Shooting and Trapping
Formerly shot by apiarists for mistaken belief that adults kill many honeybees (130).
Pesticides and Other Contaminants/Toxics
No data on pesticides for Eastern Kingbirds, but Western Kingbird (191) and Cassin’s Kingbird (192) bioaccumulate moderate to high levels of organochlorines. Pesticides likely acquired in Latin America (see 193 for hirundinids). Pesticide accumulation is possible source of egg and nestling mortality and should be studied. Rothstein (170) noted high incidence of hatching failure in kingbirds, and Graber et al. (98) noted many broods of only 1 or 2 young (suggesting poor hatching success). Eastern Kingbirds nest often in orchards, where pesticide use is still fairly common, and migrate through and overwinter in areas where pesticide use can be heavy. In New York, 1 or 2 clutches annually (out of about 80) exhibit complete hatching failure (MTM). Although other causes of mortality must be examined (e.g., chilling or overexposure to sun), biomagnification of pesticides is a possible problem. Heavy metal accumulation in association with nesting on acidified lakes of possible concern; nestlings near acidified lakes had elevated levels of mercury (168).
Collisions with Stationary/Moving Structure or Objects
Collisions with automobiles are likely an important cause of human-induced mortality in North America (MTM). Eastern Kingbirds often nest and/or forage near roads, and collisions with cars are not uncommon as birds pursue prey or fly to nests to feed young (MTM). Out of 40 native passerine species, many of which are more abundant than the Eastern Kingbird (e.g., Tree Swallow, American Robin, Red-winged Blackbird [Agelaius phoeniceus]), kingbirds ranked as the thirteenth most frequently killed species along an Ontario roadway (181). Collisions with television and radio towers rarely occur and thus not a significant cause of mortality because Eastern Kingbirds are primarily diurnal migrants (see Distribution, Migration and Habitat: Migration). The absence of Eastern Kingbirds from lists of species killed at buildings in Manhattan, New York (251 strikes; 194), 2 locations in western Illinois (415 strikes; 195), Cleveland, Ohio (271 strikes; 196), and Toronto, Ontario (3,924 strikes; 197) indicates that window collisions are not a source of mortality.
Degradation of Habitat
The most likely cause of recent population declines in eastern and central North America is loss of habitat resulting from human development, forest succession, and change within agricultural landscapes (198). Declines in the number of small farms over the last several decades has probably affected kingbirds negatively because of loss of open space and the small orchards and fencerows used heavily by many bird species, including Eastern Kingbirds (128). Likewise, forest succession has resulted in loss of significant habitat. Declines in center of range possibly related to expansion of industrial agriculture. Eastern Kingbirds will forage in row crops (117, 118, 119), but clean practices that remove perches and trees used for foraging and nesting, respectively, make agricultural habitat unsuitable. Moreover, losses of hayfields and pasture lands in states east of the Rocky Mountains associated with negative population trends in Eastern Kingbirds (198). Shelterbelts and evenly lightly forested riparian environments are critically important habitats for Eastern Kingbirds in the west (MTM) and must be protected. It is unclear if habitat loss in tropics (199) has negatively influenced kingbird populations to date, but should be monitored closely as deforestation continues.
Studies of lake acidification in Ontario indicate negative consequences of acidification for Eastern Kingbird reproduction detectable, but small in comparison to other extrinsic factors and differences among females. Eggs from nests near acidified lakes had thinner shells and lost more water, but hatching success was not impaired (168). Nestling growth (tarsus length) poorer for nestlings near acidified lakes (168).
Adult Eastern Kingbirds are very tolerant of human intrusions to nest. Nest visitation may increase nest failure rate because corvids track researcher activities, but no data exist. However, nest visitation, handling of young, and capture of breeding adults almost never causes nest abandonment. Drawing of blood from adults does not negatively affect survival (179). Return rate (61%) of migrant individuals carrying archival geolocators (4 of 10 in Oregon, 8 of 10 in Nebraska, 1 of 2 in Oklahoma, 6 of 9 in New York) is well within the range of typical annual survival; 6 of the 31 Eastern Kingbirds completed 2 full migratory flights while carrying geolocators (MTM).