May, 1921 A BIRD CENSUS AT PRESCOTT, WASHINGTON 89 
numbers, undoubtedly cause variations in the number of individuals present on 
the area, and some species would only occasionally be represented. Lowever, 
the movements on and off the area would about balance each other, and the bird 
population on the area should remain about the same; for species such as the 
Robin, present in large numbers, even the number of individuals would probably 
remain nearly constant. 
The variation in the number ot birds seen at the different periods of obscr- 
vation during the day must be due to a different proportion of those present 
being seen. It will be readily admitted that the count never included all tlie 
birds present. Many birds are very inconspicuous unless moving, and many un- 
doubtedly escaped observation by keeping out of sight in the grass or brush. 
Birds in song are especially likely to be noted. The relative activity of the birds 
at different times of the day is probably the explanation of the variations in the 
count at the various periods. 
No attempt will be made to discuss the factors which might influence the 
activity of the birds at different times of the day. It may be pointed out, how- 
ever, that, contrary to the condition usually found during periods of migration, 
the greatest number, both of species and individuals, was seen not in the very 
early hours but a little later in the morning. The number of birds seen decreases 
ayvain as the heat of the day comes on, and it seems that the activity of the birds 
was influenced, at least to some extent, by the temperature. 
It is believed that the average total number of birds seen gives very closely 
the number of individuals ordinarily present in June on the area. Probably this 
average is just a little less than the number of birds actually present. Caleu- 
lated to acres, our figures give 7.552 individual birds per acre of ground. As 
the conditions along the Touchet Valley are very similar for at least ten miles 
in either direction, it is probable that about the same number of birds per acre 
will be found all along the stream in this region. The width in the valley cov- 
ered by conditions like those on the census area would probably be on the average 
about one-quarter mile. This would give for this season a bird population of 
about 1200 per running mile along the stream. The prairie away from the vicin- 
ity of the stream has a distinctly different avifauna, and supports a decidedly 
less number of individuals per acre. 
It will be noted that nearly half the individual birds seen on the census area 
were Robins. These birds were attracted to some extent by the ripe cherries on 
the area, but they are very abundant throughout the valley, being by far the 
most abundant bird along the river. A number of other species of birds, not 
found on the census area during the time of this study, occur in the valley and 
many of them have been seen on the area at other times. In the more heavily 
forested and more secluded parts of the valley the number of individuals of the 
more retiring species is undoubtedly greater than the number recorded in this 
census. It seems certain, then, that the average number of individuals per acre 
of the different species can be only approximately determined from the results 
of this census. Probably for the more common species, which were seen on nea rly 
every trip, the results are more dependable. It would seem probable, after allow- 
ing a large margin for error, that the total number of individuals per unit area 
in the valley as determined by this study, ean be considered quite dependable. 
