Response of a forestinterior songbird to the threat of cowbird parasitism

1
Response of a forest-interior songbird to the
threat of cowbird parasitism
David Mark Bridget J. Stutchbury Dept. of
Biology, Yale University, Newhaven Dept. of
Biology, North York, Ontario Presented By Bryan
Gammons

• Identifying the Problem
• Brood parasitism by cowbirds generally lowers the
  reproductive success of host species (Payne 1977)
  and in some cases exceeds predation as a source
  of reproductive loss (Burgham Picman 1989).
• Species that have always been sympatric with
  cowbirds display a variety of behavioral defenses
  towards cowbirds including aggressive behavior
  (e.g. Robertson Norman, 1976) guarding the nest
  and cowbird egg ejection.
• Human-induced changes in the environment have
  resulted in the expansion of the brown-headed
  cowbird, Molothrus ater, onto eastern North
  America (Friedman 1929) and a dramatic increase
  in the cowbird abundance.
• Brown-headed are a unique species of birds which
  are generalist parasites, laying their eggs in
  the nests of a wide range of other species.
• New host species may be particularly vulnerable
  to reproductive loss resulting from parasitism
  because they are unlikely to have evolved
  effective defense strategies towards cowbirds.
• Cowbird parasitism is a major contributor in the
  overall decline of eastern songbirds.
• The first step in the development of a successful
  anti-parasite strategy by a potential host is the
  recognition of the cowbird as a threat.
• This study examines whether an eastern
  forest-interior species, the hooded warbler,
  Wilsonia citrine, perceives cowbirds as a threat
  to the nest.
• Forest fragmentation has allowed cowbirds to gain
  access to the interior of small forests patches,
  putting even these host species at risk.

• Results
• Female hooded warblers respond significantly more
  aggressive to the cowbird model than the veery
  model (Table 1).
• The response to the cowbird model often involved
  seet alarm calling after the model was
  discovered, followed by a prolonged period off
  the nest in which the female would remain close
  to the model and vocalize.
• Most females gave seet calls during the cowbird
  trials while few gave seet calls during the veery
  trials (Table 1).
• Females were much more likely to give passes at
  the cowbird model than the veery model (Table 1),
  however none actually struck the models.
• Females with no stimuli verses the veery stimuli
  did not differ significantly in the time off the
  nest, time alarm calling, and chip rate (Table
  1).
• Some females (52) spent over twice as long off
  the nest during the cowbird trials compared with
  the veery trials (Fig. 1), but others (12)
  remained off the nest for long periods of time
  for both trials.
• The proportion of time that females spent alarm
  calling in the presence of a cowbird model ranged
  from 0 to 97 and the chip rate fanged from 5 to
  56 per minute.
• The hooded warblers degree of aggression was
  based on
• 1. time off the nest was 100 greater for
  cowbirds than veerys.
• 2. proportion of time spent alarm calling o the
  cowbird was greater than 10.

Question To test whether an eastern
forest-interior songbird, the hooded warbler, can
recognize brown-headed cowbirds as a threat to
the nest, a mount of a cowbird or a control, the
veery, was presented to 25 incubating females.

• Discussion
• Hooded warblers clearly recognize cowbirds as a
  threat to the nest.
• Females responded to models of brown-headed
  cowbirds with close approaches, alarm calling,
  increased chip rate and increased time of off the
  nest.
• Hooded warblers are remarkable in that they are
  not historically sympatric with cowbirds, and
  have therefore developed this strong cowbird
  recognition relatively recently.
• The high rate of parasitism in this particular
  forest in the past (60) suggests that aggression
  towards cowbirds is not an effective deterrent,
  but it is possible that rates of parasitism would
  be even higher in the absence of host aggression.
• It is not know whether cowbirds abandon nest
  searching and egg laying when discovered by the
  host species.
• Male hooded warblers showed little to no
  aggression, however in similar warbler species
  which are also sympatric to cowbirds which also
  do not show signs of aggression towards cowbirds.
• In contrast many other species which the cowbird
  parasitizes, the males play a significant role in
  attacking the parasites.
• Although the experiment nearly eliminated
  cowbirds from the experimental area, yearling
  female hooded warblers did not discriminate
  between cowbird and veery models. The yearling
  females may have been hatched in populations that
  did have cowbirds so there was a good chance they
  encountered one in their past.
• All older females in our sample had bred the
  previous year in a population where cowbirds were
  not being controlled, and thus probably did have
  an opportunity to observe cowbirds around their
  nest.

Hypothesis Naïve yearlings females should show
little or now discrimination between cowbird
recognition (Briskie et al. 1992). In addition,
older females are predicted to be more aggressive
towards models than yearling females (Hobson
Sealy 1989).
A.
B.
A. Brown-headed cowbird verses B. Veery

• Methods
• The study was conducted from May to July 1991 and
  1992 in Crawford Co, Pennsylvania on a 200 ha
  mixed hardwood forest which supports about 40
  pairs of hooded warblers.
• All nests were labeled with flagging about 2m
  away, and were checked every 5-7 days.
• They presented models of a female brown-headed
  cowbird and a male veery to 25 different
  incubating females.
• They used the veery as a control because that
  species represents no threat to hooded warblers,
  are commonly found in the study area, and are
  similar in size and color to cowbirds.
• Models were attached approximately 2m from the
  nest to a branch while the female was away from
  the nest.
• They played recordings of a female cowbird call
  or a male veery song.
• In half of the trials they placed both stimuli
  equidistant from the hooded warbler and compared
  the aggressive responses.
• The trials were at least 24 hours apart to avoid
  habituation and the observer were concealed
  10-15m away.
• The observer noted 1. The females proximity to
  the model.

A nest parasitized by a cowbird.