Introduction
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Birds as carriers of propagules are major agents in the long
distance dispersal (LDD) of plants, animals, fungi and microbes
(
Green and Elmberg, 2014
).
Darwin (1859)
suggested that water
birds dispersed freshwater plants; he also demonstrated experi-
mentally that ducks were potential dispersers of freshwater snails.
As part of their migratory movements, birds often travel long dis-
tances in relatively short periods of time (
Clausen et al., 2002;
Brochet et al., 2010
). Teal (
Anas crecca
), for example, have been
known to cover an average of 100 km per day and potentially up to
1000 km within a few days, while pintail (
Anas acuta
) have trav-
elled 1,600 km within 24 h (
Clausen et al., 2002
). Even if only a
minority of individuals participate in the transporting of propa-
gules, the impact may be substantial considering the thousands of
birds travelling long distances annually, and the even larger
numbers travelling shorter distances (
Charalambidou et al., 2003;
Brochet et al., 2010
).
The successful dispersal of an organism will be governed by the
ability of its propagule to survive the transportation process
(
Figuerola et al., 2010
). Plumage is thought to play a key role in
epizoochorous dispersal. Evidence suggests that the moisture
content (humidity) found within-plumage can in
fl
uence the suc-
cess of an epizoochorous event, with higher humidities reducing
the rate of desiccation and therefore maintaining propagule
viability for longer periods (
Coughlan et al., 2015
). Currently,
however, there is a lack of empirical data in relation to bird-
mediated, epizoochorous dispersal (
Costa et al., 2014
). Additional
information is required in order to evaluate the likelihood of suc-
cessful transportation of propagules by birds, and their survival in
the associated microclimatic conditions. It is especially necessary to
gain an understanding of the temperature and moisture i.e. hu-
midity regimes that exist within the plumage of birds. Dispersal
kernel modellers need to know the conditions experienced during
*
Corresponding author.
E-mail address:
neil.coughlan.zoology@gmail.com
(N.E. Coughlan).
Contents lists available at
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Acta Oecologica
journal homepage:
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/locate/actoec
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2015 Elsevier Masson SAS. All rights reserved.
Acta Oecologica 65-66 (2015) 17
e
23
transport and the survival of propagules under those conditions.
Species-speci
fi
c dispersal distances (e.g. daphnia resting eggs,
snails and aquatic plants) may then be predicted.
Plumage is the most complex integumentary structure of ver-
tebrates and is characteristic of birds (
Stettenheim, 2000; Broggi
et al., 2011
). Plumage is comprised of unique, complex, non-living
outgrowths known as feathers (
Stettenheim, 2000; Tickell, 2003
)
which, in addition to enabling lift and propulsion for
fl
ight, also
provides for
inter alia
, thermal insulation, streamlining, water-
proo
fi
ng, social communication and camou
fl
age (
Stettenheim,
2000; Tickell, 2003; Broggi et al., 2011
). Downy feathers provide
thermal insulation by trapping air close to the skin (
Stettenheim,
2000
). Larger contour feathers also provide insulation and can be
adjusted to partially regulate body temperature (
Stettenheim,
2000
). In effect a thermal buffer is created between the bird and
its surroundings. Birds primarily lose heat to the ambient envi-
ronment by non-radiative heat
fl
ow through conduction and con-
vection (
Walsberg, 1988
) and an almost complete thermal gradient
between skin temperature and environment can be attained within
the plumage (
Davenport et al., 2004
). Feather abundance and
structure crucially determine the buffer’s ability to minimise heat
loss, set an upper limit to insulation capacity and, in part, regulate
heat loads from solar radiation (
Wolf and Walsberg, 2000; Broggi
et al., 2011
). The ability to create a thermal buffer can be essential
for avian survival when environmental conditions are unfavourable
(
Brodin, 2007
). Similarly, the loss of water vapour from the integ-
ument of an organism to the ambient environment depends on the
nature of the barrier to evaporation, the concentration of water
vapour at the evaporating surface, and the humidity of the air
(
Webster et al., 1985
). Body regions differ in their degree of insu-
lation and therefore variations in microclimate over the entire
anatomy of the body are to be expected (
Webster et al., 1985
).
