construction of bridges, culverts, and buildings that provide alternative nesting sites and have probably led to a major population expansion range-wide. Cliff Swallows are extremely tolerant of humans, frequently occupy colony sites in close proximity to people, and do not seem negatively affected by researcher activity in and around their colonies.
Homeowners sometimes object to birds nesting on their houses and will remove nests. Parasitic swallow bugs from Cliff Swallow nests pose little threat to humans and cannot persist on mammalian hosts. Fire suppression in the Great Plains (leading to proliferation of juniper trees) has caused some cliff nesting sites to become too wooded and thus unsuitable as colony sites. Both breeding and winter foraging habitat has experienced conversion from grasslands and prairies to agricultural mono-cultures, but the effect (if any) of this is currently unknown. Birds concentrate insecticides in tissues to some degree: toxaphene, various heavy metals, DDE, and PCBs were found in birds along the Rio Grande River in Texas and New Mexico (Kennedy et al. 1995, Maruya et al. 2005, Mora et al. 2007); petroleum hydrocarbons and elevated levels of trace elements (chromium, mercury, selenium) in birds along the North Platte River in Wyoming (Dickerson 2002); and mercury in coastal California (Hothem et al. 2008). Cliff Swallows may serve as reliable bioindicators for mercury and perhaps for other contaminants that transfer from aquatic to terrestrial ecosystems (Hothem et al. 2008). Colonies near the Tar Creek Superfund site in ne. Oklahoma had high levels of lead and zinc in the dried mud of the nests (C. Bridge pers. comm.).
Presence of colonies on highway bridges sometimes conflicts with road construction and bridge maintenance, with such activities causing nest loss or abandonment due to vibration, noise, or obstruction of the birds’ access. Efforts to reduce conflicts include re-scheduling highway construction to non-breeding times of the year, using plastic netting or barriers and broadcasting alarm calls to prevent settlement during construction, and installing Teflon sheeting to nest-attachment areas (Conklin et al. 2009, Delwiche et al. 2010). Most measures have had only mixed success at discouraging nesting.
Conservation Efforts
Attempts have been made to restore populations in parts of ne. U.S.; listed as Threatened for Pennsylvania and New Jersey (Kitson and McNaught 1991). Most conservation efforts, however, have been limited and localized (Krapu 1986, Silver 1993, 1995).
House Sparrow control seems to be the most effective means of increasing Cliff Swallow numbers locally and probably regionally (Samuel 1969b, Krapu 1986, Silver 1993). Trapping and shooting House Sparrows at colony sites varies in effectiveness but eventually eliminates most individuals. Knocking down all old Cliff Swallow nests after the breeding season also controls House Sparrows, by removing places for them to roost during the winter and preventing them from becoming established in the existing nests at a colony site before Cliff Swallows return in the spring. A colony in Wisconsin grew from 1 to > 2,000 nests over a 38-year period, primarily through House Sparrow control and removal of old nests over the winter (Buss 1942). Nest removal also reduces buildups of ectoparasites from year to year. Fumigation of nests with the insecticide Dibrom is effective in eliminating parasites (Brown and Brown 1986, 1996) but has not been used specifically for management to date.
An increase in painted barns in the twentieth century is believed to have reduced the population in parts of the ne. U.S. because nests are thought to adhere less well to painted surfaces (Townsend 1917, Forbush 1929, CRB). However, nests were equally likely to fall from painted and unpainted rafters at a Massachusetts colony (Silver 1993). Attaching wooden strips along a wall may enhance the structural stability of nests on buildings by providing a more secure base for the nests to be built on (Buss 1942). Artificial nests and half-nests made out of plaster or stoneware clay have been installed to provide safer nesting sites that are less likely to fall (Kitson and McNaught 1991, Silver 1993, 1995); plaster nests have also been used for research purposes (Mayhew 1958, CRB, MBB). Birds in Pennsylvania, New Jersey, and California readily occupied plaster nests, but in Nebraska plaster nests were often ignored for several years before being occupied if at all. Installation of artificial nests is unlikely to be an effective management strategy unless accompanied by House Sparrow control, because House Sparrows quickly become established in plaster nests. House Sparrows are less likely to occupy plaster half-nests (M. Silver pers. comm.). Alternative nesting structures dedicated specifically for Cliff Swallows have been constructed in California and Colorado; the latter included actual nests glued to the structure.
With House Sparrow control, local colony size can increase substantially; annual increases at one N. Dakota site averaged 97% when House Sparrows were trapped (Krapu 1986). See Buss (1942) for details on a major increase in colony size at a managed colony in Wisconsin. Removal of ectoparasites via nest fumigation can also result in colony size increases at some sites (CRB, MBB). Local increases in the number of birds nesting on barns in Massachusetts have occurred following installation of clay ledges on potential nesting substrates (Silver 1995). Alternative colony structures in California and Colorado have been readily occupied by birds. However, whether any of the conservation measures actually increase total population size over time, instead of causing mostly redistribution of birds among sites, is unknown. Attempts to entice Cliff Swallows to nest in plaster nests at Mission San Juan Capistrano, s. California, have been unsuccessful to date (CRB).
Recommendations for rehabilitating birds in captivity are given by Shaw (1992).
