The common House Sparrow is actually also facinating
Copyright: Johannes Erritzoe, 1996
As is well known the male of the Common House Sparrow has a black throat,
here called the bib. Through field studies Anders Pape Møller,
now professor in Paris, ascertained that males with large bib dominated
over males with a smaller bib both when they were seeking food and during
breeding, where the females so evidently preferred the former. He also
found that these males more often had an affair on the side in their otherwise
monogamous pairbond than the less lucky males with a smaller bib. (ref
It has often happened that a scientific result has lain unnoticed for many years considered as a mere curiosity, and then one day a new researcher digs it out into the light, because he or she can use just this information in another connection. So it also happened here. Through a study of the literature two interesting papers were recognised which both might perhaps be used to answer the question araised above:
A reasonable explanation starts to take form in the fog, but how can a female see whether a particular male if he has small or large testes ? They are, as you surely know, placed in the middle of the body.
About the middle of the sixties Johannes Erritzoe collected one thousand House Sparrows in Denmark, which were all skinned (the skin mounted in a position like a dead bird, without glass eyes but with many data recorded). This collection is unique because of the great number of skins and the uniform way all the birds have been mounted. There is said to be no more than five corresponding comparable collections in the world today (A.P.Møller in litt.). It was obvious to test this hypothesis on this collection.
All the males from the breeding season were sorted by their dates of death. The outmost length and diameter of the bib were measured and compared with the information on the labels about the size of testes and the total weight of the Sparrow, information noted for all skins, and actually the result was that males with the largest bib also had the largest testes !
Encouraged by this success, the speculation started about whether there could be an advantage for the female in choosing a male with large bib and testes. Together they have to work hard when up to 3-4 clutches young must be nursed. For such a job a beautiful bib is not enough, here a strong and healthy partner must be important. What about e. g. one of a birdīs greatest nuisances, parasites, which can weaken a bird and often cause its death ? And before it dies the parasites will be spread to the other parent and the chicks !
The immune system of birds has until today been studied very little. A work on the Barn Swallow has shown that males with the longest tail feathers (these are the status symbol among the Barn Swallows) seemingly have a better immune defence than other males. (ref 6+7) A gland in the immune defence is called bursa Fabricii. It is placed near the anus and it plays a central part as an antibody synthesizer in young birds. It disappear before the bird is sexually mature.
All the wing feathers were checked for "fault bars", transparent cross lines on the feather, which arise from starvation or other stress from the surroundings when the new feathers are growing. This examination showed that males with large bibs had fewer fault bars, were healthier and had fewer ectoparasites (some of them make holes in the feathers), and therefore also had a smaller bursa Fabricii, whereas the hard suffering individuals had a large bursa Fabricii and a small bib !
The puzzle was solved ! When the female chooses a male with a large bib, she is not only sure that he has large testes, which ensures that she lays more fertile eggs, but she is also sure that he is more resistant to parasites, and his better health will be a great advantage for her, and her offspring will inherit better genes. (ref 8)
But the story about the bib of the male House Sparrow does not end here. It is well known that birds moult their feathers every year, some even twice, when they have a breeding dress different from the winter plumage. Like e. g. Reed Bunting and Snow Bunting males House Sparrow males developed another artful and more energy-conserving moulting strategy: in autumn, when the new feathers are growing out, the feathers on the bib have a grey edge which completely hides the otherwise black feathers.
It is no longer correct to ascribe human qualities to animals (with a fancy word this is called anthropomorphism), but let us for a short moment forget this and try to put ourselfs in the place of such a little House Sparrow male, one of those, however, that in a few months will have a large bib and dominate the whole gang.
In winter field observations Anders Pape Møller saw that some males spent more time preening their feathers with their hard bills, in particular the throat feathers. This gave rise to the idea of studying the sparrow skins once more. The males were sorted chronologically and the grey edge of the throat feathers were measured at three different points.
Copyright: Johannes Erritzoe, 1996