[g] It is no less remarkable in Birds, that their Lungs adhere to the Thorax, and have but little play, than that to other Animals they are loose, and play much, which is a good Provision for their steady Flight. Also they want the Diaphragm, and instead thereof, have divers large Bladders made of thin transparent Membranes, with pretty large Holes out of one into the other. These Membranes seem to me to serve for Ligaments, or Braces to the Viscera, as well as to contain Air. Towards the upper Part, each Lobe of the Lungs is perforated in two Places, with large Perforations; whereof one is towards the outer, the other towards the inner Part of the Lobe. Through these Perforations, the Air hath a Passage into the Belly, (as in [Book I. Chap. 1. Note (b)];) that is, into the foremention’d Bladders; so that by blowing into the aspera Arteria, the Lungs will be a little rais’d, and the whole Belly blown up, so as to be very turgid. Which doubtless is a Means to make their Bodies more or less buoyant, according as they take in more or less Air, to facilitate thereby, their Ascents, and Descents: Like as it is in the Air-bladders of Fishes, in the last cited Place. [Note (i).]
[h] Such Birds as have long Legs, have also a long Neck; for that otherwise they could not commodiously gather up their Food, either on Land, or in the Water. But on the other Side, those which have long Necks, have not always long Legs, as in Swans——whose Necks serve them to reach to the Bottom of Rivers, &c. Willughby’s Ornithol. L. 1. c. 1. §. 7.
[] We have sufficient Instances of this in Geese, Ducks, &c. whose Wings, (their Bodies being made for the Convenience of Swimming,) are plac’d out of the Center of Gravity, nearer the Head. But the extending the Neck and Heads in Flight, causeth a due Æquipoise and Libration of the Body upon the Wing. Which is another excellent Use of the long Necks of these Birds, besides that of reaching and searching in the Waters for their Food.
But in the Heron, whose Head and long Neck, (although tuck’d up in Flight,) over-balance the hinder Part of the Body; the long Legs are extended in Flight, to counterpoise the Body, as well as to supply what is wanting in the Tail, from the Shortness of it.
[k] Steno thus Concludes his Myology of the Eagle, Imperfecta hæc Musculorum descriptio, non minùs arida est Legentibus, quàm Inspectantibus fuerit jucunda eorundem præparatio. Elegantissima enim Mechanices artificia, creberrimè in illis obvia, verbis non nisi obscure exprimuntur, carnium autem ductu, tendinum colore, insertionum proportione, & trochlearam distributione oculis exposita omnem superant admirationem. Steno in Blas. Anat. Animal. P. 2. c. 4.
CHAP. III.
Of the Migration of Birds.
Concerning the State of this Tribe of Animals, the first Thing I shall speak of, (by Reason God himself instanceth in it,) shall be their Migration, mention’d, Jer. viii. 7. Yea, the Stork in the Heaven knoweth her appointed Times, and the Turtle, and the Crane, and the Swallow observe the Time of their Coming; but my People, &c.
In which Act of Migration, there are two Things to me, exceedingly notable. One is what the Text speaks of, their knowing their proper Times for their Passage, when to come[a], when to go; as also that some should come when others go; and some others go when these come. There is no doubt but the Temperature of the Air, as to Heat and Cold, and their natural Propensity to breed their Young; may be great Incentives to those Creatures to change their Habitation: But yet it is a very odd Instinct, that they should at all shift their Habitation: That some certain Place is not to be found in all the terraqueous Globe, affording them convenient Food and Habitation all the Year, either in the colder Climes, for such as Delight in the colder Regions, or the hotter, for such Birds of Passage as fly to us in Summer.
Also it is somewhat strange, that those untaught, unthinking Creatures, should so exactly know the best and only proper Seasons to go and come. This gives us good Reason to interpret the מועדיה appointed times[], in the Text, to be such Times as the Creator hath appointed those Animals, and hath accordingly, for this End, imprinted upon their Natures such an Instinct, as exciteth and moveth them thus, at proper Times, to fly from a Place that would obstruct their Generation, or not afford convenient Food for them, and their Young, and betake themselves to another Place, affording all that is wanting for Food or Incubation.