House Wrens are complicated, mysterious cheaters

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I got my start in ornithology studying the love lives of House Wrens. House Wrens pair up to raise their babies in a manner compellingly analogous to the human “nuclear family;” but, like most birds, both partners also often “cheat” on each other (i.e., copulate with other birds). This means that the male wren may have chicks in other nests besides his own, and he may end up caring for chicks that are not biologically related to him. (Note: edited. The original version of this sentence had a mistake.)

This sets up a number of interesting questions, such as: why cheat on your partner? Are the chicks sired by outside birds somehow better? Do males know when they are caring for chicks who aren’t their own? The answer to the latter question seems pretty clear (no, the males do not know), but the former two are more challenging.

I spent the summer of 2009 (ooof, let’s all pretend that doesn’t sound like such a long time ago…) trying to address those questions in House Wrens. I was interested in how a chick’s paternity—whether its biological dad was its mother’s mate, or not—might affect its growth.

House Wren chicks need to grow quickly. They hatch out weighing about 1 gram, and they have about 12 days to reach 10 grams and grow all of their feathers.

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The contents of nestbox 18: four chicks hatched earlier that morning, one unhatched egg, and one egg (lower right) actually in the process of hatching.

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The just-hatched chick, still wet from the egg.

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In 8 days, that chick needs to look like this.

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In 12 days, that chick needs to look like this.

Some studies had found that chicks who were the product of their mother’s cheating grew larger than their half-siblings. This suggested that those females might be seeking out high-quality males to cheat with, and passing on those better genes to their chicks. But the issue was far from settled—some other studies had found no such effect. So I spent the summer weighing the wren chicks every day or two, and when they were old enough, collecting a blood sample to use for paternity analysis.

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Weighing a young chick.

It was a lot of fun. There’s not much like the tactile sensation of an entire brood of baby wrens squirming in the palm of your hand; and the satisfaction of being able to replace them safe in their nestbox as if I had never been there—often before the parents even noticed my presence—was enormous.

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As I worked with the wrens, I noticed something important: there were runts. Most chicks in a nest would hatch on the same day, but sometimes there were one or two chicks who hatched more than a day later. These chicks perpetually lagged behind their siblings in age (of course) and size.

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A brood of American Robins illustrating this: the chick in the middle looks about two days younger than its siblings.

This size difference is potentially a big deal, because previous work has shown that wrens generally all leave the nest when the oldest chicks are ready, regardless of whether the smallest chicks are also ready. If the smallest chicks are too small when their siblings leave, they might leave the nest before they are able to escape predators; or they might not leave the nest at all. This happened at one of my nests, in fact: a particularly small wren chick did not leave her nest when all of her siblings did. I found her dead in the nest a week later. This is the potential cost of being a runt.

Excited as I was about these questions—paternity, growth, and runtiness—my data proved tricky to analyse. I was delayed by statistical questions, and then distracted by my new junco research. My other house wren papers came out, but this one stayed on my computer, a perennial first or third or eighth draft.

When I finally cracked the statistical issues, the story the data told me was a complicated one. Chicks that were the product of cheating were not any heavier than their half-siblings. However, chicks with late-hatching “runt” siblings were heavier than chicks without any runt siblings. Being a runt is a disadvantage for the runt, but it seems to be an advantage for the runt’s siblings. There are a few possible explanations for this; for example, it may be that the parents bring food based on how many nestlings are begging, but that the runts are especially bad at competing with their big siblings for that food, so the big siblings end up getting extra food when the runt fails to secure its fair share.

Here is where it gets complicated: that benefit of having a runt sibling was greater for product-of-cheating chicks than for their half-siblings. This is a peculiar result, with too many possible explanations to get into here; but the important point is that there does seem to be something different about the chicks that are the product of cheating. They aren’t straightforwardly heavier, but they are different.

I joked to my students this fall that much of my research has been based around discovering things that do not explain cheating in birds. This paper doesn’t quite change that, but it does suggest an intriguing piece of the cheating puzzle.

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Another fun part of this research: to tell the chicks apart so that I could consistently match weights to individual chicks, I colored their legs with nontoxic marker.

Reference:

LaBarbera K, Cramer ERA, Veronese D, Lovette IJ. 2017. Paternity moderates growth benefit to house wren nestlings of having an asynchronously late-hatching nestmate. Behavioral Ecology and Sociobiology 71:53. doi:10.1007/s00265-017-2283-7

Full article available to view at: http://rdcu.be/px2D

19 thoughts on “House Wrens are complicated, mysterious cheaters

  1. Fascinating and congratulations on another paper.
    You said This means that the female wren may have chicks in other nests besides her own, and the male wren may end up caring for chicks that are not biologically related to.
    Is that the right way round? Do cheating females lay eggs in other nests? If males cheat, do they then feed chicks in the nest of the female they cheated with as well as their own? Is the one certain thing that all chicks in a nest are from that female, even by several males?

