ORIGINAL PAPER

Effects of down collection on incubation temperature,nesting behaviour and hatching success of common eiders(Somateria mollissima) in west Iceland

Thordur Orn Kristjansson • Jon Einar Jonsson

Received: 16 August 2010 / Revised: 10 December 2010 / Accepted: 27 December 2010 / Published online: 15 January 2011

� Springer-Verlag 2011

Abstract Incubating common eiders (Somateria mol-

lissima) insulate their nests with down to maintain desir-

able heat and humidity for their eggs. Eiderdown has been

collected by Icelandic farmers for centuries, and down is

replaced by hay during collection. This study determined

whether down collecting affected the female eiders or their

hatching success. We compared the following variables

between down and hay nests: incubation temperature in the

nest, incubation constancy, recess frequency, recess dura-

tion, egg rotation and hatching success of the clutch.

Temperature data loggers recorded nest temperatures from

3 June to 9 July 2006 in nests insulated with down

(n = 12) and hay (n = 12). The mean incubation temper-

atures, 31.5 and 30.7�C, in down and hay nests, or the

maximum and minimum temperatures, did not differ

between nest types where hatching succeeded. Cooling

rates in down, on average 0.34�C/min and hay nests

0.44�C/min, were similar during incubation recesses.

Females left their nests 0–4 times every 24 h regardless of

nest type, for a mean duration of 45 and 47.5 min in down

and hay nests, respectively. The mean frequency of egg

rotation, 13.9 and 15.3 times every 24 h, was similar

between down and hay nests, respectively. Hatching suc-

cess adjusted for clutch size was similar, 0.60 and 0.67 in

down and hay nests. These findings indicate that nest down

is not a critical factor for the incubating eider. Because of

high effect sizes for cooling rate and hatching success, we

hesitate to conclude that absolutely no effects exist.

However, we conclude that delaying down collection until

just before eggs hatch will minimize any possible effect of

down collection on females.

Keywords Somateria mollissima � Down collecting �Incubation behaviour � Breeding success �Nesting temperature

Introduction

Like other waterfowl (Anatidae), the female common eider

(Somateria mollissima; hereafter eider), forms a brood

patch prior to incubation (Afton and Paulus 1992). In

waterfowl, brood patch skin is modified to enhance heat

transfer from the parent to eggs, partly by moulting off

down from the breast and belly (Jonsson et al. 2006a, b).

The down that is shed from the brood patch area is sal-

vaged by the female and used to line the inside of the nest.

Nest down helps to maintain desirable heat and humidity

for the eggs during the incubation period and also conceals

the clutch while the nest is unattended (Afton and Paulus

1992; Hilton et al. 2004). However, a question is whether

the salvaged brood patch down may not be a requirement

for successful hatch, which ultimately depends on the nest

attendance and individual properties of the incubating

parent (Flint and Grand 1999; Bolduc et al. 2005; Jonsson

et al. 2007).

Over the centuries, Icelandic farmers have collected

down from eider nests during the incubation period and

replaced with dry hay as insulating material (Snæbjornsson

1998; Bedard et al. 2008). Down is considered the best

insulation material for bird nests. Conversely, hay is con-

sidered the worst but the insulation abilities reverse with

T. O. Kristjansson (&)

Department of Biology, University of Iceland,

Askja, Sturlugotu 7, 107 Reykjavık, Iceland

e-mail: thok@hi.is

J. E. Jonsson

University of Iceland, Snæfellsnes Research Centre,

Hafnargata 3, 340 Stykkisholmur, Iceland

123

Polar Biol (2011) 34:985–994

DOI 10.1007/s00300-010-0956-z

humidity i.e. wet down is considered a worse insulator than

wet hay (Hilton et al. 2004). Nowadays eiderdown is col-

lected, cleaned and used to make products such as duvets,

sleeping bags and clothes (Bedard et al. 2008). In Iceland,

the down is usually collected late in the incubation period

rather than early (Snæbjornsson 1998). Should down col-

lection have a negative effect on incubation and hatching

success of the eiders, differences will be observed in

incubation behaviour and/or associated temperature char-

acteristics between eiders nesting in down and hay-lined

nests.

