1870.] LETTER FROM MR. W. H. HUDSON. 801 
ances sounding not unlike the distant. baying of a ‘deep-mouthed’ 
watch-dog. 
“The late great increase of the large Owls in this neighbourhood 
can only be ascribed to the recent cultivation of the plains near the 
city. Within the last six years a great extent of hitherto bald shel- 
terless grounds have been enclosed, and are now yearly planted with 
wheat and maize; in the fields the Owls find shelter, and their 
favourite food in abundance, Mice, Partridges, &c. 
‘The Lechuson frequenting open plains in preference to woods, 
and hiding by day on the ground, has the colour of its plumage 
adapted to a country like the desert pampas, rough with a brown 
vegetation. But the introduction and increase of Sheep quickly 
changed the aspect of a vast extent of the plain; the long brown 
grasses disappeared, their place being taken by a tender herbage, 
short and brilliant green; the country was thus unfitted for their 
pasturage. All the wild animals have, no doubt, been greatly 
affected by this sudden change in vegetation and total destruction of 
cover. But cultivation has now partially restored the physical con- 
ditions necessary to the preservation and increase of many species 
like the Lechuson. In future descriptions I shall frequently refer 
to these changes on the pampas. 
“The gradual increase or diminution eonstantly going on in many 
species about us is little remarked; but the sudden appearance in 
vast numbers of a species not usually common is regarded by all with 
interest and wonder. When, owing to a season favourable to pro- 
pagation, a small species multiplies greatly (as often happens here 
with Mice, Toads, Crickets, &c.), we may confidently look for the 
appearing of multitudes of those birds that subsist on them. Thus, 
in the year 1856, when the earth swarmed with Mice, vast numbers 
of the then scarce Lechuson, and flocks of the Great Adjutant Stork 
(Mycteria americana) also appeared. Armies of these majestic 
white birds were seen stalking over the grass on all sides, or at the 
close of day winging their flight to the distant watercourses in a 
continuous flock ; while the night air resounded to the solemn hoot- 
ings of the innumerable Owls. However simple may be the cause 
of the first phenomenon (the sudden great increase of a Species in- 
capable of migration but exceedingly prolific), the attendant one 
appears to have been remarked with astonishment from very early 
times, and to have given rise to many conjectures. Pliny, if I re- 
member right, relates that one season in some part of Asia Minor 
the Mice increased in an extraordinary manner, but soon appeared 
‘an army of strangely painted birds’ and devoured them all. Birds 
of prey and those that subsist on large insects, and possess great 
powers of flight, without being strictly migratory, when not occupied 
with the business of propagation, are incessantly wandering in quest 
of food. They often fly high, and traverse vast distances. When 
the natural food of any one of such species abounds very much in a 
particular region, all the birds that discover it remain in it and 
continually attract to them all of their kind passing over them. It 
