An ecosystem is a fragile, complex web of life. Introduction of new animal or plant species can wreak havoc on its natural balance. When those new species are invasive, they choke out native forms of life, wiping out the diversity that makes for a healthy system.
Less than 200 years ago, there were no House Sparrows in North America. Today, they are one of the most abundant songbirds on the continent, with an estimated 150 million established in all 48 states. House Sparrows (Passer domesticus, or HOSP, sometimes called English Sparrows) have interfered with or displaced a number of native birds like bluebirds, Tree and Cliff Swallows, and Purple Martins. Invasive/nuisance species like House Sparrows generally share the following characteristics that enable their populations to explode.
Reproduce rapidly: HOSP are prolific breeders. They build nests very quickly (1-3 days) and will reuse nests. Each breeding season, they may raise 2-5 clutches, averaging 20 chicks a year. Using some conservative assumptions, one pair could theoretically increase to more than 1,000 birds over a five year period.
Effective dispersal mechanisms: In the case of HOSP, humans were the dispersal mechanism. Accounts differ, but it appears that repeated introductions occurred in various parts of the U.S. from 1850- 1874. Reasons given were to establish wildlife familiar to European immigrants, or to control insect infestations. Unfortunately, in agricultural areas, only about 4% of their diet is insects, and the remaining 96% is livestock feed, grain and weed seeds. HOSP have no recognized migration pattern, but flocks of juveniles and non-breeding adults may move 1-5 miles to new feeding areas. Not being exposed to the perils associated with migration may actually increase their survival rate.
Rapidly and easily established: HOSPs are fairly hardy birds. Unlike many birds, they eat 830 kinds of foods. They live near humans, which provides a ready source of food and nesting sites. While they prefer to nest in cavities such as a birdhouse, they will nest in protected locations such as rafters, gutters, eaves, soffits and vents, dense vines, evergreens and shrubs, commercial signs, and behind or above pipes and ductwork. Nests are often in 8-30 feet off the ground, and they forage in flocks, which affords additional protection from predators. Unlike bluebirds or Tree Swallows, they will nest in close proximity to others of their species. Because they are so adaptable, they are extremely effective at territorial control and range expansion. They live in Death Valley and in the Colorado Rockies. The only areas where HOSP are not usually found are dense forests, grasslands, alpine regions and deserts. House Sparrows even lived and bred in the coal mine shaft in England that was 2100 ft. below ground level.
Grow rapidly: HOSP eggs hatch in 11 days, birds fledge when they are 14 days old, and young are independent about a week after leaving the nest. They quickly reach sexual maturity (as early as 4-9 mos.). Compare this to the timetable for Eastern Bluebirds: 12-18 days incubation, 12-19 days to fledging, and independence a month after leaving the nest. Bluebirds do not breed until the following year.
Aggressive competitors: HOSP begin nesting in late winter and early spring, beating other migratory native birds to preferred nesting sites. They are aggressively territorial in their attachment to a nest site. They use their powerful crushing finch beak to destroy eggs, nestlings and parents of other birds, and to attack occupants of nearby nestboxes. They will overwhelm a bird feeder. They are persistent and fairly intelligent. A fast food restaurant in Australia had a set of doors that opened automatically when an "electric eye" was tripped. HOSP learned to hover in front of the electric eye until the door opened.
The combination of these factors has resulted in a very successful infestation of these "rats with wings.” In the next column, I’ll talk about methods that can be used to prevent HOSP from breeding in your birdhouses.