Different incubation temperatures have been observed in

eider nests, depending on the location of the nest. In a

comparative study in southwest Iceland, the fluctuations in

incubation temperature were much higher in exposed areas

than in sheltered ones (D’Alba et al. 2010). Similarly,

incubation temperatures in a study in North Canada were

lower at lower ambient temperatures, hardly never

exceeding 38�C (Fast et al. 2007).

Female common eiders incubate without help from the

males and thus rarely leave their nests during the incuba-

tion period, attending the nests for 90–95% of the time

(Bolduc and Guillemette 2003a). Unattended nests are in

risk of predation, and it costs the bird extra energy to reheat

the eggs upon return from incubation recesses (Brummermann

and Reinertsen 1991). Incubation recesses are more

common during early incubation period compared to

late incubation but even then the females attend the nests

for 60–70% of the total incubation time (Bolduc and

Guillemette 2003a). Eider females leave their nests to drink

fresh water (Criscuolo et al. 2000) but have rarely been

reported to leave the nests to feed (Swennen et al. 1993).

Recess duration can vary greatly but studies have reported

average recess duration of 4–17 min (Mehlum 1991;

Criscuolo et al. 2000; Bottita et al. 2003; Bolduc and

Guillemette 2003a).

While incubating the females rotate their eggs regularly,

up to 20–24 times per day (Gerasimova and Baranova

1960). This behaviour is considered to maintain the same

temperature at all sides of the eggs (Rahn et al. 1983). The

side of the egg that is close to the brood patch is probably

warmer than the side facing to the bottom of the nest. If hay

is not as good a nesting material as down, one might expect

that a female incubating on hay would have to turn her eggs

more often than a female incubating on down.

Hatching success can vary greatly among eiders but it is

usually connected to predation risk at the nesting site

(Madsen et al. 1992; Jonsson 2001; Bolduc and Guillemette

2003b; Donehower and Bird 2008), breeding experience,

body condition, timing of breeding and health status of the

female (Bolduc et al. 2005; D’Alba et al. 2010) and envi-

ronmental variables such as ambient temperature, shelter

and human disturbance. In an investigation in Alaska, the

hatching success ranged from 5 to 68% of total nests (Noel

et al. 2005), in Scotland it was 63% (Milne 1972) and in

west Iceland 70% of all laid eggs (Skarphedinsson 1996).

The aim of this study was to investigate whether down

collection (replacing down by hay) affected the incubation

temperature in the nests and the behaviour (incubation

constancy, recess frequency, turning of the eggs) of the

female eider. We used an experimental approach in which

we removed down from half the nests and replaced it with

hay (hay nests), whereas we left the down in the other half

(control nests). We then compared incubation tempera-

tures, incubation constancy, recess frequency and egg

turning between hay nests and control nests.

Materials and methods

Study site

This study was conducted from 3 June to 9 July 2006, at

Hvallatur islands located in Breidafjordur, west Iceland

(65�25079N, 22�46005W) (Fig. 1). The islands, where the

investigation was carried out, are small and the distance

from an eider nest to the intertidal zone never exceeds

20 m. The predation risk for eiders is considered low, and

only avian predators have been seen around the islands,

which are too far away from the mainland (17 km) for

arctic fox (Vulpes alopex) or American mink (Neovison

vison) to reach there (Bjornsson and Hersteinsson 1991).

The mean ambient temperature and standard deviation

(SD) during the study period was 8.9�C ± 2.3�C (min–

max;-0.7–17�C), and the mean wind speed was 5.3 m/s ±

2.3 m/s (Icelandic Meteorological Office). In Breidafjordur,

there are over 3,000 small islands with abundant food and

Fig. 1 A map of Breidafjordur west Iceland. The investigation area

(Hvallatur islands) is inside the circle

986 Polar Biol (2011) 34:985–994

123

good nesting sites for the eiders. Breidafjordur is a breed-

ing, moulting and wintering area for the eiders and supports

20–25% of the large Icelandic population (Grimmett and

Jones 1989; Jonsson et al. 2009), which is estimated as

about 850,000 birds (Gardarsson 2009).

Data collection

In the present study, 28 temperature data loggers (DST

centi 2010) were used to monitor temperature in nests and

incubation behaviour of the female eiders and hatching

success. The loggers (length = 5 cm, width = 1 cm,

weight 15 g) were placed in white film boxes of about the

same size as the eider eggs. The heat transfer from the eider

female to data loggers in boxes is considered complete

because data loggers without boxes showed the same

results as those in boxes (Kristjansson, unpublished data).

