88 
THE CONDOR 
| VOT. VIII 
mother dressed in a night cap. Eater on, when we saw them full grown, they 
got to be more owl like and dignified. 
An owl spreads terror among the small ground folk as a ghost among negroes. 
It is the owl's shadow-silent wings, his sharp, sound catching ear and his night- 
piercing eyes that make him the superior of the mouse, the mole, the gopher and 
the rat. He fans over the field with an ominous screech that sets a mouse scam- 
pering to his hole, but his ear has caught the foot-steps; those wings are swift; 
those steel trap claws are always ready; his drop is sure, his grip is death. 
From an economic standpoint, it would be difficult to point out a more useful 
bird in any farming community. Like many other birds, the barn owl deserves 
the fullest protection, but man is often his worst enemy. 
Santa Monica , Cal. 
The Percentage of Error in Bird Migration Records 1 
BY W 1 TMER STONE 
I N no branch of ornithology is it more difficult to obtain reliable data than in 
the study of bird migration. 
It is seldom that we see the actual migration in progress, and then it is 
but a small fraction of the movement that comes under our observation and that 
often under abnormal conditions. 
Consequently we are thrown back upon a comparison of the records of the oc- 
currence, or the dates of arrival and departure of birds at various points, in any 
deductions that we may make as to the direction and rapidity of their migratory 
flights. 
Without considering the possibility of error on the part of the observer there are 
many conditions which tend to impair the accuracy of such records, such as in- 
ability to be in the field every day during the migratory season, inability to 
cover the same amount of territory eacli day, and the recording by some observers 
of early stragglers which were not noted by others. 
To obviate the last, suggestions have been made to record the arrival of the bulk 
of the species; but this at once admits the personal equation into the problem, and 
I find that nearly all observers differ in their interpretation of the bulk arrival, 
especially in the case of species which are subject to a constant increase in num- 
bers from the first day that they are observed. 
The average date of arrival based on several years’ observation is more accurate 
as a basis of comparison, but even then there is a large probability of error. 
Now most of the published tables of migration consist of the records of single 
observers at scattered points along the route of travel with generally large inter- 
vals between their stations. 
Scarcity of competent observers made it practically impossible to secure a large 
number of migration records from a limited area; but the wonderful increase in the 
popular interest in bird study which we have recently witnessed has developed 
many able observers and renders the accumulation of this sort of data quite feasi- 
ble. 
It has been my privilege to study a series of local records of this sort kept at 
from 30 to 40 stations each year, all within 15 or 20 miles of Philadelphia, by a corps 
of observers organized by the Delaware Valley Ornithological Club. 
These records are suggestive both in the apparent reduction of the percent- 
/ Read at the Twenty-third Congress of the A. O. U. in New York City, November, 1905. 
