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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 2. June 2012 
FIG. 1. Section of the pond at which agonistic 
interactions were observed between two female Anhingas. 
Diagonally hatched rectangle = bridge; stippled line = 
approximate boundary of the foraging site; dashed line = 
approximate boundary of the agonistic interactions; square 
= tree with nestlings. Scale = 20 m. 
resident female chased it by alternating diving 
with swimming, and even flapping the wings, a 
behavior that caused the intruder to dive and 
retreat (Fig. 2B). The resident female vocalized 
while still in the water (Fig. 3A) and heading for a 
perch on the bank toward the site where the 
intruder disappeared. The resident vocalized from 
time to time toward the pond while perched on the 
bank or a branch near the foraging site waiting for 
its plumage to dry (Fig. 3B). If the intruder female 
again approached the foraging area, the still wet 
resident female plunged into the water and chased 
the intruder. The intruder was likely the same 
individual, as there was another mated pair on the 
opposite side ot the pond. The agonistic behavior 
was similar during all 3 days. The resident female 
and its mate were lending two nestlings on a tree 
— 150 m from the foraging site at the time of our 
observations (Fig. 1). 
The resident female promptly and insistently 
responded to playback of its own call, directing 
its vocalization toward us. This vocalization is 
a series of croaking sounds, consisting of 6-10 
notes with 4601 Hz maximum frequency, 1312 Hz 
dominant frequency, and 872 Hz minimum 
frequency (Fig. 4). The notes last 0.1-0.33 sec 
and the intervals are 0.09 to 0.13 sec in the 
recorded vocalization. The sound of this vocali¬ 
>- 
FIG. 2. (A) The resident female Anhinga (right) moves 
quickly toward an intruder female (left) that approached a 
highly rewarding foraging site. (B) The resident female 
vigorously Haps its soaked wings while chasing the 
intruder, which has just dived (concentric circles). 
zation type is roughly similar to the record 
XC5766 (http://www.xeno-canto.org/browse.php? 
query ^anhinga). 
DISCUSSION 
We initially interpreted the unusual aggressive 
behavior of the resident female as a type ot 
territorial parental care, as it fed the nestlings with 
small fishes that were abundant at the foraging 
site. However, the bird continued to vocalize at 
the foraging area for ~2 weeks after termination 
of nesting. Tims, parental care was not the main 
cause ol the aggressive behavior even if care of 
young may have initiated it. Instead, we regard the 
aggressiveness of the resident female as a detense 
of a particularly rewarding foraging site, as the 
pieces of bread supplied by people promoted and 
maintained a high fish density near the bndge 
(Sazima 2007). Anhingas vocalize at breeding 
sites or at perches (Orta 1992, Fredenck and 
Siegel-Causey 2000). although vocalization at a 
foraging site seems unrecorded to date. The 
resident bird’s vocalization toward its playback 
