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.