July, 1921 THE FLOCK BEHAVIOR OF THE COAST BUSH-TIT 125 
While the birds were in the oaks, a California Jay swooped in among them, caus- 
ing great alarm for a moment, until the intruder was identified. This reaction to Jays 
I have frequently noticed, although as far as I know the larger bird does not harm 
the Bush-tits, and they manifest no fear of a perching Jay. 
About noon one bird flew across the road to the clump of oaks marked Y, and 
was followed by 138 others: Three more followed hurriedly a little later. I did not 
observe the birds further. 
Thus it is seen that the flock when first observed was composed of 16 
individuals, that 5 were added while the birds were in the thicket, and that 4 
later left the flock, so that the number remaining when observations were 
discontinued was 17. This affords a good example of the puzzling fluctua- 
tion in numbers which I have above mentioned as leading me to conclude that 
- the flock organization is relatively loose and that birds probably pass from 
one flock to another with considerable frequency. 
Doubtless the reader, if he has had the patience to follow the observa- 
tions above recorded, has been struck with the lack of system or directioi 
and the unnecessary retracing of routes manifested by the birds, their behavior 
in this respect being somewhat suggestive of that which Mark Twain has 
attributed to ants in his classical essay on that subject. The method of prog- 
ress of the birds may be analyzed as follows: 
The flock is foraging, let us say, in the outer foliage of an oak tree. The 
twigs and leaves are examined in quite a thorough manner, the birds inspect- 
ing them from above, or clinging, as they do frequently, upside down and 
examining the under surfaces. Presently some individual finds the forage 
poor; no more scale insects or aphids are to be found in its immediate vicinity ; 
it begins to look about in search of fresh fields and pastures green. Yonder 
is a clump of chaparral that looks promising. A few yards of open space 
must indeed be traversed in order to reach it, and Bush-tits have a native 
abhorrence of open spaces; they are natural agoraphobiacs. But hunger is a 
strong stimulus. The bird hesitates a moment, then darts out and with hur- 
ried, undulating flight crosses to the chaparral. 
Now other individuals of the flock find food beginning to run short in 
tne oak fohage. They too see the near-by clump of chaparral; they have seen 
their companion make the flight successfully; they hear his notes, perhaps 
indicating that he has found food; they themselves are encouraged to make 
the venture. 
Now the impulse spreads; in groups of two or three or five, others dart 
across from the oak to the chaparral, until shortly the whole flock has moved 
to the new location. 
I would not attempt to maintain that all the steps I have indicated here 
pass as successive ideas through the minds of the birds. I have merely out- 
lined the impression which their behavior gives to the observer. The analysis 
of what goes on in an avian mind is a problem which the comparative psychoi- 
ogist does not regard with appreciable cptimism. But of the following ob- 
jective facts we may, I think, be certain: 
1. The flock moves from place to place by what may be termed the 
spread of impulse. An individual bird, moved no doubt by the hunger instinct, 
takes temporary leadership, and is followed to a new location by the others. 
There are no regularly assigned leaders, though probably the most venture- 
some birds assume the leadership most often. 