The data loggers measured the nest temperature every 30 s,

starting when they were put into the nests and stopped

when removed following hatching or nest abandonment by

the female. The clutch size in the nests ranged from 3 to 5

eggs, and incubation had lasted from 0 to 20 days in the

nests when the study started.

When the investigation started, the incubation stage of

the 28 nests ranged from 0 to 20 days. However, 4 females

abandoned their clutch after the loggers had been put into

the nests (incubation stage 0–17 days), leaving our sample

size at 24 nests with data loggers. These 4 females were not

included in the present results as the reason for nest

abandonment is not related to the nesting material, but

rather to the disturbance when inserting the data logger. In

every other nest, all the down were replaced with hay

(hereafter hay nests, n = 12), whereas the other half was

not manipulated but used as controls (hereafter down nests,

n = 12).

Incubation curve

Incubation stage was estimated both by (1) calculating

the specific density of the eggs and using it to calculate

the incubation day from an incubation curve and (2) by

counting from the day eggs hatched backwards until the

day the data logger was put into the nest. The average incu-

bation period was 24 days both in hay nest and in down nest

depending on the incubation curve (see Appendix).

To estimate the laying dates of eggs, an incubation curve

was calculated using a technique, which states that egg

density (g/cm3) changes during incubation (Hoyt 1979;

Furness and Furness 1981). The volume of the eggs was

calculated by the rule that the shape of the eggs is the

same (Furness and Furness 1981). It is therefore enough

to know the length and width of the eggs to calculate the

density. With this technique both laying and hatching

dates of unknown eggs can be calculated with 95% accuracy

(Hario 1983).

Interpretation of temperature data

The relationships between observed temperature changes,

incubation recesses and rotation of eggs were confirmed by

using a video camera (Pelco 35 mm) filming a single eider

nest (n = 1) containing a temperature data logger, from 2

to 5 June 2007 (72 h). The insulation material for one and a

half days was down (from 2 June at 12:00 to 3 June at

21:45) replaced by hay (from 3 June at 21:45 to 5 June at

12:00). The incubation stage was 15 days when filming

started. The temperature fluctuations read from the data

loggers were compared to the actual behaviour of the eider

observed on the videotape. This comparison confirmed our

interpretation of the temperature data with respect to tim-

ing, duration and frequency of incubation recesses. The

eiders treated the data loggers as they were eggs, turning

them through the whole incubation period.

The behaviour of the females was recorded based on the

temperature data from the data loggers. A sudden tem-

perature decrease and the duration of this decrease shows

when, and for how long time, an incubation recess takes

place. Rising temperature, followed by a steady tempera-

ture reading of 33–34�C, indicates when the female has sat

on the nest again. Conversely, small temperature fluctua-

tions indicated the rotation of the eggs as the females

rotated the data logger as well as her eggs. We considered

that a female took a recess when the temperature dropped

more than 3�C and the slope was continuous and steep. At

the study site, sun exposure was not considered a critical

factor concerning nest temperature. The mean ambient

temperature was only 8.9 ± 2.3�C during the incubation

period, and data from abandoned nests showed that tem-

peratures never exceeded 18�C. A rotation of the egg was

considered when the temperature drop in the nest was

0.5–3�C and lasted for a shorter period (see results).

Data analysis

SeaStar model (Star-Oddi 2001) was used to read the

temperature data from the DST data loggers. Our statistical

tests were one-tailed, looking for a negative effect, because

we expected down collection to lower incubation temper-

ature and increase rates of temperature loss during incu-

bation recesses. We only report nonparametric tests if

assumptions of their parametric counterparts were not met.

Statistical comparisons were made using a Welch two

sample t test when the data passed a normality test (tem-

perature comparison for average and highest values, incu-

bation stage comparison), otherwise a Mann–Whitney

U-test when normality failed (frequency of incubation

Polar Biol (2011) 34:985–994 987

123

recesses and their duration) (Sokal and Rolf 2001).