Many organisms will exploit the plumage and its thermal buffer
zone for dispersal, or as a permanent habitat. Birds are host to many
parasites including a wide range of medically and veterinarily
important ectoparasites (
Clayton and Moore, 1997; Atkinson et al.,
2008
). The composition of epifauna found within the plumage is
governed by many variables, including
inter alia
, host behaviour,
ecology, anti-parasite defences and parasite competition (
Clayton
and Walther, 2001
). Ectoparasites living within or beneath the
plumage will be directly exposed to plumage microclimatic con-
ditions (
Moyer et al., 2002
). For instance, the distribution of lice
(Phthiraptera) (
Clayton, 1991; Clayton et al. 2008
) and mites
(Arachnida) (
Bonser, 2001; Pence, 2008
) between, and within,
feathers will be in
fl
uenced by microhabitat properties such as hu-
midity and temperature (
Mestre et al., 2011
). In his seminal work
Yu. S. Balashov (1968)
observed the behaviour and survival of ticks
under varied conditions of relative humidity and temperature. He
hypothesised that bird parasitizing species with long feeding pe-
riods will have a greater potential for dispersal, and that migratory
movements of birds can account for the wide distribution of some
Argas
species and other bird parasites. Indeed, it is the opinion of
Hoogstraal et al. (1961, 1963, 1964)
that there is an in
fl
ux of African
tick subspecies into Europe and northern Asia in the spring, and an
ef
fl
ux in autumn, facilitated by transmigrating birds (
Balashov,
1968
).
In this study the microclimate within the plumage of mallard
ducks was examined. The mallard is the most abundant and
widespread dabbling duck species in the world, with a global
breeding population estimated to be at least 18 million birds
(
Delany and Scott, 2006; S
€
oderquist et al., 2013
). The species is
comprised of both sedentary and migratory populations, though
birds in both groups will frequently intermingle (
Delany et al.,
2006
). As an economically important quarry species, wild stocks
are heavily augmented with farmed birds (
Guillemain et al., 2010;
S
€
oderquist et al., 2013
). Given their ecology, water birds in partic-
ular are considered leading contributors to bird mediated dispersal
(
Costa et al., 2014; Green and Elmberg, 2014; Coughlan et al., 2015
).
We hypothesised that a microclimate exists within the plumage,
where temperature and humidity remain reasonably constant,
which can facilitate plant propagule survival. In this study surface
temperature and humidity within the plumage of mallard (
Anas
platyrhynchos
) were measured. To link the identi
fi
ed microclimatic
regimes with propagule survival, we examined the survival and
viability of a common aquatic plant
Lemna minor
under similar
conditions to those observed within the mallard plumage. The re-
sults reveal the relative suitability of different areas within the
plumage for propagule survival.
2. Methods
2.1. Examination of temperature and humidity
Five game-farm reared mallards were acquired and kept in a
large, out-door, free-range enclosure (15 m
3m
3 m), which
included a housing unit for shelter and an arti
fi
cial pond. The
mallards were investigated in the spring of 2013 on three separate
occasions; in each experiment three birds were sampled. All birds
were sampled at least once, no bird was examined twice on a single
occasion. Subsequently,
fi
ve more birds were added to the enclo-
sure. All ten individual mallards were investigated in the spring of
2014, each bird on a separate occasion. All birds had been out of
water for at least 120 min before monitoring of the microclimate
within the plumage commenced. Birds were held in the hands of an
operative, while a second operative examined the microclimatic
conditions. Restraint of the birds in this manner did not interfere
with the examined anatomical areas, nor cause stress as the birds
were used to handling.
A dual hygrometer/thermometer (Fisher Scienti
fi
c, 11-661-18,
Waltham, MA, USA) was used for the measurement of plumage
temperature (
C) and relative humidity (%RH). The portion of the
instrument probe which needed to be inserted within the plumage
was 20 mm long and 13 mm wide. The end of the probe, which
protects the sensors, is plastic with 8 slots, each 3 mm wide. Areas
such as the breast and back were sampled with the probe held at
acute angles to the plumage to allow for full immersion of the probe
within it. Readings were taken from as close to the skin as possible.
The probe remained in place until microclimatic conditions were
observed to recover and stabilise. This took several minutes. The
birds were examined in the
fi
eld, ambient temperature and relative
humidity were recorded in both 2013 (11.2
±
1.1
C; 69.4
±
4.8% RH)
and 2014 (10.2
±
0.7
C; 70.6
±
1.3% RH) (mean
±
SE) at the time of
the experiments.
