Keith Kridler of TX asked an interesting question.

If House Wrens are so adept at removing eggs and newborn chicks from nestboxes of bluebirds, why have they not exterminated House Sparrows in yards where people do not monitor their nestboxes?

I would guess that:

  1. There are so many House Sparrows (HOSP) that House Wrens (HOWR) can not make a dent in their population
  2. Lots of HOSP do not nest in cavities, which is where HOWR prefer to nest.
  3. Male HOSP are FABULOUS about defending their box, rarely leaving it unattended. (I think bluebirds can learn the importance of this too after losing some eggs).
  4. BOTH the male and female HOSP incubate the eggs (unlike bluebirds and Tree Swallows, where only the female incubates) so the eggs are rarely unattended.
  5. HOSP have a longer breeding season than HOWR, so they have time to get their numbers up.
  6. Maybe HOWR are afraid of that crushing HOSP beak?

More Information:

We tend to forget that these are wild creatures living in a harsh world.
Only the smartest and strongest are expected to survive and raise young.
Anything less and the species as a whole becomes weak and soft and they WILL be replaced with a stronger more adaptable species.

– Keith Kridler, Bluebird_L 2009


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