A Chi-square test (Sokal and Rolf 2001) was used for

comparison of incubation recesses night vs. day (to

examine for diurnal pattern) and for absolute hatching

success. All these analyses were done using the SigmaStat

3.1 program, except for a linear mixed effect model (PROC

MIXED, Littell et al. 1996, SAS Institute 2001), which was

used for comparison of nest cooling rate in down and hay

nests. We calculated the nest cooling rate as the temperature

change that occurred during a recess divided by the length of

that recess (min). The square root was considered to nor-

malize the data when looking at relation between incubation

days and the nest cooling rate. The nest cooling rate was used

as a dependent variable, treatment (down vs. hay) and

incubation day were fixed effects, and individual nests

(which equal individual females) were a random effect.

The absolute hatching success in the two nest types

(down and hay nests) was calculated by comparing the

number of nests and eggs where hatching succeeded

completely or partly (one or more egg lost) to the number

of unsuccessful nests (abandoned). The relative hatching

success was also calculated for relative proportion of

hatched eggs in each nest type with a GLM model

(Crawley 2007 (a\ 0.05)), in which treatment (down or

hay) and incubation stage (as a covariate) were the

independent variables. We used these two methods to

calculate hatching success, and both methods yielded very

similar findings.

Results

Nest temperatures

In the incubation period, mean temperatures were similar

in down (31.5�C, SE = 0.5) and hay nests (30.7�C,

SE = 1.0) (t test; t = 0.72, P = 0.242, df = 14), where

hatching succeeded, despite considerable temperature

drops when the females left their nests (Figs. 2 and 3). The

lowest mean temperature observed in a hay nest was

25.5�C, caused by one long recess, when the temperature

went down to 9.8�C in a short period (Fig. 3).

The mean highest temperatures (down: 38.2�C, SE =

0.7, hay: 37.4�C, SE = 0.6) and lowest temperatures

(down: 16.8�C, SE = 1.4, hay: 14.2�C, SE = 1.2) were

calculated. The mean of the highest temperature observa-

tions (t test; t = 0.81, P = 0.215, df = 14) and the lowest

temperature observations (t test; t = 1.44, P = 0.086,

df = 14) in the two nest types was similar.

When the females left their nests, the temperature

decreased steadily until they returned to their nests. The

mixed model indicated that nest cooling rate was not

related to treatment (F = 1.601,14; P = 0.227) or to

incubation day (2.491,207; P = 0.116). Average nest cool-

ing rates were 0.34�C/min (SE = 0.06) and 0.44�C/min

(SE = 0.06) in down and hay nests, respectively.

Recesses frequency

The average recess frequency and standard error (SE) per

24 h was similar in successful down nests (1.3 times,

SE = 0.1) and successful hay nests (1.4 times, SE = 0.1)

(Mann–Whitney; t = 7084.00, n = 80, n = 97, P =

0.459. The females left their nests 0–4 times per incubation

day, both in down nest and in hay nest (Table 1). Similarly,

the average recess frequency in unsuccessful down (1.8

times, SE = 0.2) and hay (1.7 times, SE = 0.1) nests was

Fig. 2 Mean temperature, 95% confidence interval and extreme

values in down nests where hatching succeeded for common eider

(Somateria mollissima) in Hvallatur Islands summer 2006

Fig. 3 Mean temperature and 95% confidence interval and extreme

values in hay nests where hatching succeeded for common eider

(Somateria mollissima) in Hvallatur Islands summer 2006

988 Polar Biol (2011) 34:985–994

123

similar during this period (Table 1) (Mann–Whitney;

t = 1297.00, n = 26, n = 66, P = 0.224).

However, the average frequency of incubation recesses

in all successful nests (down and hay) (1.4 times,

SE = 0.1) was lower than the frequency in all unsuccessful

nests (1.7 times, SE = 0.1) (Mann–Whitney; t =

14173.00, n = 92, n = 177, P = 0.002). Comparison

between successful and unsuccessful nests within both hay

(Mann–Whitney; t = 6021.50, n = 66, n = 97, P =

0.195) and down (Mann–Whitney; t = 1678.50 n = 26,

n = 80, P = 0.018) showed a difference between the two

groups.

The frequency of the incubation recess at day (08–200

clock) and night (20–080 clock) in nests where hatching

succeeded was compared to investigate whether or not the

eider showed a diurnal pattern. No significant difference

was observed in frequency in day and night time

(X2 = 0.16, df = 1, P = 0.69).

