Bluebird and Small Cavity Nester Conservation
Sialis - Bluebirds and other small cavity nesters
 
bluebirds

eggs

House Sparrow
Proliferation

Male house sparrows. Photo by Bet Zimmerman
House Sparrow Eggs. Photo by Bet Zimmerman.

House Sparrows "average three broods per mating season with each brood containing four to seven eggs, with 20 offspring a year average. ... If unchecked, a breeding pair can grow to over 2,000 birds in two to three years." (From Bird Barrier America, Inc., http://web.birdbarrier.com/BirdBarrier/Site%20Pages/Sparrow_id.htm.)

Densities may be as high as 3108-3367 birds/mi2 (1,200 - 1,300 individuals/km2) around dwellings associated with livestock; average of 518/mi2 (200/km2) in rural areas (Dyer et al. 1977). 

House Sparrows probably live 1-3 years in the wild, to a maximum of 13 years. (McLoughlin indicates that one captive sparrow lived 23 years. According to a UMBC Bluebird Trail paper, existing records suggest that bluebirds can live up to 10 years in the wild, although the average life span is 3 to 7 years.)

I did some math (note that I have not validated these numbers scientifically.)

Assuming:

  • average of 20 chicks per year per pair
  • 50% of chicks survive (I've seen statistics of 40-60% mortality for HOSP)
  • Half of chicks are female, half male (so 2 birds=1 pair), and they don't mind inbreeding
  • they don't mate till year 2 (although they can reach sexual maturity at 9 mos. per McLoughlin)
  • they only mate for one year and then die

Then THEORETICALLY:

2 birds (one pair) could quintuple the population in one year, and could theoretically multiply to 1,250 birds in 5 years.

Year 1: 2 birds (one pair: one male, one female)
Year 2: 10 birds (first pair died, but produced 10 offspring that lived = 5  new pairs)
Year 3: 50 birds (5 pairs bred, produced 10 chicks per pair)
Year 4: 250 birds (25 pairs from previous year x 10 chicks per pair)
Year 5: 1,250 birds (125 pairs from previous year x 10 chicks per pair)

I don't even want to THINK of the worst case scenario.

Most people do not quit fighting Asian beetles, rats, cockroaches, fleas, mosquitoes, mold, dust, or grime by saying "Oh, there's so many of them, that nothing can be done." Persistence will win.
- Dottie Rooseboom

It is a battle, and a battle that can be won.
- Paula Ziebarth

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Last updated May 5, 2022. Design by Chimalis.

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