    • Whoops! Thank you for this; you’re right, and I’ve fixed it. Females may very rarely lay eggs in other nests (“egg dumping”), but that’s not what I meant to bring up. Males both have eggs in other nests AND may care for unrelated chicks.

        • Yes! That is indeed the interesting connection. Many more species are facultative brood parasites than you would think. The question is, what triggers (or favors) the switch to obligate parasitism? A crucial step would be the transition from seeking hosts of your own species to seeking hosts of other species.

          I’m actually waiting to hear back from an application to a postdoc about brood parasitism. It’s a favorite topic of mine.

          • That’s funny. I can safely say that brood parasitism creeps me out and kind of makes me angry. Yeah, weird. I know and clearly anthropomorphising.

            That brings to mind a passing question- if a species is parasitic, are they exclusively parasitic or would they, or at least the female, be able to hatch, feed and raise her own clutch? Nest building could be less important since they could maybe use one someone else is done with.*

            * Last year a messy-ish nest was built on our porch light and I think a clutch was raised. We often used a side door to not upset them; what I saw looked Flicker or Mourning Dove size. I never looked in the nest. The nest was tidied- new birds that looked like robins moved in- eggs laid and one was pushed to the edges of the nest and a parasite was laid. I really wanted to take that out of there but didn’t. We’re in the Front Range of Colorado.

          • A lot of people share your feeling! I have a fondness for brood parasites because they lead to a lot of interesting evolutionary dynamics (such as how many brood parasite chicks evolve to mimic their hosts’ chicks), and also because it strikes me as a bit funny how much trouble they go to to avoid parental care. I’m not convinced it’s that much easier to find 30 nests, watch them until the timing is JUST right, and lay an egg in each one, than it is to lay 4 eggs on your own schedule and then raise them.

            Regarding your question: depends on the species. Some species are obligate brood parasites, and cannot raise their own eggs. Others are facultative brood parasites, and can both parasitize other nests and raise their own chicks.

            Even non-brood parasites will sometimes use abandoned nests of other birds; I’ve heard that juncos love old robin nests, although I’ve never seen it myself. It would be pretty funny if a robin took over a dove nest as you suspect: doves make really messy, minimal nests while robins build pretty nice nests.

          • I use my phone too much for blogs and WordPress doesn’t work as advertised. The 2nd nest was very tidy but the house was painted so there wasn’t a 3rd iteration. The painters put it alongside the house and I wonder sometimes if a clueful person could identify it.

  2. Its amazing how the chicks grow so quickly,Unlike us we grow quite slow compared to these birds! I wonder if the cheated on male ever gets into fights with the male the female cheated with?

    • The males will certainly fight if they encounter each other – the territory-holder will fight any male he catches in his territory. The cheating male (and the female) have to be sneaky to avoid detection.

  3. I have a question. My husband cleaned out the nest box that Carolina wrens housed this year. In the box he found underneath a bunch of sticks….two bodies in a nest…I am guessing were about 8 days old by looking at the photos provided. What are the possibilities as to why they did not survive?

    • I’m afraid I won’t be able to pin anything down exactly, but if you found bodies then they weren’t eaten by a predator (the most common cause of chick mortality). Carolina wrens almost always raise more than two chicks, so whatever happened, it probably didn’t happen to all of the chicks; this also rules out possibilities like the parents dying, since in that case I’d expect there to be 4-7 dead chicks in the nest.

      It may be that these were “runt” chicks who didn’t grow as fast as their siblings, or that they had something wrong with them, and that the parents abandoned them when the bigger/healthier chicks left the nest. I’ve seen that happen with house wrens. Brown-headed cowbirds often parasitize Carolina wren nests, laying an egg which hatches into a very demanding cowbird chick; it’s possible that there was a cowbird chick who monopolized the parents’ feeding so much that two of the wren chicks starved.

  4. We watched a pair of Carolina Wrens build a nest about 30″ above ground in an empty open top planter. 4-5 eggs were laid a few days later. About the same time, one of the wrens showed up with a missing tail…we found several long feathers scattered around the yard, a couple quite a distance from the nest on the other side of the house. Next day one of the eggs lay on the dirt outside the nest with a hole punctured in it. Next day another egg was on the dirt, but was whole…two or three remained in the nest. We didn’t notice either of the parents for several days, but could hear loud calls. One day the one with the tail appeared briefly then flew away. Another day or two went by and one of the remaining eggs was smashed on the walkway. The last one disappeared a day later. There were never any signs of a chipmunk or squirrel scruffing up the dirt or nest. Could it have been another bird?

