MIGRATION. 13 
CHAPTER II. 
The migration of birds, although a most interesting subject, 
is yet very imperfectly understood, and reliable data from 
different countries and places are greatly wanted to elucidate 
it. Without doubt caused by the absence or abundance of 
food, which in turn is caused by difference of temperature, 
the passage of birds in these parts begins with most species 
almost to a day in the spring, usually lasting for about 
three weeks, though some, as the Hoopoe and the Swallows, 
are more irregular in their first appearance ; and with these 
the migration lasts throughout a longer period. 
Few (indeed hardly any birds) do not migrate or shift their 
ground to some extent. I can name very few which do not 
appear to move, viz. Griffon Vulture, Imperial Eagle, Eagle- 
Owl, Blue Thrush, all the Woodpeckers, Tree- Creeper, Black- 
headed Warbler, Dartford Warbler, Crested Lark, Chough, 
Raven, Magpie, Red-legged and Barbary Partridges, and the 
Andalucian Quail. Generally speaking, it seems to me that 
in the vernal migration the males are the first to arrive, as 
with the Wheatears, Nightingales, Night- Herons, Bee-eaters ; 
but this is a theory which requires more confirmation. Some 
species, as tlie Neophron and most of the Raptores, pass in 
pairs. 
Most of the land-birds pass by day, usually crossing the 
Straits in the morning. The waders are, as a rule, not seen on 
passage ; so it may be concluded they pass by night, although 
I have occasionally observed Peewits, Golden Plover, Terns, 
and Gulls, passing by day. 
The autumnal or return migration is less conspicuous than 
the vernal : and whether the passage is performed by night, 
or whether birds return by some other route, or whether they 
pass straight on, not lingering by the way as in spring, is an 
open question ; bu.t during the autumn months passed by me 
at Gibraltar I failed to notice the passage as in spring, though 
more than once during the month of August, which I spent 