Recess duration

Recess duration and standard error (SE) were similar in

successful down and hay nests (Mann–Whitney; t =

17758.00, n = 130, n = 147, P = 0.320), (Table 1). The

female left the nest from 10 min to 2 h with an average for

45 min (SE = 2.1) and 47 min (SE = 2.3) in down and

hay nests, respectively (Table 1). The average incuba-

tion constancy was 97% in down nests and 96% in hay

nests.

In the eight unsuccessful nests, recess duration was

always longer than the longest duration in successful nests.

In these nests, the maximum recess duration was from 100

to 1,440 min and these long recesses were in the middle of

the laying period (investigation period). Therefore, the last

absence was not counted as a recess (Table 1). In seven of

the nests, the female left dead eggs, but in one nest the eggs

were eaten by predators.

Egg rotation

Egg rotation was similar in successful down and hay nests.

On average, a female in a down nest rotated her eggs 13.9

times per 24 h, (SE = 0.5), whereas a female incubating in

hay 15.3 times (SE = 0.5) (t test; t = -1.56, df = 14,

P = 0.07). The average egg rotation did differ between

unsuccessful down and hay nests (Mann–Whitney;

t = 888.00, n = 26, n = 61, P = 0.009) and was on

average, 11.2 (SE = 1.2) in down but 14.4 (SE = 0.6)

times in hay per 24 h, respectively.

Hatching success and incubation state of the females

In successful nests, incubation stages were similar in down

(7–20 days) and hay nests (7–16 days) when the investi-

gation started (t test; t = 0.07, df = 14, P = 0.473). No

difference was observed between hatching success in down

(0.60) and hay (0.67) when relative hatching success was

investigated using a GLM model (P values \ 0.462) nor

when absolute hatching success was calculated, down

(0.55) and hay (0.67) (Table 2).

Incubation stage was similar in the unsuccessful down

(0–16 days, mean = 5, SE = 1.8) and hay nests (1–9 days,

mean = 9, SE = 3.4) when the investigation started (t test:

t = 0.91, df = 6, P = 0.199). However, the incubation

stage when data loggers were placed differed between

successful (n = 18, mean = 12, SE = 1.3) and unsuc-

cessful nests (n = 8, mean = 7.5, SE = 1.9) excluding

difference in nesting material. The unsuccessful females

Table 1 Number of incubation recesses per 24 h, the average frequency ± SE, range of recess duration and average duration ± SE of eiders

nesting in successful and unsuccessful down and hay nests

Recesses/24 h Successful down nests Successful hey nests Unsuccessful down nests Unsuccessful hay nests

No of recesses 0–4 0–4 0–5 0–5

Average recess frequency ± SE 1.3 ± 0.1 1.4 ± 0.1 1.8 ± 0.2 1.7 ± 0.1

Range of recess duration (min) 10–110 10–120 30–1320 15–1440

Average recess duration ± SE (min) 45 ± 2.1 47 ± 2.3 141 ± 33 93 ± 20

Table 2 Hatching success (number of nests and number of eggs

hatched) in down and hay nests in Hvallatur, Breidifjordur in 2006

Nr of nests Total All hatched Partly hatched Abandoned Robbed

Down 12 5 3 3 1

Hay 12 8 0 4 0

Total 24 13 3 7 1

Chi-test P = 0.18 X = 4.84, df = 3

Nr of eggs Total Hatched Left and robbed

Down 49 27 22

Hay 46 31 15

Total 95 58 37

Chi-test P = 0.22 X = 1.51, df = 1

Polar Biol (2011) 34:985–994 989

123

had incubated for shorter period than the successful

females when the investigation started (t test: t = 2.53,

df = 22, P = 0.01).

Video filming

Temperature fluctuations in the eider nest, which was

filmed for 72 h (2–5 June), are shown in Fig. 4. The

incubation recesses seen on the video and the temperature

drops from the data logger in the nest matched very well.

The rotation of the eggs was also observed from the video

as movements of the female on the nests and matched the

smaller temperature fluctuations from the data loggers as

well. In the first day of observation (2 June), the female left

the nest for 45 min and the temperature drop was 5.8�C.

The other eight observed fluctuations in these first 12 h

(0.5–3�C) were caused by egg rotation. On June 3, the

female left the nest 4 times, the temperature decrease was

from 3.2 to 11.1�C and the duration of the recesses was

14–70 min. The longest duration and the largest tempera-

ture decrease occured when the down was replaced by hay.