    • It certainly could have been a bird; the egg with the hole in it is especially incriminating, and makes me suspect that another wren did this. (Other birds may kill eggs in order to eat them, like jays, or kill them by smashing them, like house sparrows, but the hole strongly suggests a wren to me.) They may do this is order to try to take over the nesting spot, if they want it for themselves; or they may do it in the hopes of courting the female wren and making her be their mate, instead of her original mate. (When I was studying house wrens, I went to check on a nest one day and found all of the chicks dead with neat holes in their skulls – certainly the work of another wren.) The missing tail and tail feathers suggest a scuffle, probably the parent wrens trying to defend their nest. Wrens can be quite fierce!

      • Thanks…I did suspect another bird because of the unique spot they chose to nest…almost inaccessible to a rodent…well, unlikely at least.
        I am always on the lookout fir sparrows and was advised to kill them or their eggs/chick’s because they are destroying/chasing our songbirds away.
        We have a nest of House Wrens on opposite side of house and they are extremely territorial, aggressive and crabby. They’re on their 2nd brood of the season and are all over the yard. They nest yearly in the same Japanese brazer we have on a shelf by the front door…They use the same one…I rarely clean it out…should I? I did see a tiny egg smashed on the walkway there…identified it as a House Wren egg by pictures. Catbirds are nested nearby them in the nest formerly occupied by Robins until they fledged about 2 weeks ago. Cardinals are in 2 places on property and are very crabby if we stand near or walk by…happening often because they are in the tall yews at the end of driveway by the back walkway.
        This year we have a pair of Baltimore Oriels nesting in the tall pines or oaks along our property. I put out an oriel feeder (mandarins oranges and grape jelly which they haven’t discovered yet…or don’t like the location).
        Our hummingbirds are quite timid and I’m jealous when I see pictures of people feeding them from their hands. They tolerate us sitting on the patio 8-10 feet away as long as we don’t move.
        It’s great connecting with a bird expert!
        Thanks for your response.
        Suzanne

        • Wow, it sounds like you’ve made your property into fabulous bird habitat! Your orioles may be less excited about the feeder right now because they’re focussed on finding bugs to feed their chicks; generally birds are more attentive to feeders when they’re not actively breeding. Although once the chicks leave the nest, the parents might bring them to the feeder.

          Here’s a good source for the question of whether to clean out old nesting material: https://nestwatch.org/connect/blog/to-clean-or-not-to-clean-your-nest-box/

          It is indeed legal to do whatever you want with house sparrows, which are invasive and pretty aggressive toward native birds. (It’s not *required* to kill them – I’m a soft heart and wouldn’t – but it’s reasonable and defensible from a conservation standpoint.) However, if you ever do, make sure you’re certain of your species ID! We have lots of lovely native sparrows, many of them declining, and you wouldn’t want to accidentally hurt one of them.

  5. I have a wren nest on my porch. They are lovely little babies in a hanging plant and away from predators. I have peeked and watched them grow. We had a large dying tree cut down in the front yard…away from the nest, but there was obviously commotion. I haven’t seen the parents since the tree people came and I am so worries. It’s been 12 hours. Is this nature, and I leave them alone? I am a fisheries biologist, so I have no experience with young birds. The are feathering out but still small. I feel helpless. Your material is fascinating, so I thought I would ask an expert. I am selfishly in love with these little guys…Thank you.

    • Hmm, I’d be surprised if the parents got permanently spooked by that at this late stage in the nest; usually they’re loathe to abandon the nest when chicks are older. Still no sign of the parents?

      If the chicks have really been abandoned, they won’t survive at this stage. Do you have a wildlife rehabilitation center you could get to? (Some are closed due to covid, but many are not. If you’re in a major metropolitan area there is almost certainly one in the area.) They would be best equipped to raise the chicks. If you’re sure the parents are gone, getting the chicks to a rehab center ASAP is the best option.

      If that’s not possible, as a last resort – and I should say, technically illegal – you could try to raise them yourself. They will need lots of insect food, which you can find at pet stores: mealworms, killed (by drowning or smooshing their heads), crickets (also dead), and waxworms (live is okay) are all good. They’ll be a 3-4 week commitment at this stage. For the first week or so they should be fed every 1-2 hours during daylight. Then over time you can cut back, and start putting food in the cage for them to pick at and experiment with. It will take them a good while to learn how to forage for themselves: they’ll be flying before they’re proficient at foraging.

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