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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 2, June 2011 
Mayfield nest survival in this stand was the lowest 
among eight stands that we monitored from 2007 to 
2010 (37% vs. 61% for seven other stands 
combined). 
OBSERVATIONS 
We watched a female Cerulean Warbler 
(hereafter Female A) between 1230 and 1405 
hrs EST on 19 May 2010 aggressively attack an 
incubating female Cerulean Warbler (hereafter 
Female B) and destroy eggs in the nest. We found 
this nest on 10 May 2010 during the egg-laying 
stage, most likely after one egg had been laid 
(based on timing of behavioral observations at the 
nest). The nest was 16 m high in a cucumber 
magnolia (Magnolia acuminata). We observed 
Female B incubating for the first time on 15 May 
2010; Hie egg destruction occurred on the fifth day 
of incubation, Female B’s mate was a fourth-year 
male banded in 2008 which had returned to this 
approximate location each of the following two 
breeding seasons. We monitored the interactions 
through a Kowa TSN-821 spotting scope 
equipped with a 2Q-60X eyepiece. We also 
attached a Nikon Coolpix P5100 digital camera 
to the scope and recorded much of what is 
described. The video is available at http://www. 
youtube.com/watch?v=rq2zUSVVhAs. 
We were able to distinguish between the Iwo 
females involved because Female B appeared to 
be a typical female Cerulean Warbler while 
Female A possessed several distinctive character¬ 
istics including a light breast band (which we have 
seen in other female Cerulean Warblers, but it is 
relatively unusual) and darker than normal 
streaking on the sides and flanks. The female- 
female interactions began while Female B's mate 
was fighting with another male nearby (it is 
unknown if this male was paired). Female A was 
observed watching the male-male interactions and 
became physically involved in the fights several 
times (including tumbling to ground with the 2 
males), while Female B remained on her nest. 
Female A also behaved in a manner reminiscent 
of a territorial male, repeatedly flicking her wings. 
Female A approached to within 3 m of the nest 
where Female B was incubating on three occa¬ 
sions over a 10-niin period as the two males 
interacted aggressively. Female B chased Female 
A from the nest tree on each approach. On the 
fourth approach. Female A moved within 1 m of 
the nest. Female B again chased Female A, 
however Female A did not retreat; instead the 
two birds fell to the ground together, their feet 
extended and clasped together for several seconds, 
This same interaction occurred three more times 
over the next 10 min, after which Female B 
returned to the nest to incubate. Two min later, 
Female A Hew directly at Female B while she was 
on her nest. Female A used her feet to grab the 
wing of Female B and hung upside-down unde: 
the nest for several seconds until Female B flew 
off and the two birds tumbled to the ground 
together. Similar interactions were repeated five 
times; each time Female B was pulled or struck 
off the nest, tumbled to the ground with Female A. 
and then returned to incubate. After the sixth 
instance where Female A displaced Female B. we 
observed Female B sitting in a low sapling with 
one of her wings drooping slightly. 
Subsequently, Female B did not return to her 
nest; female A returned twice and simply looked 
into the nest. However, on her third trip to the 
abandoned nest. Female A stuck her head into the 
nest and made several pecking motions which 
were accompanied by the sound of eggs being 
punctured (heard from the ground below). Female 
A then fiew and 10 min later returned, repeated 
the puncturing motions, and also appeared to 
ingest/drink something from inside the nest 
(possibly yolk from broken eggs). Female A 
returned to the nest 5 min later, once again 
appeared to consume something, and then wiped 
her beak on the side of the branch. Female A 
returned once more (I min later) and perched nett 
to the nest for 30 sec, looking around vigilantly 
Female A then flew and we observed no activity 
at the nest for the next hour. We returned to the 
nest every 1-3 days for the next 30 days and did 
not observe another bird at the nest or in the nest 
tree. We observed the banded male singing in the 
same territory many times, and observed him 
feeding fledglings on 6 July 2010, but no other 
nest was found and neither female was observed 
again. The nest was still intact as of 6 July 2010. 
DISCUSSION 
Egg destruction is rarely observed and it ^ 
difficult to infer the adaptive nature (or lack 
thereof) of this behavior. The female Cerulean 
Warbler that destroyed the eggs of a conspcciti- 
could have done so as: (1) an adaptive sexual 
strategy. (2) a strategy to reduce local competition 
for resources. (3) an exploitation strategy, or (4 1 a 
pathological behavior with no adaptive value. The 
male occupying this territory helped raise a brood 