Twelve temperature fluctuations that were observed these

24 h were caused by rotation of the eggs. On June 4, there

was only one recess, lasting 20 min and the temperature

dropped 5.5�C. In these 24 h, the female rotated the eggs

20 times (0.8–4�C). From midnight until 12:00, the last day

(5 June), the female did not leave the nest and turned the

eggs only once.

Discussion

Nest temperatures

In this study, average nest temperature during incubation

was similar in successful nests with down (31.5�C) and hay

(30.7�C). Likewise, the mean highest and lowest temper-

ature recorded in the two nest types was similar. Average

nest temperatures were similar to those reported from

southwest Iceland, where the average temperature was

31.4�C in natural nests but 32.4�C where man-made shel-

ters surrounded nesting sites (D’Alba et al. 2010). The

average central egg temperature among eiders over the

incubation period has been estimated as 33.6�C (Rahn et al.

1983). The observation that females can maintain similar

average temperature in a hay nest, compared to a down

nest, implies that they can compensate for poor insulating

material with their body heat. The down is thus not a

critical factor for incubation when the female is on the nest.

Rather, the incubation constancy along with heat transfer

from the female to the eggs is probably more important

than the nest material. A future research question is whe-

ther females must expend extra energy to maintain nest

temperatures in a hay nest. Such extra heat loss through the

brood patch potentially can induce catabolism of energy

reserves (Jonsson et al. 2006a).

Heat at the brood patch region of the eiders has been

documented from 39.1�C up to 40�C (Rahn et al. 1983;

Criscuolo et al. 2001). In this study, brood patch temper-

ature was not measured but the average highest tempera-

tures both in hay and in down nests were similar to brood

patch temperatures reported by Rahn (1983). In the present

study, the lowest temperature observed in successful nests

was only 9.8�C but the highest was 41.8�C. These extreme

values lasted for a short time of the total incubation period,

or in 42 min the nest temperature was \15�C and for

20 min it was [40�C. In chickens (Gallus domesticus),

embryos do not develop at egg temperature below 26�C

(physiological zero temperature) or above 40.5�C (upper

lethal temperature), these critical values are believed to

apply to all bird species (White and Kinney 1974; Webb

1987). Our results suggest that the embryos were resistant

to short-term temperature fluctuations. When the females in

this study left their nests, temperatures decreased linearly

but the nest cooling rates did not differ significantly

between down and hay nests. However, the nest cooling

rate was 29.4% faster in hay nests than in down nests.

Thus, a higher sample size could have detected a signifi-

cant difference between hay and down nests, although the

variation within groups ranged 0.53 in down (min; 0.00,

max; 0.53, median; 0.16, 25%; 0.10, 75%; 0.22) and 0.89

(min; 0.01, max; 0.90, median; 0.19, 25%; 0.13, 75%; 0.28)

in hay nests.

Recess frequency

Replacing down by hay did not affect recess frequency of

the female eiders in the present study. In 16 nests where

hatching succeeded the females left the nests 0–4 times per

Fig. 4 Temperature fluctuations in an eider (Somateria mollissima)

nest, which was video recorded in Hvallatur Islands summer 2006.

The dots point where the female left the nest for recessing and the

arrow when down was replaced by hay

990 Polar Biol (2011) 34:985–994

123

24 h (average 1.4 times). The 8 eiders, which did not hatch

their clutch, took incubation recesses more frequently than

those with successful nests (average 1.7 times). In these

nests, the temperature fluctuations were higher and average

recess duration was much longer. In a study conducted in

Canada, the eiders only left their nests on average 0.5 times

per 24 h and the high predation risk of the nesting site was

considered the main reason for this low recess frequency

(Bottita et al. 2003).

In waterfowl incubation, constancy is generally high and

eiders are reported to have one of the highest average

incubation constancies among ducks (Afton and Paulus

1992, Bolduc and Guillemette 2003a). The eider is the

largest duck in the Northern Hemisphere; larger species

generally have higher incubation constancies than smaller

ones because they have higher fasting endurances than

smaller species (Jonsson et al. 2006b; Jonsson et al. 2007).

