- 1The post-nuptial recrudescence of sexual display in male birds in two species (Erithacus rubecula and Corvus frugilegus) occurs after the regeneration of the exhausted lipoid Leydig cells of the interstitium and there is evidence that the same applies to other species.
- 2Although the collapsed post-nuptial testis remains at or near its minimum size it is extremely active. A new tunica albuginea is formed inside the old collapsed and distorted testis wall. The new generation of endocrine Leydig cells become lipoidal and cholesterol-positive and appear to secrete sufficiently to stimulate the late summer and autumn display characteristic of many species. Meanwhile moult ends and the tubules gradually lose their metamorphosed lipoids. As these are cholosterol-positive they too may have an influence on post-nuptial sexual activity. The refractory period of photoexperimentalists draws to an end. In some species post-nuptial sexual behaviour may occasionally lead to occasional autumnal or winter reproduction if weather and food conditions remain propitious.
- 3Testis collapse, tubule metamorphosis and interstitial regeneration are part of the internal rhythm of the organ and are unrelated to climatic events such as daylength or light variation.
- 4Among many British birds post-nuptial display is partially or sometimes almost wholly extinguished in normally severe December weather. There next follows the pre-nuptial leap in gametogenetic activity that begins, according to the individual species and locality, from December until February. The last lipoids vanish from the testis tubules and there is an interstitial advancement. The ovaries show macroscopic changes and pairing occurs in species that have not already done so.
- 5It is not known whether this late-winter pre-nuptial leap in gametogenetic activity is stimulated by external factors or whether it is a spontaneously occurring part of the internal rhythm in one or both of the sexes. There is evidence suggesting that in the case of the robin neither daylength or light increment cause it.
- 6After its pre-nuptial reactivation the sexual cycle is governed by the week to week environmental experiences, including behavioural interactions, of the pair. Sunshine as opposed to mere lengthening days is a potent stimulus; excessively dull, cold weather and hunger are inhibitors.
- 7In the species studied the male cycle advances more rapidly than that of the female; in several British passerines spermatozoa appear while the oocytes are still minute and while the woods are still almost bare. There is evidence which suggests that the environmental stimuli that allow fertilization, and thus keep the sexual cycle of the species anchored in time, principally operate through the female and so ovulation occurs at a time that will enable the young to appear at about the same period as the traditional food harvest. Such stimuli will have to be determined from species to species.