Sunday, 30 July 2017

Variation in size in Barn Swallow nestlings

     The second broods of Barn Swallows (Hirundo rustica) are now well advanced and we have banded the birds from numerous nests already.
     Recently, we extracted six nestlings from a nest and noted a significant size difference between the birds. Three were distinctly bigger than the other three, feather development was more advanced and it appeared that two different hatch dates were involved.  Judging that the smaller birds were too small to band, we banded only the three larger birds. Unfortunately we did not weigh them, being more concerned with getting the birds back to the nest as quickly as possible.I should add here that we have adopted the practice of returning the young birds back to the nest individually, as soon as we have processed them, rather than waiting until all the nestlings have been dealt with. In this fashion, they seem to find their place in the nest more readily and align themselves with each other in a way that does not threaten any of them being pushed out of the nest. (See picture below of the nest when all occupants have been returned).

     I mused to Kevin at the time that egg dumping might have been a factor in this size discrepancy, although I was unaware of this practice which is common among other families (waterfowl, for example) being noted in Barn Swallows. 
     Today we encountered this phenomenon again. The pictures below are not of great quality, but you can clearly see that the bird at the bottom left is substantially smaller than its siblings, with feather development less advanced.

     In fact, here are the weights of the five nestlings: 9.3 grams, 18.7 grams, 17.3 grams, 19.0 grams, 16.7 grams. The slight variation in the bigger four is easily explained by normal factors in nestling development, such as some youngster being more aggressive and getting a higher proportion of feedings from the adults, but the fifth bird is clearly in a different category. Furthermore, it was healthy, squirming around in my hand when I returned it to the nest, gaping right away for food - and it was promptly fed by one of its parents. There was nothing to indicate anything other than a robust youngster, less advanced than the other nest occupants, but entirely commensurate with the progression we have observed in other birds.
     Here is information gleaned from the literature which might shed some light on this condition.
     From Swallows & Martins (1989), Angela Turner and Chris Rose:
     Barn Swallows are nearly always monogamous, but there have been records of males pairing with two females  and, in colonies, males often copulate with females other than their own mate.
     Thus, it is possible that a second male may have inseminated a female after she has been incubating for several days which then causes her to lay an egg which hatches later than the initial clutch.
    From Handbook of the Birds of the World (2004), Volume 9, Josep del Hoyo, Andrew Elliott, David Christie, editors.
     Socially monogamous, occasionally polygynous, extra-pair paternity common, c. a third of nestlings in European studies, 22% in Canada (Ontario). Egg dumping by conspecifics occurs (3% in Spanish study, with nest-owner male sometimes the father.)
     Again, it is possible then, that a male copulated with a female other than his mate, and that female deposited her egg in the nest while the host female was absent.
     If anyone has anything to add to this account we would very much welcome your comments.
     As a total aside, you can observe the amount of horsehair attached to the nestlings when we retrieve them from the nest. This material seems to be a bit of a mixed blessing. It is readily available and seems to be favoured by the swallows for nest construction but it can sometimes be lethal. We found one young bird hanging from a nest totally entangled in horsehair - and dead. On several occasions we have been able to disentangle birds whose movement was seriously restricted and doubtless would have starved to death, unable to move freely and obtain food.


  1. My own experience with horsehair is that very occasionally it can cause a young bird to become entangled in the nest or close environs. The fact that you are monitoring and examining the nests will perhaps reduce the chances of that happening since you will remove any loose material that might cause entanglement.

    More of a problem here is the fact that many of the new generation of horse owners do not like swallows near their horses. They claim the swallows spook the horses and cause a mess. So it is not unusuial for them to prevent swallows from entering stables or to keep removing attempts at nest building until the swallows give up. I just hope their stables become infested with insects that the swallows would otherwise consume.

    1. To say that the swallows spook the horses, or any other livestock for that matter is simply not true. There are horses in the barn where we do our banding and they are totally unconcerned. In fact, I have bemoaned the fact that the very first time I went in, when I didn't have a camera with me, two Barn Swallows were perched on the back of one of the horses. One cannot deny that they make a bit of a mess but a few minutes of maintenance each day takes care of that. The joy of having the swallows around is more than adequate compensation for "our" farmer. We are grateful for his enlightenment.

  2. Interesting post David, thanks for sharing. Diane

  3. Hello David!:) Although I have always believed these birds to be monogamous, your reasoning about the fifth bird, makes a lot of sense as to why there are sometimes much smaller birds in a swallows nest. They are packed tightly aren't they! I didn't know they used horse hair in building their nests. Do you know David, in Portugal we believe it to be lucky if a Swallow or House Martin makes a nest on our houses. I only wish the horse owners saw it that way were you are.

  4. I always learn so much by reading your posts and I appreciate it!

  5. This post was very interesting David. Thank you, I learned something knew today.

  6. I have so much to learn about birds. Have a good new week!

  7. What an amazingly informative post David, and thank you for droping by mine.
    All the best, Gordon.

  8. A fascinating post, David. Until I read your two 'references' my thoughts were along the lines of the second suggestion. I'd be interested to hear if you come up with any conclusive information.

    My love to you both - - - Richard

  9. Beautiful images David, and the egg boxes, excellent post.

  10. Good work. I always feel a bit unworthy to even comment on these kind of posts, because I certainly have nothing to add -- except admiration for your efforts and pictures -- and thanks for what I learn from you.

  11. Gran trabajo amigo mío, estoy aprendiendo mucho contigo. Gracias por compartir David, un abrazo desde España.

  12. Interesting!
    Often it's said that dog hair is one of the favourite nesting materials, but then it's also warned that the hair of a (really) long-haired dog can be dangerous.

  13. This was so interesting to read. Where we go in Wales there are a lot of swallows and they're like bats too quick to catch on camera. They just swoosh past you, not bothered by people or dogs! At home they don't come that close. I know a lot of people don't like the mess they make and disturb their nests which is such a shame.
    Have a wonderful week :)

  14. David, I just came over to say thanks for the invite, and you would be welcome to join us, any time you are in Portugal.:)

  15. Hello, interesting post on the swallows. I am thankful that these bug eating birds are doing so well there. You are doing great work with the Swallows. BTW, the Wood Thrush seems to be doing great breeding in the woods next to my house. I hear them singing often from my deck. Happy birding. Enjoy your day!

  16. Thank you David for chooseing to follow my blog.
    All the best, Gordon.

  17. Hello David,
    It is indeed a pity that 3 birds are so small and the other 3 have grown up. When feeding, the big ones are moving forward to get the parents' feed first.
    You did not weigh the 3 little ones but I hope these little swallows will get it.
    Dear greetings,
    Helma xxx

  18. Hello David,
    Beautiful and interesting post is this.
    In the spring, I will keep my dog's hair when trimmed.
    I put the hair in the garden and within 15 minutes everything is gone.
    I have a small dog, so also little hairs.
    Best regards, Irma

  19. Very interesting, thank you for the references!
    Very fun the egg carton for the nestlings. They stay warm and do not fall off.

  20. Hi David

    Fantastic pictures of this beautiful nature.
    Beautiful all the animals you show.

    Groettie from Patricia.

  21. You always provide interesting posts ... thank you.

    All the best Jan

  22. Maravilloso, que fotos tan tiernas, me encantan. Saludos.