On balance, the effects of humans on Cliff Swallows are probably positive. Breeding habitat has been enhanced by the widespread construction of bridges, culverts, and buildings that provide alternative nesting sites and have probably led to a major population expansion range-wide. Cliff Swallows are extremely tolerant of humans, frequently occupy colony sites in close proximity to people, and do not seem negatively affected by researcher activity in and around their colonies.
Homeowners sometimes object to birds nesting on their houses and will remove nests. Parasitic swallow bugs from Cliff Swallow nests pose little threat to humans and cannot persist on mammalian hosts. Fire suppression in the Great Plains (leading to proliferation of juniper trees) has caused some cliff nesting sites to become too wooded and thus unsuitable as colony sites. Both breeding and winter foraging habitat has experienced conversion from grasslands and prairies to agricultural mono-cultures, but the effect (if any) of this is currently unknown. Birds concentrate insecticides in tissues to some degree: toxaphene, various heavy metals, DDE, and PCBs were found in birds along the Rio Grande River in Texas and New Mexico (Kennedy et al. 1995, Maruya et al. 2005, Mora et al. 2007); petroleum hydrocarbons and elevated levels of trace elements (chromium, mercury, selenium) in birds along the North Platte River in Wyoming (Dickerson 2002); and mercury in coastal California (Hothem et al. 2008). Cliff Swallows may serve as reliable bioindicators for mercury and perhaps for other contaminants that transfer from aquatic to terrestrial ecosystems (Hothem et al. 2008). Colonies near the Tar Creek Superfund site in ne. Oklahoma had high levels of lead and zinc in the dried mud of the nests (C. Bridge pers. comm.).
Presence of colonies on highway bridges sometimes conflicts with road construction and bridge maintenance, with such activities causing nest loss or abandonment due to vibration, noise, or obstruction of the birds’ access. Efforts to reduce conflicts include re-scheduling highway construction to non-breeding times of the year, using plastic netting or barriers and broadcasting alarm calls to prevent settlement during construction, and installing Teflon sheeting to nest-attachment areas (Conklin et al. 2009, Delwiche et al. 2010). Most measures have had only mixed success at discouraging nesting.
Conservation Efforts
Attempts have been made to restore populations in parts of ne. U.S.; listed as Threatened for Pennsylvania and New Jersey (Kitson and McNaught 1991). Most conservation efforts, however, have been limited and localized (Krapu 1986, Silver 1993, 1995).
House Sparrow control seems to be the most effective means of increasing Cliff Swallow numbers locally and probably regionally (Samuel 1969b, Krapu 1986, Silver 1993). Trapping and shooting House Sparrows at colony sites varies in effectiveness but eventually eliminates most individuals. Knocking down all old Cliff Swallow nests after the breeding season also controls House Sparrows, by removing places for them to roost during the winter and preventing them from becoming established in the existing nests at a colony site before Cliff Swallows return in the spring. A colony in Wisconsin grew from 1 to > 2,000 nests over a 38-year period, primarily through House Sparrow control and removal of old nests over the winter (Buss 1942). Nest removal also reduces buildups of ectoparasites from year to year. Fumigation of nests with the insecticide Dibrom is effective in eliminating parasites (Brown and Brown 1986, 1996) but has not been used specifically for management to date.
An increase in painted barns in the twentieth century is believed to have reduced the population in parts of the ne. U.S. because nests are thought to adhere less well to painted surfaces (Townsend 1917, Forbush 1929, CRB). However, nests were equally likely to fall from painted and unpainted rafters at a Massachusetts colony (Silver 1993). Attaching wooden strips along a wall may enhance the structural stability of nests on buildings by providing a more secure base for the nests to be built on (Buss 1942). Artificial nests and half-nests made out of plaster or stoneware clay have been installed to provide safer nesting sites that are less likely to fall (Kitson and McNaught 1991, Silver 1993, 1995); plaster nests have also been used for research purposes (Mayhew 1958, CRB, MBB). Birds in Pennsylvania, New Jersey, and California readily occupied plaster nests, but in Nebraska plaster nests were often ignored for several years before being occupied if at all. Installation of artificial nests is unlikely to be an effective management strategy unless accompanied by House Sparrow control, because House Sparrows quickly become established in plaster nests. House Sparrows are less likely to occupy plaster half-nests (M. Silver pers. comm.). Alternative nesting structures dedicated specifically for Cliff Swallows have been constructed in California and Colorado; the latter included actual nests glued to the structure.
With House Sparrow control, local colony size can increase substantially; annual increases at one N. Dakota site averaged 97% when House Sparrows were trapped (Krapu 1986). See Buss (1942) for details on a major increase in colony size at a managed colony in Wisconsin. Removal of ectoparasites via nest fumigation can also result in colony size increases at some sites (CRB, MBB). Local increases in the number of birds nesting on barns in Massachusetts have occurred following installation of clay ledges on potential nesting substrates (Silver 1995). Alternative colony structures in California and Colorado have been readily occupied by birds. However, whether any of the conservation measures actually increase total population size over time, instead of causing mostly redistribution of birds among sites, is unknown. Attempts to entice Cliff Swallows to nest in plaster nests at Mission San Juan Capistrano, s. California, have been unsuccessful to date (CRB).
Recommendations for rehabilitating birds in captivity are given by Shaw (1992).