By Lauren Flesher
They’re stealthy. They’re quick. They have far too many legs. They’re spiders, and they’ve been vilified for centuries. They’re not all bad, though. Spiders perform a great many ecosystem services, from agricultural pest control to disease prevention. They’re especially beneficial to native birds. Here are some key reasons a true bird lover should leave that spider alone.
Spiders as Bird Food
Spiders are like an energy bar for birds—the bite-sized packs of protein are incredibly nutritious. This is especially important during migration, when birds are physically taxed and need much more food to keep going. Many species of smaller birds, particularly hummingbirds, will also take advantage of a sticky spider web and steal the captured insects from the silk. It’s the bird equivalent of a convenience meal!
Spiderwebs and Nests
Many species of birds use spider silk when constructing their nests. It has the advantage of being strong while also being flexible. As the nestlings grow, the nest can stretch with them, and the silk allows for easy repair in the case of damage.
Spiders as Nest Pest Control
House Wrens have discovered an ingenious way of keeping mite levels under control. While the nests of other birds are ridden with the parasites, the House Wren places spider egg sacs inside her nest. When the egg sacs hatch, the baby spiders will have an abundance of mites to eat, and the House Wren nestlings will be thankful. Talk about a win-win.
More About Spiders and Birds
Spiders aren’t always a bird’s best friend, though. The Goliath Birdeater Tarantula does not prey primarily on birds, but it won’t pass up the opportunity if a bird stumbles onto its path. Thankfully, it can only be found in the northern swamps of South America, but even in the United States, bird-eating spiders exist. Songbirds will occasionally become stuck in spiderwebs and die there, because their struggles to escape only further entangle them. Sometimes a spider will cut the bird free, then repair its web.
Because web repair is costly in time and in nutrients, spiders such as the garden orb weaver will weave a visible pattern into their webs, called a stabilimentum. Many arachnologists believe the stabilimentum serves as a warning to birds and other large animals that the web is there, thus saving the spider the trouble of repairing it when a large animal comes crashing through. It has been documented, however, in rare cases, that a golden orb weaver will take advantage of a snared bird, encase it in silk, and eat it. These predation methods are mostly opportunistic, and always involve smaller birds such as hummingbirds and kinglets.
When it comes to spiders, we may want to squash them on sight, but clearly the benefits of having them around outweigh the dangers. So take the time to hug your creepy-crawly neighbors today…or maybe not.
Is it a good idea to imitate the wren and introduce spider casings, (or spiders), into nests we are monitoring, such as an Eastern Bluebird nest? It sounds like it would be a more wholesome solution than using pyrethrin, which I have read is recommended for parasite prevention in a nest-box. The pyrethrin solution I have emits such a pungent odor that I stopped using it, fearing the effect the fumes would have on the young.
Thanks! Interesting article.
Hi Paula. I don’t usually see mites in my bluebird boxes; normally just the blowfly larvae which are too big for baby spiders to eat. But even if you do have mites, NestWatch doesn’t recommend introducing anything into a bird’s nest. We recommend just letting the birds handle it, as the parasites aren’t usually lethal. Nature knows best.
Thank you, Robyn. You’re right, I think the second and third broods get the blowfly larvae in our boxes, but mites are not likely a problem, thankfully. Seeing these larvae in the bottom of the nest was a cause of concern to me at first, but the bluebirds have fledged many a healthy chick without the use of pyrethrin.
Is it the male or the female house wren who adds the spider egg cases to the nest?
Hi Chris, Male House Wrens typically build several nests for females to choose from, and spider egg cases have been recorded in these male-built nests. However, in a study from 2005, the presence of these egg sacs did not enhance a male’s ability to attract mates, nor did they affect the number, mass or condition of offspring produced from the nest, so it’s still unknown why the males might add them to a nest (when only one that he built may actually host young/have mites). However, after pairing, the female largely takes over nest-building for her selected nest, first finishing the stick platform and nest cup – at which point she may or may not add egg sacs, then lining the cup with soft material. In short, either sex may add spider egg sacs to the nest.
I was wondering about these tunnel spiders that seem to be all over the wooded grounds in Esst Texas. I have one in my planter that the wren is in nesting on her 6 eggs. The spider is black and as large as her head. He is very allusive so I can’t get a photo. Are these okay?
Hi Debra, There are few bird-eating spiders, and none are known to live in North America. I’m not sure of the exact species you’re seeing, but most spiders in North America typically eat insects, and this one should not be harmful to this nest. In fact, spiders are a favorite food of Carolina Wrens (though, we agree such a large one is unlikely to be on her menu).
Hi – Wondering if you could please forward to me the 2005 reference that is mentioned above in a post about testing hypotheses of using spider mite egg cases in wren nests?
Hi Tom, Here is the reference for that study: Eckerle, K. P. and C. F. Thompson. (2005). Addition of arthropod cocoons to House Wren nests is correlated with delayed pairing. Behavioral Ecology 16 (1):1-7. https://doi.org/10.1093/beheco/arh122
I have a spider living in the top of my nest box. Will it deter birds from using the box? If so, should I remove the spider?
Hi Phil, Since spiders are food for many birds, it’s unlikely that the birds will mind, but it’s also hard to predict (there are no studies I know of that have looked at birds choices between boxes with/without spider nests). It’s probably fine to leave the box as is. If the box isn’t having luck this season, then you can always attempt to remove the spider later – but be careful to wear gloves to protect from any potential bites.