10.25338/B8SW5G
Eadie, John
0000-0001-9573-2703
University of California, Davis
Lyon, Bruce
University of California, Santa Cruz
Data for: Environmentally-driven escalation of host egg-rejection
decimates success of an avian brood parasite
Dryad
dataset
2020
avian brood parasitism
Egg Rejection
environmental perturbation
host-parasite co-evolution
Host behavior
black-headed duck
South American coots
conservation behavior
British Broadcasting Corporation (D. Attenborough's Life of Birds)*
National Geographic Society
https://ror.org/04bqh5m06
#5099-93
2020-10-21T00:00:00Z
2020-10-21T00:00:00Z
en
https://doi.org/10.1093/beheco/araa084
97397 bytes
4
CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
The black-headed duck (Heteronetta atricapilla) of South America is the
only known avian obligate brood parasite with precocial offspring. In
Argentina, it relies on two species of coots as primary hosts, which
typically reject 35-65% of duck eggs. We show that environmentally-driven
increases in host egg-rejection behavior lead to substantial reductions in
the reproductive success of the brood parasite. Episodes of flooding and
vegetation loss caused dramatic shifts in host egg rejection behavior,
resulting in rejection (85-95%) of almost all duck eggs. Coots respond to
fluctuating water levels by building up their nest, raising their own eggs
but leaving duck eggs behind. Coots can apparently recognize parasitic
duck eggs, but large-scale rejection is triggered only when hosts must
actively make a choice. We use a simple population model to illustrate the
unique demographic challenges that black-headed ducks face with their
parasitic lifestyle, and to explore the potential impact of
environmentally-induced escalation of egg rejection. Using best available
estimates for key vital rates, we show that obligate parasitism may
provide a demographically precarious existence for black-headed ducks,
even under benign environmental conditions. Environmentally-mediated
increases in egg rejection rates by hosts could impact significantly the
viability of this enigmatic species of brood parasitic duck. Our results
demonstrate that egg rejection rates are not fixed properties of host
populations or individuals but are strongly influenced by social and
ecological factors. Shifts in these environmental drivers could have
important and unforeseen demographic consequences for brood parasites.
Field observations and experiements in Argentina in 1993, 1994 and 1997.We
conducted systematic surveys of the marshes every two to four days on foot
or by canoe to find potential host nests and detect brood
parasitism. Nests were identified to species by observing birds on or near
nests. Parasitism was easily detected because the duck eggs differ
dramatically from the eggs of both major hosts. Environmental effects on
egg rejection were determined using four methods: natural cases of brood
parasitism by the ducks (Mal Abrigo Gull and Tern Marshes and Real Viejo
Marsh A, all 1993), experimental parasitism with real duck eggs swapped
among host nests as soon as the eggs were laid (Real Viejo Marsh B 1993),
experimental parasitism with painted domestic chicken eggs whose length
and width (length: 50.5-63.5 mm, width: 41.3-47.6 mm, n = 314 eggs)
overlapped the size range of duck eggs (Cari Lauquen 1994, Real Viejo
Marshes A and B 1997), and experimental parasitism with real coot eggs
from nests of conspecifics (Cari Lauquen 1994, Real Viejo Marshes A). The
diversity of approaches was employed originally to examine different
aspects of the host-parasite interaction (e.g. Lyon and Eadie 2004, 2013),
but when the environmental changes occurred, we realized that we could
leverage these experiments to evaluate how the changing environmental
conditions affected rejection behavior of the hosts. We used nominal
logistic regression analysis to evaluate the effects of flooding, species
of host, and wetland site on rejection rates of parasitic eggs in 1993.We
used a Fisher’s exact test to contrast rejection rates before and after a
second flood event in 1997 on one wetland (Real Viejo Marsh B) in an
experimental study of egg rejection; this comparison involved a single
wetland (Marsh B), one host species (red-gartered coots) and a single egg
type (painted hen egg). We used nominal logistic regression analysis to
evaluate the effects of vegetation loss, wetland site, and egg type on
rejection rates of parasitic eggs. For each wetland we pooled all egg
rejections in that year.
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