On the swift’s scuttly wingless fly friend

May 28, 2015 • 3:52 pm

by Matthew Cobb

Earlier today I posted a video of a pair of swifts that met up for the first time in 9 months. In the comments, Mark Sturtevant pointed out that on the male (the front bird) you can see an ectoparasite scuttling about:


Here’s the video again. Go to 50 seconds, which is about the first moment you can see the parasite, as a white blob on the back of the male’s neck. It starts scuttling around and can be clearly seen at around 1:10.

Chat on the comments and on Twitter identified the beast in question as probably Crataerina pallida, a wingless fly like the wingless bat louse fly Jerry posted the other day. Morgan Jackso (aka @bioinfocus) has gone on to pose some interesting questions about this particular fly over on his website Biodiversity in Focus.

You should head over there to see the full argument, but Morgan’s starting point is that this species does not lay eggs, but instead gives birth to live maggots, that presumably overwinter as pupae in nest boxes:

This leads us to an interesting question of how this louse fly got onto this bird! The fly was already aboard the bird when it entered the box (if you watch closely you can see a white blob that moves around neck is first visible at 0:06, immediately after the male bird approaches the sitting female). This means that one of two things happened: either the male bird has in fact carried its little parasite friend down to Africa and back (something that neither Hutson nor Walker & Rotherham (2010) believe to be the case) (and assuming this was the first nestbox that the bird stopped in, which I take to be the presumption of the ornithologists who posted the video and stated it shows a male reuniting with its mate from last year in last year’s nestbox), or alternatively, the male bird did stop for a time in another nestbox where it picked up its little hitchhiker, and then proceeded on to its longterm mate.

Curiouser and curiouser. Even more curiouser, as we were chatting about this on Tw*tter, the BBC programme Springwatch showed a video of their swift cam, showing a brooding female, who also had one of these beasts scuttling about on her. Lewis Spurgin, who ID’d the beast on our post, said:


If you have expert knowledge on this, please chip in below, and on Morgan’s site!

19 thoughts on “On the swift’s scuttly wingless fly friend

  1. Why don’t the birds eat the hitchhikers? By which I mean… why don’t the birds groom each other for the noms?

    1. Only a guess here, but the fly might stay on the upper back where the bird cannot easily reach. If the fly detects a probing beak coming its way it would just scuttle out of the way. They seem pretty agile.

        1. It’s easy to forget that animals don’t “think” like we do: unless something triggers a genetic instinct in the animal, it might well ignore it. Some animals have the, “attempt to eat anything that moves” instinct, and some don’t. It’s also possible that the fly has some disagreeable substance in it that has, over generations, provoked a, “don’t peck that” response.

          1. Well, considering that we’re animals, that isn’t exactly right. But your speculation about something disagreeable seems plausible.

    1. I was prompted to search out the “other” lyrics to a well-known hymn :

      All things bright and beautiful,
      All creepy things that crawl …
      Some despised, some hungered-for,
      The Lord God made them all.

      Each pit-bull jowl that opens,
      Each killer bee that stings,
      God made their gnawing molars,
      God made their spiny wings.

      I feel like eating some rotten milk. “Cheese” to you. Shallow-fried locust? Anyone?

  2. I’ve found these on house martins too. When I was a kid, a house martin fell out of the sky, bounced off our house roof and landed at my feet, dead. It had several of these parasites on it.

  3. I have no expert knowledge, but I googled Crataerina pallida and took a look at the pictures. All I can say is, “It’s another triumph for Intelligent Design!” I mean, if you were designing a parasite (and what kind of designer wouldn’t?), wouldn’t you give it a pair of useless wings? Of course you would. The evidence for a intelligent designer just continues to pile up. Maybe not a brilliant designer, yes, but intelligent. Some kind of idiot god, perhaps, or god child… or some alien who was constructing our biome for a school project but put it off until the night before and had to improvise a lot. But, technically, intelligent!

  4. That the male had another nestbox in which it picked up the little hitchhicker raises the question about whether this one is keeping another female swift on the side.
    ‘Is that a louse fly on your collar?? Where have you been? You said you flew to Africa!’

  5. I don’t do the tw**er enough to even know how to “head over there and see the full argument”. I do wonder, though, why it is considered unlikely that the bird “in fact carried its little parasite friend down to Africa and back”. Sounds like a reasonable possibility to me. What do they think, it’s going to fall off? If the fly can live long enough for the journey, I don’t see any reason it couldn’t make it.

  6. I don’t get the assumption that the fly was already on the swift when it entered the box. Why couldn’t it have acquired it as it entered the box? From the point of view of flies competing for access to hosts, wouldn’t that be the ideal opportunity to climb aboard?

  7. It’s the “scuttly” part that’s creepiest!

    I suppose quick movement is an advantage to a bird parasite, given their (the birds’) extensive preening. Otherwise one thinks ectoparasites would want to hang on for dear life on these avian acrobats.

    It’s really amazing to think that such a large parasite wouldn’t have devastating effects on nestlings. Not that that would be terribly adaptive for the parasite.

Leave a Reply