Daf Yomi Beitza 10a: Rashi and Aristotle In Accord On Brooding Birds

Let my preface this by stating that IANAO (I am not an ornithologist). Prompted by a post yesterday on dafnotes.

Rashi, Beitza 10a, d"h בריכה:

קובי"ד בלע"ז שהיונים יולדים בריכה בכל חדש וחדש והן שני גוזלות זכר ונקיבה ומפסיקין בתקופת טבת וחוזרים ויולדים בניסן

Thus, a brood of doves consists of two chicks each month, one male and one female, they stop laying at the winter solstice and begin again at the vernal equinox.

Rashi was likely citing the scientific knowledge of his day (rather than stating this be-ruach ha-kodesh), and that knowledge was likely based on empirical evidence such as a lot of observation.

We find a similar statement in Aristotle's History of Animals, in Book 6:

Very few birds propagate within their first year. All birds, after once they have begun laying, keep on having eggs, though in the case of some birds it is difficult to detect the fact from the minute size of the creature.

The pigeon, as a rule, lays a male and a female egg, and generally lays the male egg first; after laying it allows a day's interval to ensue and then lays the second egg. The male takes its turn of sitting during the daytime; the female sits during the night. The first-laid egg is hatched and brought to birth within twenty days; and the mother bird pecks a hole in the egg the day before she hatches it out. The two parent birds brood for some time over the chicks in the way in which they brooded previously over the eggs. In all connected with the rearing of the young the female parent is more cross-tempered than the male, as is the case with most animals after parturition. The hens lay as many as ten times in the year; occasional instances have been known of their laying eleven times, and in Egypt they actually lay twelve times.

It is a separate question whether all of this can be confirmed true ornithologists today.

The laying of one male and one female seems a bit strange, but does make sense. Whereas gender is more or less determined in humans by the father (who gives either X or Y, which combines with the X in the egg -- although the release of hormones of the mother also have some final say in the decision), the sex of the offspring in birds is determined by the mother. See here. A female has genotype ZW while the male has ZZ. Thus, the male sperm contributes a Z, and the female contributes, in the ovum, either a Z or a W, which determines the gender. On average, we would thus expect to see half male and half female offspring, as probability bears out.

But that should not mean that a brood cannot have two males or two females.

Note also that the mother's hormones play a role here as well, in that they can tilt the scales in favor of the chromosome for female in the ovum. See this article here.

By experimenting with domestic chickens, they have determined that the presence of higher-than-normal levels of the hormone progesterone during the first meiosis -- the cell division that divides the sex chromosomes and genetically determines the sex of an offspring -- produces significantly more females.

...

If hormones play a role in determining gender in the ovum, then perhaps there is some natural process that will bias it towards producing male followed by female, or vice versa.

What about taking off from laying from the winter solstice until the vernal equinox? Aristotle has something similar:

The hens lay as many as ten times in the year; occasional instances have been known of their laying eleven times, and in Egypt they actually lay twelve times.

And when they begin laying:

Young hens are the first to lay, and they do so at the beginning of spring and lay more eggs than the older hens, but the eggs of the younger hens are comparatively small.

The beginning of spring is the vernal equinox (ver=spring).

DafNotes notes:

Rashbam in Bava Basra (80a) states that they have offspring every month of the year except for the month of Adar.

Sheorim Mitzuyanim B'halacha suggests that it might depend on the climate of each particular area.

This would match what Aristotle spoke about, of different locales having different instances of egg laying.

It seems to me that Rashbam is rather close to Rashi. Rashbam states they lay every month except for Adar. The month after Adar is Nissan. So both agree (with Aristotle) that they resume laying in Nissan. The difference is how many months they pause.

Bringing in the modern science. (e.g. here) What causes a bird to release eggs from the ovaries? It is hormones released by the pituitary gland. Since hormones control this, it makes sense that being excited by the presence of a rooster can stimulate egg production, or, if excited by the ground, unfertilized eggs. This does not necessarily accord with the gemara's statement that whatever has tashmish by day only gives birth by day, vs. getting excited by the ground where laying can then happen at night. Except that perhaps it takes a certain amount of time from when the egg is released from the ovary until it forms and is finally laid. Again, IANAO.

One thing that regulates the hormones released by the pituitary in birds is light. (This may also work in to the birds laying by day/by night issue.) The more sunlight, the more hormones, and the more likely they are to lay. So saith the modern experts.

This may well work into the observation of Aristotle, Rashi, etc.. In winter, the days are shorter, so there is less exposure to sunlight. As the length of days extend, there is more sunlight, so the birds resume laying. If Aristotle stated that in Egypt, they lay 12 broods, presumably 1 per month with no break, this may be because the length of days at that latitude is such that they get enough sunlight.

This also may well account for the difference between Rashi and Rashbam. They may be citing observations from different locales. (To my mind, one cannot say that Rashi and Rashbam are speaking of different climates -- where I change this to latitudes -- and are speaking from direct observation, because both of them lived in France.) At different latitudes, there would be different length of days, which would play out with different times of year that hens would stop laying.