Multiple areas were examined on all individuals in order to
compile an overall map of the temperature and humidity ranges
found on the birds. The anatomical areas (see
Fig. 1
) investigated
included the posterior neck (dorsal), the centre breast, the centre
back, either side of the tail within the undertail coverts (crissum),
both wings under the postpatagium, and within the crural plumage
of the inner legs. Readings were taken at the wing with the probe
positioned under the postpatagium and with wings naturally fol-
ded closed against the body. Postpatagium and crural plumages
were examined in 2014 only. The external atmospheric tempera-
ture and relative humidity were also measured both before and
after the experimental protocol had been completed.
2.2. Examination of plumage depth
To quantify differences in plumage structure across the anatomy
of the mallard, depth of plumage was examined using vernier
N.E. Coughlan et al. / Acta Oecologica 65-66 (2015) 17
e
23
18
callipers. The stem of the callipers was gently inserted into the
plumage and positioned as close to bare skin as possible. The stem
was held gently and as perpendicular as possible against the skin.
The base of the vernier callipers was then brought into contact with
the outermost plumage and a measurement was recorded. This was
done for each of the examined areas of plumage on
fi
ve randomly
selected individuals. No bird was sampled twice. In the case of the
inner crural, the stem was inserted into the pit of the leg. Plumage
feather type composition was also judged by the gentle back-
combing of plumage by hand.
2.3. Lemnaceae cultures
Axenic cultures of
L. minor
were maintained on half-strength
Hutner’s growth medium in 100-ml magenta vessels in a
controlled environment growth room (
Lahive et al. 2011
). The
standard conditions for plant culturing were 16-h light: 8-h dark-
ness at a light intensity of 50
m
mol m
2
s
1
(cool white
fl
uorescent
lamps) and a temperature of 22
±
2
C.
L. minor
had been collected
locally (Blarney, Co. Cork, Ireland).
2.4. Desiccation and survival experiments under ex situ conditions
Following the method outlined in
Coughlan et al. (2015)
,
L. mi-
nor
colonies were removed from the magenta vessels and excess
media gently was removed using
fi
lter paper. Damp colonies were
then spread out: (a) on glass plates, representing severe drought
stress; (b) in plastic containers between two layers of two to four
feathers of mallard plumage (obtained from the breast, back,
crissum and neck of dead mallard provided by game-hunters) or (c)
in plastic containers between two layers of two to four feathers of
mallard plumage resting on a double layer of damp to saturated
fi
lter paper. Samples were returned to fresh growth medium at
regular 1-h intervals for up to 25 h to quantify drought survival and
viability under stagnant air conditions. At each time point, six
replicates, comprised of four colonies of two fronds, were taken.
Control samples were directly transferred between magenta ves-
sels, without exposure to drought. Resumption of growth after
drought stress was measured as an increase in the number of col-
onies, fronds and biomass after 7 days under standard growth
conditions. The relative humidity and temperature were recorded
at regular intervals within the containers from within the plumage,
using a dual hygrometer/thermometer (Fisher Scienti
fi
c,11-661-18,
Waltham, MA, USA). If humidity began to decrease, very small
amounts of deionised water were added to a far corner of the
container and its lid was closed until humidity had recovered. If
humidity began to increase, the lid of the container was removed.
The experiments were conducted under ambient room tempera-
ture (21.7
C
±
0.74SD, range
¼
20.6
C
e
23.1
C).
2.5. Speci
fi
c humidity calculations
Relative humidity (RH%) is a percentage of the maximum
possible carrying capacity (absolute humidity) of the air at a given
temperature. The maximum absolute humidity will vary at
different temperatures. When calculated as speci
fi
c humidity a
measurement of water content in grams per kilogram of air is
produced. Speci
fi
c humidity (1) was calculated under standard at-
mospheric pressure, 1013.25 hPa, using the observed relative hu-
midity and temperature.
Speci
fi
c humidity
Q
¼
1000
P
v
=
ð
1
:
6078
Pb
0
:
6078
P
v
Þ
h
gkg
1
i
(1)
where:
Pv
¼
Partial pressure of water vapour [Pa] See Eq.