The average incubation constancy in the present study was

97% in down nests and 96% in hay nests. Bolduc and

Guillemette (2003a) recorded 99.5% incubation constancy

for eiders nesting in Denmark and related the relatively

high incubation constancy to high predation rates from

avian predators (Bolduc and Guillemette 2003a).

In the present study, there was no difference observed

between the frequencies of incubation recesses between day

and night hours. The incubation period of eiders in Iceland is

May–June, when there is light for 24 h a day. The main

predators in the present study, the gulls (Larus spp.), are

active for 24 h a day during that time of the year. It has been

demonstrated in Canada (Bolduc and Guillemette 2003a)

and the Netherlands (Swennen et al. 1993) that eiders leave

their nests more often during nights (during dark hours) than

days to avoid predation from avian predators. However, in

Alaska, most recesses of spectacled eiders (S. fischeri) were

observed between 10:00 and 22:00 when predators were

most active but the ambient temperature was warmer than

that during night time (Flint and Grand 1999).

Recess duration

In the 16 successful nests, recess duration was similar in

down and hay nests, females recessed on average 45 min,

each recess ranging from 10 to 120 min. In a study con-

ducted in Svalbard, the eiders left their nest for only

4–7 min each time (Mehlum 1991; Criscuolo et al. 2000),

in Denmark for an average of 14 min (Bolduc and

Guillemette 2003a) and in Canada for 17 min (Bottita et al.

2003). The longer recess durations observed in our study

may be explained by different ambient temperature and/or

different predation risk at the nesting sites. In Svalbard,

snow covered the nesting grounds for half of the incubation

period and the ambient temperature was most likely lower

than in the present study. Eiders probably take shorter

recesses in cooler atmosphere due to the risk of egg cooling

and a high energy expenditure caused by reheating the eggs

to optimal temperature (Mehlum 1991; Criscuolo et al.

2000). Birds incubating in northern hemisphere spend up to

50% more energy in the incubation period compared to

individuals of the same species incubating at a lower lati-

tude (Piersma et al. 2003; Eichhorn et al. 2010).

In our study, females left their nest for a longer period in

each session as incubation progressed. Most eiders use

their recess to drink fresh water and preen (Criscuolo et al.

2000) but the long recess observed here could indicate

other activities as well. Eiders have been seen feeding

during incubation (Swennen et al. 1993; Flint and Grand

1999) and as food is abundant at the study site and the nests

always close to the sea; it is possible that some of the

recess time was used to feed. The absence of mammalian

predation and protective efforts against avian predators at

our colony may explain to some extent longer incubation

recesses giving the female an opportunity to feed or loaf. In

our study, the eiders covered their nest when they left on an

incubation recess, which is a widespread behaviour among

waterfowl (Afton and Paulus 1992; Hilton et al. 2004;

D’Alba et al. 2010).

We present our findings based on successful nests.

However, in all the eight failed eider nests, one or more

recess was longer than the longest recess observed in

successful nests, causing larger temperature fluctuations.

Such long recesses (up to 24 h) in failed nests occurred

halfway through the incubation period. Unsuccessful

females returned from these long recesses and incubated

for some days, probably on dead eggs, before abandoning

their nests. High temperature fluctuations in the nest while

the eider takes incubation recess may lengthen the incu-

bation period, even delay development and reducing

probability survival of the embryo (Olsen et al. 2006).

Long recess duration is considered to increase the risk of

nest failure more than high recess frequency (Rauter and

Reyer 1997).

The unsuccessful eiders abandoned their nest early in

the incubation period. Inserting of the data logger could

have had a stronger impact on the eider during early

incubation (Bolduc and Guillemette 2003b), or this was an

artefact of poorer quality females (younger and lighter)

initiating nests later than those of better quality. It has been

reported that fitness of the eider is strongly related to egg-

laying dates (Spurr and Milne 1976; Bolduc et al. 2005),

which points towards this conclusion.

Egg rotation

There was no difference observed in egg rotation between

successful hay- and down-nesting females. Females rotated

their eggs on an average of 14–15 times each 24 h in both

Polar Biol (2011) 34:985–994 991

123

nesting materials, even though the material it self might

have different ability. It is considered that birds rotate their

eggs in nests to maintain equal temperature and moisture at

all sites of the egg and even out the temperature difference

between the bottom of the egg, furthest from the brood

patch, and the top, closest to the brood patch (Rahn et al.

