160 
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 1. March 20/2 
pattering of >50 re-entering grebes occurred. 
Two or three more waves by other groups 
followed immediately. Five minutes later, 232 
grebes were counted inside the EW canal. 
Only the continuous and contiguous calls of the 
birds were heard for —20 min. Ten birds then 
initiated a fourth round of pattering in waves. This 
time, the grebes had no common general direc¬ 
tion. The grebes more in the central part of the 
observed area moved towards the dam, those 
already closer to the eastern end paltered into a 
more southwestward to westward direction. 'The 
population present divided into two groups. About 
100 grebes were clustered near the dam and 
another 100 were scattered over the upper western 
third of the EW canal. The space in between both 
groups remained mostly empty. The western 
group started immediately to swim eastward while 
the eastern group slowly dispersed. The groups 
soon melted and spread over the empty space that 
had separated them. 
Perhaps five additional pattering flights of up to 
4-5 grebes were observed in between the different 
mass pattering and pattering flights. It was not 
known whether these were premature attempts to 
initiate a wave or whether they were unrelated to 
the mass movements. 
The observations ended at ~ 1700 hrs and 257 
grebes were counted in the EW canal (26 in the 
connecting corner square to the NS canal), 65 
were present in the lower half of the NS canal and 
347 in the upper half. Only five additional Eared 
Grebes were detected at the mouth to sump I A. 
Other parts of the sump close to the English 
Channel were empty of Eared Grebes. A count of 
the birds at 0700 hrs on the following day totaled 
exactly 400 individuals, 269 less than the previous 
count. Only 77 grebes were observed inside the 
EW canal (28 in the connecting corner square) 
while the NS canal had 323 grebes. Three hours 
later, 126 Eared Grebes were recorded in the EW 
canal and 337 in the NS canal. The two counts on 
26 May revealed quite differing numbers of 
grebes. The EW canal held 131 to 180 grebes 
less and the entire English Channel held 206 to 
269 grebes less than on the afternoon of 25 May. 
Eared Grebes had arrived at Tule Lake Rt 
in the course of the previous 2-3 weeks. I as: 
that shortly after arrival, their wing muscles 
•till in good (light condition on 25 May 
intense practicing could not have explained 
mass pattering. Most birds were actively courting, 
hut the group pattering did not appear to tv 
related to pair bonding. There is also no reason to 
believe the grebes tried to divert an aerial predate 
with common flight activity as several instance* 
ol Bald Eagles (Haliaeetus leucncephalus) ap¬ 
pearing in High! over the grebes or even trying a 
catch in the canal did not trigger much reaction. 
Birds pattering to escape a pursuing conspecificor 
to flee possible danger incited alarm at the most to 
a handful of other Eared Grebes in their 
immediate vicinity. The generalized pattering by 
larger groups of Eared Grebes observed appeared 
unrelated to courtship, aggression, fear or predator 
presence. A similar or comparable behavior by 
Black-necked Grebes (P. n. nigricoUis) in Europe 
has not been reported. 
There is comparable agitation in Silvery Grebes 
(P. occipitalis) during migration towards breeding 
areas. Fjeldsa (1982) noted that Silvery Greks 
show high restlessness and form long lines that 
move back and forth on a lake from where, in the 
subsequent night, at least part of the population 
departed. He termed this pre-migratory restless¬ 
ness. Movements of a group of 70 Silvery Grebes 
at Laguna Las Encadenadas, Argentina, in De- 
cember 2006, were not limited to swimming, but 
included sudden quasi-simultaneous take-offs ol 
individuals more at the rear end of the line. Some 
flew up, reaching a height of -2 m, possibly to 
avoid collision with the birds preceding them. 
They landed again in front of the group that was 
moving in one direction. The grebes at the rear 
end acted similarly. The group changed direction 
as it approached the shore, but continued 
swimming in a line, and pattering and flying from 
the back to the front (Konter 2009). 
Eared Grebes at Tule Lake Refuge all swam 
actively into the same direction, although they did 
not form one line. They showed pattering and 
pattering flights in waves and repeated the 
directed group movements. .4 priori the compar¬ 
ison of total counts of grebes inside the English 
Channel on the following day strongly sugge'^ 
least a major portion of the population had leti ihc 
area. Zugunruhe seems an appropriate character 
ization for the Eared Grebes' behavior. Additional 
pre-departure activities at Tule Lake Refuge 
including group diving, submerging and surfacing 
in near unison as reported by Jehl and Henry 
(2010) were not obvious. The grebes' diving and 
swimming seemed to be predominantly related to 
feeding, except the dives after mass pattering 