(2)
Pb
¼
Total or barametric pressure
¼
101,325 Pa
Partial pressure of water vapour
P
v
¼
f
Psat
(2)
where:
Pv
¼
Partial pressure of water vapour [hPa]
f
¼
Relative humidity (%)
Psat
¼
Saturation vapour pressure [hPa] See Eq.
(3)
Saturation vapour pressure
Psat
¼
6
:
1078
10
7
:
5
T
T
þ
237
:
3
(3)
where:
Psat
¼
Saturation vapour pressure [hPa]
T
¼
temperature [
C]
2.6. Statistical analysis
Data were analysed using one-way ANOVAs with the
post-hoc
Tukey HSD in SPSS (version 20; SPSS Inc, Chicago, IL, USA). Multiple
ANOVAs with different dependent factors were performed. Tem-
perature and speci
fi
c humidity were both examined separately for
each body location, with data from each individual area taken as a
dependent factor and year of experiment as the random factor.
Subsequently, plumage temperature and speci
fi
c humidity (exam-
ined separately) were tested across the body surface, with
measured data from all combined body locations as the dependent
factor for both. Plumage depth was likewise calculated with body
locations as the random factor.
L. minor
survival and viability were
examined with the random factor of time and the dependent factor
of the numbers of colonies, fronds and the biomass. All factors were
examined separately for each experimental humidity.
Post-hoc
analysis was conducted for all ANOVAs which compared more than
two groups.
Fig. 1.
Relative humidity (% RH), mean
þ
SE, observed within the plumage of mallard
duck. Neck, back and breast, n
¼
19. Wing and crural, n
¼
20. Crissum, n
¼
38.
N.E. Coughlan et al. / Acta Oecologica 65-66 (2015) 17
e
23
19
3. Results
Ambient air conditions of temperature and the calculated spe-
ci
fi
c humidity at the time of the experiments in 2013 (11.2
±
1.1
C;
5.6
±
0.3 g kg
1
) and 2014 (10.2
±
0.7
C; 5.5
±
0.3 g kg
1
)
(mean
±
SE), were not statistically different; temperature
(F
1,17
¼
0.697,
P
>
0.05), speci
fi
c humidity (F
1,17
¼
0.15,
P
>
0.05).
Likewise the temperature and speci
fi
c humidity pro
fi
les on the
mallards were found to be similar at each of the body locations
sampled in 2013 and 2014 and so the data from these experiments
were pooled (
Table 1
).
Temperature and speci
fi
c humidity were higher in both the
postpatagium and crural plumage (temperature F
5,129
¼
33.366,
P
<
0.01; humidity F
5,129
¼
55.741,
P
<
0.01), compared with other
locations on the birds (
Fig. 2
). The postpatagium and crural plum-
ages were similar in relation to speci
fi
c humidity, but the temper-
ature at the former location was signi
fi
cantly higher. In the case of
temperature, the neck and crissum were also found to be margin-
ally different (F
5,129
¼
33.366,
P
¼
0.044) from each other. The neck,
back, crissum and breast plumage of the mallards were similar in
relation to speci
fi
c humidity. Temperature was similar at the neck,
back and breast of the birds.
The postpatagium displayed both the highest temperature
(33
±
0.8
C) and speci
fi
c humidity (19
±
0.93 g kg
1
), the centre
back the lowest humidity (10
±
0.72 g kg
1
) and the crissum the
lowest temperature (21
±
0.66
C) (mean
±
SE). All other areas
showed very similar values of speci
fi
c humidity and temperature
(
Fig. 2
).
Speci
fi
c humidity found within the plumage was on average
1.8
e
3.5 times greater than ambient humidity. Average speci
fi
c
humidity of each body location was examined against the average
exterior speci
fi
c humidity. Mean relative humidity values (
±
SE)
were highest at the inner crural and crissum (72.1
±
3.4 and
72.8
±
2.1%) and lowest at the centre back (58.4
±
2.9%) (
Fig. 1
).
While examining the birds it was noted that %RH often
decreased with increasing temperature, particularly at the post-
patagium and inner crural. Speci
fi
c humidity was found to increase
with increasing temperature (
Fig. 3
).
