118 NATURAL HISTORY OF SELBORNE. 



KAII NOMINA. USUALLY APPEARS ABOUT 



8. Swallow, Hirundo rustica : April 13. 



4. Martin, Hirundo domestica : Ditto. 



or retarding the period of its coming on, which are very worthy the attention of the naturalist. I 

 have already hinted the proximate cause which appears to induce migration in the spring: in sup-* 

 port of which opinion it will be found that those species which winter in our own island, and linger 

 here till very late in the season, as redwing and especially fieldfare thrushes, and certain fringil- 

 lidee, have hardly any internal sexual development till a week or so before they leave us ; while, on 

 the other hand, our various summer visitants are always in perfect breeding condition at the time 

 of their first arrival iu their summer haunts. The most allied species often differ remarkably as 

 to the time in which they are first excited by the genial influence of the season, so much so that ^ 

 this frequently amounts to a specific character ; and it appears to be a very general rule for those 

 species which inhabit in summer far to the north to be very late in being thus constitutionally 

 affected, as may be exemplified by a comparison of our resident thrushes with those which 

 only winter with us, and is as strikingly shown in the case of the closely allied chaffinch and 

 bramblefiach of authors, the former of which assumes its breeding aspect early in March, whereas 

 the latter, which is not known to breed within the British isles, does not exhibit this change 

 until about six weeks afterwards, as I have ascertained by examining numerous specimens late 

 in April. We have next to consider the influence of the moon and of the wind, temperature 

 being, at least in spring, a consideration of very trivial import, for we do not find that the 

 warmest weather in early spring hastens much the departure of the redwings and fieldfares, and 

 summer has of course long set in in the regions to which our spring visitants retire before most 

 of them commence returning to their breeding haunts. It is only after moonlight nights that 

 small migratory land-birds are observed to settle on the rigging of ships, and most abundantly 

 when the moon is near the full, in perfect accordance with which it may be remarked that birds 

 of passage, when confined in a cage, remain comparatively quiet so long as they continue in the 

 dark, but no sooner is a light introduced into the room where they are kept than the very pecu- 

 liar migrative restlessness is sure to be exhibited, and this although plentifully supplied with 

 the food to which they have been always accustomed sufficiently intimating that deficiency of 

 sustenance is not, as many suppose, an element in the mystery of migration. The London bird- 

 catchers even refer to the almanack to ascertain the time when the nightingales will be over, cal- 

 culating on their arrival a day or two before full moon in April ; but the state of the weather 

 must also be to a certain extent taken into consideration, as migratory birds arrive usually with a 

 favourable gale, which materially assists'their flight. The prevalence, however, of southerly breezes 

 during the spring months, and of gales from the opposite direction in autumn, so accordant to 

 the necessities of birds of passage, must be numbered among the endless examples, every where 

 so obvious to the student of natural history, of the admirable manner in which all the parts of 

 the one grand and universal system act so beautifully together, each conducing to the benefit of 

 the whole, though appearing perhaps comparatively isolated on a superficial view. In autumn, 

 the migrative impulse is often developed without any apparent cause ; for certain species, as the 

 swift and the adult cuckoo, leave us for a warm region during the heats of summer, the lattar 

 during its annual moult, and both when their food would seem to be most abundant. In those 

 birds, however, which remain with us till late in autumn, decrease of temperature tends 

 perceptibly to bring it on, as on the first frosty night all the inmates of an aviary will become 

 restless, though perfectly quiet the night preceding ; and the extreme susceptibility they after- 

 wards show to thermometrical changes is not a little remarkable, the impulse being always more 

 or less exhibited during the winter months, according to the degree of temperature. I shall now 

 conclude with giving a list of the summer birds of passage, ranged somewhat in the order in 

 which they arrive in Surrey, the period of full moon being supposed to occur about the middle of 

 the mouth. About the third week in March will appear, if the weather prove favourable, a few 

 avant- couriers of the 



Dark-footed or chiff-chaff pettychaps, Sylvia loquax, 



Fallow-chat, Saxicola cenanthe, 



Bank-swallow, Hirundo riparta, 



and perhaps also of the 



Blackcap-fauvet, Ficedula atricapilla, 



Tree pipit, Anthus arboreus t 



Wryneck, Torquilla vulgarist 



' and 



Song pettychaps, Sylvia 1lu i odiat 



