118 
NATURAL HISTORY OF SELBORNE. 
RAll NOMINA. USUALLY APPEARS ABOUT 
8. Swallow, Hirundo rustica : April 13. 
4. Martin, Hirundo domesUca : 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 inouv own island, and linger 
here till very late in the season, as redwing and especially fieldfare thrushes, and certain /ringiZ- 
lidxB, 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 in 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 
bramblefinch 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 sprijig, a consideration of very trivial import, for we do not find that the 
warmest weather in sarly 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 ynay be remarked that birds 
of passage, when confined in a cage, remain comparatively quiet so long as they continue in the 
dark, hut 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 latter 
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 month. 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 loquaxy 
Fallow-chat, Saxicola oenanthey 
Bank-swallow, Hirundo ripariut 
and perhaps also of the 
Blackcap-fauvet, Ficedula atricapillat 
Tree pipit, Anthus arboreusy 
Wryneck, Torquilla vulgaris, 
and 
Song pettychaps, Sylvia melodia, 
