INTRODUCTION 25 
with such meteoric swiftness. There is absolutley no proof that any of these 
plover pass from Labrador to Brazil at one flight, nor do we know even ap- 
proximately the time taken for the trip as performed. Presumably they 
do make flights of 300 to 400 miles without rest, since they are often seen 
passing Bermuda without resting, and have probably flown continuously 
since leaving the North American coast; yet should they tire there is no reason 
whatever why they should not rest upon the sea at any time except during 
storm, and it is well known that in case of heavy storms thousands of them 
do alight on the Massachusetts coast, as well as on Bermuda, while the species 
lingers regularly for weeks on some of the West Indian Islands. 
Doubtless one reason why migrating birds fly at great heights in fair 
weather is because it is easier. So long as the breeze is in the right direction 
they utilize it to the full, but if compelled by circumstances to face a wind 
they find it far easier to fly near the surface where they can take advantage 
of such eddies and counter currents as may appear, and where at all events 
the velocity of the wind will be less than at higher levels. Another and 
possibly the strongest reason for flying at great elevations lies in the fact 
that the view thus obtained is of paramount importance to the migrant. 
At a height of two miles a bird in clear weather commands a view of an area 
nearly 200 miles in diameter, so that under favorable conditions, it might 
cross any of our great lakes, or even reach South America by way of the 
West Indies without ever being out of sight of land. Should the weather 
become unfavorable, however, after a start has been made, the little travellers 
must make the best of a bad matter and meet the emergency as best they 
may. Were they gifted with the extraordinary powers popularly assigned 
them they would rarely or never be caught and overwhelmed as they so often 
are. Were they able to fly even seventy-five miles an hour, they could flit 
to safety on the first appearance of bad weather, well knowing that a few 
hours of clear weather would enable them to recover all the ground lost. 
In spite of the general belief that birds are able to foretell meteorological 
changes and forestall dangerous storms, the weather is very largely responsible 
for the terrible destruction which thins their ranks and brings death to so 
large a proportion of our bird population every year. 
In the winter of 1894-95 most unfavorable conditions in the south nearly 
annihilated the Bluebird and so far reduced the number of Robins reared in 
this section of the north that their normal numbers were not regained for 
at least six years. 
It is clear that, on the average, as many birds die each year as are born, 
else the individuals of any species would increase in numbers year by year 
until they became intolerably abundant. As it is, some species increase for 
a time and then their numbers fall away again. There is a constant oscilla- 
tion in numbers with occasional decrease to such a point that recovery becomes 
impossible, and the species becomes extinct. Ordinary minor oscillations 
may occupy but a few years, but more often apparently one or more decades 
may be required for the completion of such a cycle. : as 
We have no time to go into the causes of mortality among birds, but it is 
evident that a very large percentage of the young and inexperienced birds 
which start south in autumn never return to their birthplace—probably having 
perished from the dangers of the trip. As a rule the southward journey is 
made—or at least might be made—with comparative safety. There is no 
need of great haste, food is abundant and the travellers are moving always 
toward regions of increasing warmthand superabundant food. On the 
northward trip on the contrary, the birds often are leaving safety and abund- 
