ILLINOIS AUDUBON SOCIETY 29 



There are various interesting details connected with the migratory 

 nights of birds which merit attention, but the limits of this article forbid 

 more than a brief reference to a few of them. Among the questions which 

 naturally arise when one thinks of these wonderful journeys are the fol- 

 lowing: At what time of the 24 hours do they fly? How fast and at what 

 height? To what extent is their rate of progress influenced by food condi- 

 tions? How potent are wind, temperature, and barometric conditions in 

 determining their movements? Do they make fairly uniform progress of 

 30, 40, or 60 miles a day, or do they make longer single flights of several 

 hundred miles and then tarry a few days before making another consider- 

 able advance? Do migrants return to their home of the preceding year? 

 We will attempt to partially answer some of these questions. 



Anyone who is on the watch at the right time of the year may see birds 

 in migration, since many of the larger strong-flying birds are known to mi- 

 grate by day, and so also do some of the smaller ones such as swallows, 

 chimney swifts, and night hawks, that are especially expert on the wing. 

 That very many travel by night is shown by their destruction at lighthouses, 

 by observations through telescopes directed towards the moon, and they may 

 often be heard in great numbers calling to each other when flying low on 

 cloudy nights. Birds of most species migrate at night. 



In 1905 Professor Stebbins and Dr. Carpenter of the University of Illi- 

 nois, with the use of two telescopes directed towards the moon, succeeded in 

 developing a method for determining the height of migrating birds which 

 is far more accurate than any in use before. Their observations seem to 

 show that most birds migrate at considerably less than a half-mile above 

 the earth. 



The speed at which birds fly while migrating has been accurately deter- 

 mined in but a very few instances. The results do not warrant conclusions 

 that migrating birds often fly over 50 miles per hour, but indicate that the 

 majority of our smaller birds average much less than that rate of speed. 



A study of the daily records made in the months of February to May 

 inclusive, during the years 1903 — 1918, at Urbana, Illinois, furnish ample 

 evidence that there is a great lack of uniformity in the amount of migra- 

 tion activity on successive nights. On some mornings we have found large 

 numbers of new arrivals belonging to as many as 1 5 or 20 species not pre- 

 viously seen that season, and such movements are very likely to be preceded 

 and followed by several nights of very little activity. Such extensive move- 

 ments or bird waves, as they are called, are evidently independent of any 

 particular food conditions, since they commonly involve birds as unlike 

 in food habits as are the green herons, black and white warblers, and fly- 

 catchers. Extensive bird waves commonly occured with us, while records 

 were being kept, at the end of February, soon after the middle of March, 

 near the end of April, and early in May. The early May movement was the 

 greatest of all, and at its height, we expected to list 70 to 80 different 

 species per day and see multitudes of individuals. A study of the weather 

 maps of such times of migration activity reveals a close correlation 

 between bird waves and special weather conditions. The greatest 

 flights of night migrants have taken place at times when the weather maps 

 have shown the near approach from the west, of an area of low barometric 

 pressure, with the accompanying rise in temperature. a::d southerly winds. 