1983). Another function for egg turning is to maximize

utilization of the albumin by the embryo. Turning increases

diffusion of protein and ions from the albumin to the yolk

and prevents the embryo from adhering to the inner shell

membrane (Deeming 2009). The egg rotation in this study

was unrelated to nesting material indicating that the female

could keep the same temperature at the bottom of the nest

in both nest types.

Hatching success

Our findings indicate that replacing down by hay have no

impact on relative hatching success in eiders nests as the

success was similar in down (0.60) and hay nests (0.67)

when the proportion of hatched eggs was investigated using

a GLM model. However, these results point towards a better

hatching success in hay than in down, which may indicate

effects of a small sample size resulting in low statistical

power.

Females with failed nests had incubated for shorter time

and recessed more often and for a longer time than the

females where hatching succeeded, which might indicate

younger females as the older and heavier lay eggs earlier

than young and light ones (D’Alba et al. 2010). Bolduc

et al. (2005) found out that nesting success was principally

related to female characteristics rather than to nest site

characteristics, they suggested that eiders rely on nest

attendance rather than on nest concealment to protect their

nests. Nesting close to shore may shorten incubation

recesses and improve hatchling survival when leaving the

nest.

The methodology to evaluate the behaviour of nesting

female eiders using temperature data loggers, as done in

the present study, seems to work quite well. The data

logger in the one nest, which was video recorded, showed

that all behaviour of the eider matched the temperature data

from the logger. Data loggers are useful in behaviour

investigations where the ambient temperature is much

lower than the average nest temperature, as in the present

study. In this case, the nest attendance, incubation recess

and its duration become obvious as temperature fluctua-

tions. In a comparable study in the United States, the

behaviour of ducks was monitored by using data loggers at

the same time as video recorder and also here the results

confirmed the temperature fluctuations well (Hoover et al.

2004).

Conclusion

In this study, we were looking for overall effects of down

collection on productivity of the eider population by

looking at some critical variables that might help under-

standing the mechanism driving any overall effects. It does

not appear that down collection has any substantial effects

on productivity, incubation temperature, recess frequency

or recess duration, thus females can compensate for down

loss while incubating on the nests. The strongest effect,

however, occurred as increased rates of nest cooling during

incubation recesses. This high cooling rate in hay-lined

nests may increase embryonic mortality and reduce

hatching success of eiders incubating in hay. However,

because of our low sample size we may have failed to

detect effects that actually exist. Nonetheless, even if we

assumed that our effect sizes are real, the overall effects of

down collection on population dynamics would be small

and the practice of delaying the collection of down until

late incubation would reduce the effects to less than what

we estimated in this study.

Collection of eiderdown is believed to be non-harmful to

the species (Snæbjornsson 1998; Bedard et al. 2008), and

down is considered the best insulation material for eiders

but no study has been conducted on whether or not removal

of the down and not replacing it at all has any impact on

breeding success. A larger study sample in which eiders

nesting in hay and down would be weighted at the begin-

ning of the incubation and at hatching would be a good

future study to compare nesting material and energy

expenditure of the female. Likewise it would be informa-

tive to compare fitness of the eiders both early and late in

the nesting period relating to breeding probability the fol-

lowing year.

Acknowledgments Financial support for this work was partly

provided by The Icelandic Centre for Research (Rannis) and the

Icelandic Eider Farmers Association. The authors wish to thank

the owners of Hvallatur in Breidafjordur for allowing us access to the

eider colony and Vilhjalmur Thorsteinsson at the Mar. Res. Inst. in

Reykjavık for lending the temperature data loggers and assistance

with transferring the data to the computer. We thank Snæbjorn

Palsson for his help with statistical tests and Tomas G. Gunnarsson,

Arnthor Gardarsson, Pall Hersteinsson, Larry Jocobson, and Gudrun

Thorarinsdottir for comments that improved earlier drafts of this

manuscript. We also thank the reviewers Paul Flint, John C. Coulson

and Flemming R. Merkel for their improvements to this manuscript.

Appendix

The incubation curve for common eider (Somateria mol-

lissima) nesting in Hvallatur Islands summers 2005–2006.

The dots show the egg density on known incubation days,

992 Polar Biol (2011) 34:985–994

123

the 95% confidence interval is shown for the dots and the

regression line.

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