Switch to diester preen waxes may reduce avian nest predation by mammalian predators using olfactory cues
1 Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB
Den Burg, Texel, The Netherlands
2 Centre for Ecological and Evolutionary Studies, University of Groningen,
PO Box 14, 9750 AA Haren, The Netherlands
* Author for correspondence (e-mail: reneer{at}nioz.nl)
Accepted 6 September 2005
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Summary |
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Key words: uropygial gland, preen wax, camouflage, olfaction, nest predation, sandpiper, Calidris canutus
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Introduction |
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Red knots that departed on, or arrived after, the first part of a non-stop
migratory flight of several thousand km did not secrete diester preen waxes.
Birds of the same population, ready for the second part of the migratory
journey to the High Arctic breeding grounds, did, however. Therefore, it was
suggested that secretion of diester preen waxes was not related to
long-distance flights per se but that the timing of the compositional
preen wax shift was apparently related to breeding activities
(Piersma et al., 1999).
Our first hypothesis, that diester waxes enhance plumage colouration and
function as an individual quality signal during courtship
(Piersma et al., 1999), is not
the only explanation for the observed shift in preen wax composition, as
spectral measurements of plumages of red knots before and after the shift to
diester preen waxes showed no difference in colouration
(Reneerkens and Korsten, 2004
).
Furthermore, the secretion of diester preen waxes continues during incubation,
with a return to monoesters when the chicks hatch. A similar shift to diester
preen waxes during incubation has already been found in wild-type and
domesticated mallards Anas platyrhynchos
(Jacob et al., 1979
;
Kolattukudy et al., 1987
),
which are also ground-breeders. For species whose males do not incubate, the
shift to diester preen waxes is limited to the incubating females
(Reneerkens et al., 2002
).
This indicates that the diester wax cocktail fulfils a specific function
during incubation, but that function during this crucial phase is unknown
(Reneerkens et al., 2002
).
Ground-nesting birds are particularly vulnerable to loss of their clutch to
predators (Whelan et al.,
1994), which can greatly influence the population dynamics of
ground-nesting birds (Blomqvist et al.,
2002
). Because of their high molecular mass, diesters are less
volatile than monoesters and might thus be more difficult to detect by
olfactory-searching predators. In this study we tested this hypothesis using a
sniffer dog trained to locate different amounts of pure mono- or diester preen
waxes.
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Materials and methods |
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Under a fume hood we pipetted 0.5 ml of the solution to square metal rods that were lying on clean aluminium foil. The solution was equally spread over two sides of the square rods using a pipet, such that the side with waxes never touched the aluminium foil. Control rods were applied with 0.5 ml pure ethyl acetate. After evaporating off the volatile solvent, the metal rods were kept in airtight glass jars. Rods and glass jars were boiled in water for 10 min and washed without detergent in a dishwashing machine before use. Metal rods and glass jars were never touched and always handled using a pair of metal pliers.
Sniffer dog
We trained a 6-year-old female German shepherd dog to locate different
amounts of both mono- and diester waxes. The dog had positive health
certificates on stamina and had been recommended for breeding. Initially, the
dog was taught to sniff systematically a row of six plastic tubes mounted 1 m
apart on a wooden board, to locate the metal rod applied with smell of the
dogs owner at a randomly chosen position and be rewarded for it by being
allowed to play with the rod for some time and by compliments from the
trainer. The dog trainer applied his own smell to the rod by touching it and
keeping it in his pocket. Control rods remained untouched and were placed in
the remaining locations. After the dog had located the rod with the smell of
the dog trainer convincingly several times, human smell was replaced by 1 mg
mono- or diester preen waxes. To get an idea of the amounts at which the dog
started to fail locating the preen waxes, the amounts of preen waxes on the
rod were gradually decreased during the training procedure. Training took
place from January 2003 to February 2004, and the actual experiments on four
different days during the period February to July 2004 in familiar
surroundings, in the garage of the dog-owner.
The experiment was performed with different amounts of mono- and diester preen waxes, between 0.24 and 15.6 µg. The intention of the experiment was to examine whether detection probabilities are equal for the same amounts of preen wax. Under natural conditions, the quantity of wax molecules in the air will depend on the distance from the source. The amounts of preen wax used in this qualitative experiment, in which the dog sniffed the rods at a distance of only a few cm, therefore do not need to reflect the natural amounts expressed by birds. The order of sessions with respect to composition (mono- or diesters) and amount of preen wax was randomised. Dog and trainer were unaware of the location of the treated rod. If it had smelled the preen wax, the dog would take the metal rod (see supplementary material). On failing to locate the rod with wax, the dog continued systematically searching the row of tubes, sometimes up to 30 times before giving up. On giving up, the dog often started searching elsewhere in the room where the experiments were carried out. The dog never indicated a finding of preen wax on control rods, i.e. never made a mistake.
The success with which the dog located the wax was scored for wax
composition and amount. Each combination of wax composition and amount was
tested 20 times (280 experiments in total) over 4 days, on each of which all
combinations of wax amounts and composition were tested five times. Detection
chance (Pdetection) was analysed using a logistic model
ln(Pdetection/1Pdetection)=a+bxamount,
which was fitted to the data by iteration
(Crawley, 1993). Factors
(amount of wax, wax composition and their interaction) were added separately
to the model and a
2 test was used to estimate whether the
addition of factors caused significant reductions of deviance.
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Results |
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Discussion |
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Predation of eggs by mammalian, olfactory-searching predators largely
determines the reproductive output of young sandpipers
(Blomqvist et al., 2002). This
severe natural selection has led to the evolution of cryptic plumage and egg
coloration to conceal nests and incubating birds from visually searching egg
predators (Solís and De Lope,
1995
; Jukema et al.,
2003
). It has long been known that many mammalian predators rely
on smell to locate prey (Whelan et al.,
1994
), and although folk wisdom relates that snipe and quail are
impossible to detect by hunting dogs as long as they are on their nest, to the
best of our knowledge, this is the first experimental test of olfactory
crypsis as a potential complementary anti-predation strategy. Future research
should reveal if our findings using a single dog can be generalised to natural
predators in a field situation where detection probabilities also depend on
distance from the incubating bird, wind conditions and habitat
characteristics.
Seasonal shifts in preen wax composition are presumably the result of a changing balance between costs and benefits of the production, and use of diester rather than monoester preen waxes. In the non-breeding season sandpipers live in large flocks and can fly away from mammalian predators. Their reliance on monoesters during times when olfactory crypsis is unimportant suggests that the production or use of diesters carries a cost. A greater understanding of the energetic costs and of functional properties, such as anti-parasitic aspects or waterproofing, of mono- and diester preen waxes is necessary to better understand seasonal shifts in preen wax composition.
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Acknowledgments |
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Footnotes |
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References |
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