3.1. Examination of plumage depth
Mallards displayed signi
fi
cantly different depths of plumage
across the different areas (F
5,24
¼
66.64;
P
<
0.0001) (
Fig. 2
). The
postpatagium and the back had the most shallow plumage, while
the crural and the crissum had the deepest plumage. The crissum
and posterior neck were marginally non-signi
fi
cantly different
(
Table 2
). Each of the examined areas differed in their composition
of feather type’s viz. downy (plumulaceous) and contour feathers
(pennaceous). Some areas, for example the crural and crissum, had
greater amounts of downy than contour feathers, while other areas,
such as the back and neck, had more contour feathers (pers. obs.).
3.2. Desiccation and survival under ex situ conditions
L. minor
colonies were removed from the magenta vessels and
spread out: (a) on glass plates; (b) in plastic containers between
two layers of two to four feathers of mallard plumage; and (c) in
plastic containers between two layers of two to four feathers of
mallard plumage resting on a double layer of damp to saturated
fi
lter paper. Under these three experimental conditions the relative
humidity (RH) was 47.2
±
1.4%, 66.9
±
2.2% and 98.8
±
0.3% (max
99.9%), (mean
±
SE) respectively. Mean temperature (
±
SE) was
21.2
±
0.1
C, 21.6
±
0.1
C and 21.6
±
0.2
C respectively. When
colonies were exposed to a low RH,
L. minor
viability decreased
signi
fi
cantly with increasing time outside the medium (
Fig. 4
). The
longer
L. minor
was kept outside the medium, the lower the
number of colonies (F
4,25
¼
48.87;
P
<
0.01), fronds (F
4,25
¼
62.19;
Table 1
Temperature and speci
fi
c humidity pro
fi
les on the mallard ducks were found to be
statistically similar at each of the body locations sampled in the spring of 2013 and
2014.
Body location Temperature Speci
fi
c humidity
Posterior neck (F
1,17
¼
1.7,
P
>
0.05) (F
1,17
¼
0.896,
P
>
0.05)
Centre back (F
1,17
¼
0.685,
P
>
0.05) (F
1,17
¼
0.591,
P
>
0.05)
Centre breast (F
1,17
¼
1.558,
P
>
0.05) (F
1,17
¼
0.47,
P
>
0.05)
Postpatagium 2014 only 2014 only
Inner crural 2014 only 2014 only
Crissum (F
1,36
¼
3.492,
P
>
0.05) (F
1,36
¼
1.7,
P
>
0.05)
Fig. 2.
(A) Temperature (
C) and (B) speci
fi
c humidity (g kg
1
), mean
þ
SE, observed
within the plumage of mallard duck. Posterior neck, centre back and breast, n
¼
19;
postpatagium and inner crural, n
¼
20; crissum, n
¼
38. (C) Plumage depth (mm),
mean
þ
SE, observed in different areas the surface anatomy of mallard duck (n
¼
5).
Corresponding symbols indicate statistical similarity, otherwise each anatomical
location is statistically different from all others. Dashed line represents ambient
temperature (A) and speci
fi
c humidity (B).
N.E. Coughlan et al. / Acta Oecologica 65-66 (2015) 17
e
23
20
P
<
0.01) and biomass (F
4,25
¼
196.71;
P
<
0.01) produced by fronds
returned to the medium. No viability was detected when
L. minor
had been retained for more than 120 min out of the medium
(
Fig. 4
). In contrast, when colonies were kept under a more mod-
erate humidity, viability was retained for a considerably longer
interval. Even after 5 h outside the medium, viable fronds were
noted (
Fig. 4
). The number of colonies (F
6,35
¼
20.69;
P
<
0.01),
fronds (F
6,35
¼
35.11;
P
<
0.01) and biomass (F
6,35
¼
58.15;
P
<
0.01)
produced following return to growth medium all displayed a very
gradual decline with increasing time out of the medium. When
L. minor
was kept under high RH, an initial decline in colony
number, frond number and biomass occurred after fronds had been
removed from the medium for an interval of 1 h. Thereafter colony
and frond numbers and biomass were stable (
Fig. 4
).
4. Discussion
4.1. Mallard plumage displayed anatomical variations in
microclimatic regime
In this study mallards showed a consistent microclimate (tem-
perature and humidity) within their plumage when exposed to
identical ambient conditions. However, some anatomical variation
was observed. For example, the postpatagium had the highest
(mean) temperature and speci
fi
c humidity, while the centre-back
had the lowest humidity and the crissum the lowest temperature.
