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 1)
The question arose, why a female should prefer such a faithless male only because he has a larger bib ?
Is that a marvel of beauty for a little insignificant sparrow virgin, or is the explanation, as so often in nature, a little more complex ?

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:
Burrows and Titus wrote already in 1939 that the bigger the testes in chickens were, the more sperm was delivered in each copulation , (ref 2), and in 1974 two other scientists stated that the more sperm was spent in each mating the greater was the chance of fertilization, which for the female is of great importance because of the cost of producing an egg.
The chance of fertilization in a sperm-competition connexion is therefore greater for a male with large testes and he will fertilize more eggs than a male with small testes. For the female the result will be the very best if she colpulates with more than one male. (ref 3)

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 !
The solution of the mystery had advanced a little more. It was also a fine validation of the theory of Darwin, which stated that the most powerful survive and get most descendants. (ref 5)

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.
The bursa Fabricii was also registered in the above mentioned collection. Therefore it was also possible to check this hypothesis by means of the collection. The following feature was examined:

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)
On the reverse of the medal it must be noted that several other studies of various bird species have shown that males with the clearest coloured feathers participated less in bringing the chicks food than males with duller colours. (ref 11)

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.
In the course of the winter and spring this grey edge will be worn off, and the beautiful bib comes to light again in all its glory. This pause gives the males a much needed rest after the fatiguing breeding work when they do not fight with other males about the females' favour, but can concentrate on foraging and the very energy demanding moulting. Besides the more insignificant plumage offers better protection against enemies. (ref 9)

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.
I think he will keep to himself on the edge of the group, depressed at his own miserableness, and he will be thinking very hard about what to do so that he can soon get back on the arena to power, glory and lots of women. At least it would be very understandable and Sparrowlike if he think like that.

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.
Now it was possible to sort the birds week by week according to the wear of the feathers, and eureka ! It turned out that the males with the largest testes, which we already knew would later get the largest bib, these males had the greatest wear on the throat feathers, which would only be possible by their preening their bibs !
The males with smaller testes and therefore lower status had no impatienceto unveil their true identity to the females, which is the sex that chooses partners. (ref 11) This must be an advantage for them, because they do not fight so much with other males and therefore have more time for feeding. However, it is probably not strengthening for their health, because the best defence against parasites in the feathers is frequent preening. (ref 12)

So sly can a little grey sparrow be. Who would have thought that?

Copyright: Johannes Erritzoe, 1996

References:

1. Møller, A. P. 1987: Variation in badge size in male House Sparrows Passer domesticus: evidence for status signalling. Animal Behav. 35:1637-1644.
2. Burrows, W. H. & H. W. Titus, 1939: Some observations on the semen production of the male fowl. Poultry Science 18:8-10.
3. Martin, P. A. & P. J. Dzuik, 1977: Assessment of relative fertility of males (cockerels and boars) by competitive mating. J. Reprod. Fert. 49:323-329.
4. Møller, A. P. & J. Erritzoe, 1988: Badge, body and testes size in House Sparrows Passer domesticus. Ornis Scandinavia 19 (1):72-73.
5. Darwin, C. 1859: On the Origin of Species by Means of Natural Selection. J. Murray, London.
6. Saino, N., A. P. Møller, & A. M. Bolzern, 1995: Testosterone effects on the immune system and parasite infestations in the Barn Swallow (Hirundo rustica): An experimental test of the immunocompetence hypothesis. Behav. Ecol. 6 (4): 397-404.
7. Møller, A. P. 1991: Parasites, sexual ornaments, and mate choice in the Barn Swallow. in: Loye, J. E. & M. Zuk, (eds.): Bird-Parasite Interactions. Oxford University Press, Oxford. pp.328-343.
8. Møller, A. P. Kimball, R. & J. Erritzoee, 1996: Sexual ornamentation, condition, and immune defence in the House Sparrow Passer domesticus. Behav. Ecol. Sociobiol. 39:317-322.
9. Møller, A. P. 1989: Natural and sexual selection on a plumage signal of status and on morphology in House Sparrow Passer domesticus. J. Evol. Biol. 2:125-140.
10. Møller, A. P. & J. Erritzoe, 1992: Acquisition of breeding coloration depends on badge size in male House Sparrows Passer domesticus. Behav. Ecol. Sociobiol. 31:271-277.
11. Møller, A. P., R. Dufva, & J. Erritzoe, 1998: Host immune defense and sexual selection in birds.J. evol. biol. 11: 703-719
12. Clayton, D. H. 1991: Coevolution of avian grooming and ectoparasite avoidance. in: Loye, J. E. & M. Zuk: Bird-Parasite Interactions. Oxford University Press, Oxford. pp.258-289.