London Starlings and Seasonal Reproduction in Birds
Authors: ROWAN WILLIAM 
Published: August 2010

.Winter egg production in domestic poultry cannot be considered equivalent to the sexual activity induced in winter in birds or mammals with brief breeding seasons normally restricted to spring, even though both may depend on the administration of artificial light.

The function of the pituitary as an intermediary organ in the activation of the gonads appears to be firmly established.

Increased temperatures are without effect on the development of the gonads of homoiothermal vertebrates, although they may be essential to the act of breeding.

The essence of the light experiments with which this paper deals is embodied in the relative length of day to which the animals are exposed, passage from a short to a longer day inducing development of the sex‐organs, while a curtailed day results in regression.

The manner of administration produces varying results, the gradual addition of slight increments being essential to normal development in at least some forms, birds as well as mammals.

Wave‐length, beyond certain limits of the spectrum, is of doubtful significance. The experiments of Bissonnette in this connection, with special reference to red light, are reviewed at length and his conclusions questioned.

Ultra‐violet rays appear to exert a recognizable accessory effect on gonadal development, but are complementary to the visible rays which are the primary stimulating elements.

Intensity, provided it is sufficient to induce wakefulness, appears to be of little further significance, since its increase beyond a certain relatively low point is immaterial. There is no accelerating effect coincident with enhanced intensity, generally speaking, but results have shown some variation. If reduced to a point at which sleep is not prevented recrudescence of the organs does not occur in birds, though in ferrets this is probably not the case, but the evidence is conflicting.

Experiments specifically designed to demonstrate that the effective receptor is the eye (thus stimulating the pituitary, which in turn activates the gonads) have failed, or, where seemingly convincing, are open to question. Evidence that the skin of the body acts as a receptor is doubtful.

It is possible to induce winter development of the testes in juncos by means of compulsory exercise forced on the birds in almost total darkness, the light used being below the point at which it begins to be effective by itself. Bissonnette's failure to corroborate these results with starlings is attributed to the nervous temperament of this species. His birds showed protracted fright and muscular exhaustion during the entire running of the experiment.

With specific reference to London starlings, the available evidence, most of it circumstantial, suggests that their premature sexual development is due to the environment in which the birds find themselves at their nightly roosts in contrast to country birds. The facts cannot be explained on grounds of age or racial differences. The same individuals apparently use the same roosts and the same perches on those roosts continuously for long periods during the winter.

The slightly higher temperatures of London may certainly be disregarded as significant, even though they may account for the adoption of the roosts themslves.

That the light falling on the birds from the street lamps is a subsidiary factor in keeping them awake seems probable, but it seems equally probable that light of this intensity would of itself fail to keep the birds awake and equally fail to induce sexual development.

That a combination of illumination and traffic disturbance induces wakefulness in London starlings is evident, since they may be heard twittering more or less continuously till the theatre crowds have gone home and the traffic diminishes substantially some time after midnight. The primary disturbing element appears to be the traffic, for at 4 in the morning, when it has almost entirely subsided, the roosts (such as have been watched at all events) are quiet, although the lights continue to burn through the night.

It is suggested that the additional period of wakefulness imposed nightly on the London as against country starlings must be looked upon as the crucial element in inducing premature development of the gonads during the winter months.