Elevated temperatures corresponded to decreased relative hu-
midity but increased speci
fi
c humidity (
Fig. 3
).
Plumage depth does not on its own explain the anatomical
variation in microclimate. The postpatagium and crural plumage
displayed a very similar microclimate, however these areas differed
substantially in plumage depth (
Fig. 2
). Feather type, density and
depth differ anatomically, therefore variation in insulation is ex-
pected (
Webster et al., 1985; Porter et al., 2000
). Certainly the
centre back, observed to have some of the shallowest and the least
dense plumage of the examined areas, showed the lowest speci
fi
c
humidity of the different anatomical areas. Future studies should
more speci
fi
cally examine variation in microclimatic conditions
due to plumage type. Distance from skin is also likely a determining
factor of microclimatic conditions within the plumage. The density
of feather elements is lowest near the skin and greatest at the
Fig. 3.
Speci
fi
c humidity (A) was observed to increase with increasing temperature
(y
¼
0.669x
3.253), while relative humidity (B) was observed to decrease with
increasing temperature within the plumage of mallards. (y
¼
0.633x
þ
82.54).
Table 2
Plumage depth of the mallard ducks was found to vary signi
fi
cantly with anatomical
position on the body.
Body location Depth Signi
fi
cance
Centre back Most shallow (F
5,24
¼
66.64;
P
<
0.0001)
Postpatagium Most shallow (F
5,24
¼
66.64;
P
<
0.0001)
Inner crural Deepest (F
5,24
¼
66.64;
P
<
0.05)
Crissum Deepest (F
5,24
¼
66.64;
P
<
0.001)
Fig. 4.
Formation of new colonies (A), fronds (B) and biomass (C) by drought-stressed
Lemna minor
(mean
±
SE). Plants were drought stressed for up to 2 h at 47.2
±
1.4%
(
A
), 6 h at 66.9
±
2.2% (
–
) and 25 h at 98.8
±
0.3% (
:
) (mean
±
SE). Mean tem-
perature (
±
SE) was 21.2
±
0.1 0.2
C, 21.6
±
0.1
C and 21.6
±
0.2
C respectively. Af-
terwards, four colonies of two fronds were returned to the medium and growth
assessed after 7 days (n
¼
6).
N.E. Coughlan et al. / Acta Oecologica 65-66 (2015) 17
e
23
21
feather
e
air interface (
Porter et al., 2000
). Some areas, such as the
crissum, are covered by a deep and dense plumage. It is likely that
in such circumstances microclimatic measurements are those of
the plumage rather than that at the level of the skin. Increased heat
transfer from the birds’ skin likely accounts for the higher surface
temperature and speci
fi
c humidity observed at the postpatagium.
This is seen once againwithin the inner crural, where the probe was
in direct contact with the tarsus. At these locations the probe was
both in close contact with the skin and well sheltered by plumage.
Thus a gradient across the plumage microclimate is presented here.
The level of the skin is likely best represented here by the post-
patagium and crural examinations. The middle inter plumage zone
being represented by the other examined areas.
4.2. Plumage appears capable of buffering against low ambient
humidity
Moyer et al. (2002)
found within-plumage RH to be highly
correlated with ambient RH for captive pigeons, and concluded that
plumage does not act as an effective humidity buffer. A somewhat
similar trend between ambient and plumage RH was also observed
in our study of mallard ducks. However, in this case, speci
fi
c hu-
midity was found to be signi
fi
cantly higher within-plumage than in
the ambient environment. Speci
fi
c humidity within the plumage
was on average 1.8
e
3.5 times greater than ambient speci
fi
c hu-
midity. Higher speci
fi
c humidity corresponded with higher within-
plumage temperatures. It appears that plumage can buffer the
surface microclimate of mallard ducks against low ambient hu-
midity. There is likely a species-speci
fi
c element that explains this
difference; waterfowl plumage probably has greater insulating
properties than that of pigeon, due to differences in plumage
structure corresponding with their respective ecologies (
Dove and
Agreda, 2007; Dove et al., 2007; Rijke and Jesser, 2011
).
