From Leonard Jenyns 27 February 1868
Darlington Place | Bath
Feb. 27— /68
My dear Darwin,
I was pleased to have your letter, it being so very long since I heard any thing of you direct from yourself;— since we last met it is positively more than twenty years!1
I hope you have not thought me long in answering, but I was rather busy when it came;—also, having attended but very little to Zoology since I left Cambsh. in 1849,2 I had to collect my thoughts, & brush up recollections on the subject.— For myself, I shd have said, in the first instance, that the pheasant was the only polygamous bird in our British lists,—but I find Montagu, whom I look upon as a good authority, states both the Wood & Black Grous to be also polygamous though the Red grous is monogamous;—the first of these indeed is not now a British bird, but it was formerly. Selby too, who lived quite among the grouse in the North, & must have had good opportunities of knowing,—mentions the Black as polygamous & the Red as monogamous.3
That the pheasant is really polygamous, I shd be inclined to infer from two circumstances,—first that mentioned by Montagu—that “the female carefully hides her nest from the male”, who he says—often disturbs her when incubating;—4 This does not look like pairing;— the second is the fact, often formerly mentioned to me by the keeper at Bottisham Hall,5 & also mentioned by Yarrell,—that two or more hens will sometimes lay in one nest,—until sometimes there is quite an accumulation of eggs;6— how incubation is arranged among them in such case I have no idea,—but this again would hardly be, if the cock interested himself much in the affair of bringing up the young. I should think it was an unnatural habit—contracted only in preserves where the birds are numerous.— But what again of the Cuckoo?— There can be no pairing here strictly so called, as evidenced by the marks you suggest; & if the male only serves one female, his business in this country would soon be despatched,—whereas he remains till the beginning of July at least, & moreover keeps up his note thro’ May & the greater part of June, wh. I imagine is a pretty good sign that his feelings & desires connected with the breeding season are not entirely at an end;—&, further, that there are females to whom he might be welcome even very late in the Spring would appear from what Montagu says—that he has “killed this sex as late as the 26th. of June, from which he took a matured egg”—not yet deposited.—
Possibly with some species of birds,—whether they are polygamous or monogamous may depend upon the circumstances under wh. they are placed, for the same author observes that—“wild ducks pair,—but when domesticated one male will serve several females”.7
I am not sure that I quite see the drift of your remark, that you “suppose such birds as blackbirds & bullfinches, in which the sexes differ in colour, pair strictly”;— it would seem as if you considered there was a connection between pairing & such difference of plumage in the two sexes;—yet in the Raven, wh. is said to pair for life, & in the Carrion crow & Magpie, wh. go in pairs all the year,—there is scarce any difference in plumage excepting that the female is not quite so bright & glossy;—while in the wood & black grouse, which as above stated are polygamous, the sexes are very different.—8
With regard to your last question, I conceive it is very difficult to estimate the relative numbers of the two sexes in any species of bird, at least during the breeding season or to say which sex preponderates.— It is a remarkable, tho’ well-known circumstance,—that in the breeding-season, when birds that strictly pair might be supposed in pairs,—(i.e. if the sexes are in equal proportion)—if one sex be killed the other generally finds a second mate to take its place—and quickly too;— I remember having read somewhere, but I cannot put my hand upon the place, of a gentleman who, in the case of a pair of magpies that had built near his house, shot one of the sexes (I forget which) seven times in succession, the place of the one killed having been supplied as often as that, before there was a stop put to it.—9 That birds sometimes, under such circumstances, have a difficulty in procuring another mate, or are unable, I gather from cases in which the song of the male is protracted very much beyond the usual time, as in that of the nightingale mentioned by me—p. 88 of “Observns in Nat. Hist.”—10
Might it not be possible, in the case you put—of females being in excess over the males—that a cock blackbird or bullfinch shd. be stimulated to pair (itself) a second time—with a hen that had not paired before;—just as some species are thought to have very often, or always, more than one brood in the season, tho’ under what circumstances has not perhaps been correctly ascertained?
Or, without any disproportion in the sexes, there might be a disproportion in the numbers sufficiently matured for nidification, by reason of the young of the previous year having been hatched some very much earlier than others,— amongst these might be a few which would just breed or not according as they were able or not to get mates late in the spring or summer, when the great bulk had been served, but they themselves were not ready sooner.— I merely throw out these crude suggestions as contingencies—which possibly may arise & help to explain some of the apparent anomalies, or difficulties, connected with the subject on which you write. But cast them at once aside, if not worth attending to.— My letter is long enough, & it is time to stop.
I am very glad to find you speaking of yourself as so much better in health than formerly. I sincerely hope you may be able to continue your labours, & bring them, without check or hindrance, to whatever end you set before you as the goal to be reached ultimately if possible.— I have just got your last work on the variation of domestic animals & plants,—which I shall look at with great interest.—11 I am in tolerable health myself, though not very strong,—& unequal to much work at a time either of body or mind.— I am not a quarter so industrious as yourself. Of late years I have amused myself chiefly with our British plants, & during last season,—as well as at this present time,—especially with the Mosses.
But I take great interest in our Field Club, founded by myself—some 13 or 14 years ago,—& flourishing well, as regards both numbers & work done,—tho’ I am not strong enough to join it in its excursions.— I am also just now very busy in getting together a Local museum in the building of the Bath Lit. & Scient. Institution.—12
It will never, however, be anything approaching to the admirable museum got up by Henslow at Ipswich.—13
Believe me, | My dear Darwin, | Your’s very Sincerely | L. Jenyns.
P.S. I have only just by accident caught sight of your question at the end relating to the horns in the males of the Lamellicorn beetles, wh. I am not prepared to answer just now.—14
On polygamous birds and the pairing of birds. Late singing of males. [see Descent 2: 107.]