22 
EIGHTH REPORT. 
means of knowing whether such journeys (in the case of land birds) 
are undertaken volutarily or not, but doubtless in very many cases the 
birds are swept off the coast by heavy winds and comparatively few of 
their numbers ever reach a safe refuge. It is a significant fact, as 
pointed out by Mr. Hensliaw many years ago,* that when land birds 
come aboard vessels at any distance from land they invariably die from 
exhaustion, no matter what care be taken of them.' 
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 to 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 para¬ 
mount 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 tlie weather conditions 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 fiv 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 destruc¬ 
tion which thins their ranks and brings death to so large a proportion 
of our bird population every year. 
If 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 oscillation in numbers with occasional decreases to such a 
point that recovery becomes impossible, and the species becomes ex¬ 
tinct. Ordinary minor oscillations may occupy but a very few years, 
but more often apparently one or more decades may be required for 
the completion of such a cycle. Several well-marked examples of such 
cycles have fallen under my own observation. I saw the Passenger 
Pigeon pass through one of its minor fluctuations apparently culminat¬ 
ing between 18 <0 and 1875, after which time it rapidly declined until 
now it trembles on the utmost verge of extinction. 
In the winter of 1894-95 most unfavorable weather 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. 
We have not time to go into tiie causes of mortality among birds, 
* Bull. Nutt. Orn. Club, VI. 194. 