Quantitative studies on desiccation of biota were pioneered by
Edney (1951)
, but were conducted under conditions of negligible
air
fl
ow rate. Therefore,
Edney (1951)
allowed shells of moist
microclimate to persist around the test material. A study by
Kensler
(1967)
revealed that air-
fl
ow dissipates the moist boundary layers
between an organism and the true external environment, hence
causing greatly enhanced water loss at RH values
<
100%.
Kensler
(1967)
also showed that size of the tested organism was impor-
tant
e
small animals dried out much more quickly than larger
animals when exposed to moving air of reduced RH. This was
con
fi
rmed by
Davenport and Vahl (1983)
. However, bird plumage
appears to be ideally suited to minimizing air
fl
ow over small
propagules; its microporosity means that the air within the
plumage is probably stagnant for most of the time, allowing moist
shells to persist around damp propagule material.
4.3. The impact of desiccation stress depends upon the level of
humidity exposure
In relation to dispersal, plumage provides a means of attach-
ment and ensnarement for organisms and propagules (
Raulings
et al., 2011
).
Coughlan et al. (2015)
demonstrated that
L. minor
manually placed between feathers on a live duck can be retained by
free-roaming birds for over 2 h. Any propagule that is entangled
between feathers will be exposed to the microclimate of the
plumage. In keeping with the
fi
ndings of
Coughlan et al. (2015)
on
the survival of desiccation stressed
Lemna minuta
, desiccation
stress was found to depend upon the level of humidity exposure. At
a low humidity (47.2
±
4.7% RH), viability of
L. minor
decreased
rapidly, and no viability was observed after 120 min outside the
aqueous medium (
Fig. 4
). In contrast, when exposed to moderate
humidity (66.9
±
8.7% RH), viability was retained for a much longer
period (
Fig. 4
). Even colonies that had been out of the medium for
6 h displayed some growth. We also found that under conditions of
high humidity (98.8
±
1.4%)
L. minor
retained viability for in excess
of 25 h. Extrapolating these survival data to the temperature and
humidity levels measured between the feathers of living birds
(
Fig. 2
), survival for up to 6 h can be anticipated, especially in
crissum and breast plumage. Survival within the crural and at the
postpatagium would potentially be longer. However, there is also
likely to be increased risk of dislodgement at the postpatagium
during
fl
ight, as well as changes in microclimate caused by venti-
lation induced by wing beat.
Thus, mallard plumage can provide a suitable humid microcli-
mate in which
L. minor
(an aquatic plant with limited capability to
regulate transpiration) can potentially survive for extended periods
of time. At speeds of 65 km h
1
mallards easily cover long distances
and cross geographical barriers (
Cabot, 1977
). Yet, the conditions of
fl
ight, such as the effects of wind speed and wing beat on the
plumage microclimate, need to be considered in more detail if we
are to further our understanding of bird-mediated dispersal. Data-
loggers attached to birds would make it possible to record micro-
climatic conditions in
fl
ight. More examination of plumage micro-
climatic conditions of bird species occupying contrasting ecological
niches is also required. Nevertheless, together with the evidence of
entanglement and retention described by
Coughlan et al. (2015)
,
our measurements of microclimatic parameters make it likely that
mallards and other waterfowl contribute to dispersal of
L. minor
.
Waterfowl are also likely dispersers of many other aquatic organ-
isms. Known temperature and humidity regimes within the
plumage microclimate, combined with known species desiccation
tolerances, will allow dispersal kernel modellers to accurately
assess dispersal distance and propagule survival. In a study by
Barnes et al. (2013)
aquatic plant species
Elodea canadensis
,
Egeria
densa
,
Myriophyllum aquaticum
,
Myriophyllum heterophyllum
, and
Potamogeton richardsonii
all retained viability after 3 h of desicca-
tion exposure at 25
C and 40
±
8% RH (
±
SD). Mallard ducks could
potentially relocate over 200 km within a 3 h period.
Van Leeuwen
and van der Velde (2012)
examined desiccation tolerance of 13
common aquatic snail species. Almost all snail species survived
48 h of desiccation at 10 and 20
C under 80
e
85% RH. Based on the
observed microclimate data, mallard ducks could facilitate long
distance dispersal of the aforementioned species. This
fi
nding
needs to be considered in the context of the dispersal of invasive
alien species, and their ecological management.