In the column “Since You Asked” in every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior. Here’s a question from our October 2015 issue:
When cleaning out my nest boxes after the breeding season, I found numerous small white orbs in a box used by House Wrens. What were they? — Dawn Ware, Nashville, Tennessee
They were spider-egg cases. A male House Wren typically constructs several nests that are inspected by potential mates. (The female chooses the nest and completes building it.) The male commonly places spider-egg cases in the nest during the initial construction.
It has been thought that when the spiders hatch, they eat mites that might otherwise parasitize the nestling wrens, but this has been observed only in the lab. Experiments have indicated that the number of mites does not differ in nests with or without spider-egg sacs.
Another theory is that the orbs serve as ornamentation intended to tempt a female to choose a particular nest (and mate), but one study found that males that put spider-egg cases in their nests actually took longer to acquire a mate.
So far, researchers have not been able to find any benefit to having spider-eggs sacs in the nest for either adults or young wrens. The exact purpose of the behavior remains a mystery.
About Julie Craves
Julie is supervisor of avian research at the Rouge River Bird Observatory at the University of Michigan Dearborn and a research associate at the university’s Environmental Interpretive Center. She writes about her research on the blog Net Results, and she maintains the website Coffee & Conservation, a thorough resource on where coffee comes from and its impact on wild birds.
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