Cygnus (2015) 1:79-90
DOI [21304114, 21298039, 21723492, 21400188, 21522056]
RESEARCH ARTICLE__________________________________
Climate change has a possible impact on the timing of breeding
events for the Dusky Moorhen (Gallinula tenebrosa)
Natasha Lutz • Lauren Pullella • Natasha Vermeulen • Alara Woods •
Joshua Wesson
Received: 22 October 2015 / Accepted: 28 October 2015
Subject Editor: Blair Bentley, Manuscript Editor: Nicola Mitchell
Abstract
Climate change is increasing temperatures globally. Warmer temperatures have
been linked with an increase in the duration of the breeding season of some birds. The
purpose of this study was to determine whether warmer temperatures are shifting the nesting
season of the Dusky Moorhen (Gallinula tenebrosa). It was predicted that increased
temperatures would cause an earlier onset and later conclusion of the breeding season of G.
tenebrosa. Phenological data for G. tenebrosa was obtained from ClimateWatch and
compared to historical nesting data from the Atlas of Living Australia (ALA).
The
ClimateWatch data were analysed in terms of the timing of breeding behaviours such as
‘courting and mating’, ‘nest-building’, ‘nesting’, and ‘presence of chicks’. Particular focus
was placed on the ‘nesting’ and ‘nest-building’ activities as there was existing literature
relevant to these breeding behaviours. Climate data from the Bureau of Metrology was used
to establish a climatic context for the results. The results were inconclusive; ClimateWatch
data showed that warmer temperatures could be causing an earlier onset and later conclusion
of the nesting season of G. tenebrosa. The peak in nest-building evident in the ClimateWatch
data was consistent with the time indicated by the published literature on this species.
However, the irregularity of the citizen scientist activity, and limited time span and size of the
ClimateWatch data set meant the impact of warmer temperatures on the phenology of G.
tenebrosa could not be definitively determined.
Keywords – Dusky Moorhen, Gallinula tenebrosa, breeding phenology, temperature, climate
change, ClimateWatch.
1 Introduction
Over the past century, the global climate has been changing at an unprecedented rate, with
consequences on the Earth’s ecological systems (Plummer et al. 1999). Australia’s surface
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temperatures have increased by approximately 0.7 degrees since 1910, a figure consistent
with global trends (Chambers et al. 2005). Abiotic factors such as temperature affect the
timing of avian phenology. A warming climate could cause the breeding season of some
birds to split into two peaks, with one peak occurring earlier than historically observed and
the other later (Schaper et al. 2012). However, not all species of birds show a change in the
timing of phenological activities (Visser & Both 2005).
Bird species reproduce when environmental conditions will promote the greatest survival of
offspring. The main selection pressure on avian reproduction is food availability (Martin
1987). Determining the impact that climate change has on the environment in which the
organism lives is important in understanding possible shifts in the phenology of the species.
Although there is a general understanding of the possible ecological consequences of climate
change on the phenology of Australian birds, the impact on individual species is mostly
unknown (Chambers et al. 2005).
The citizen science program ClimateWatch (2015) was developed to increase the available
data on the effect of climate change on selected Australian ‘indicator species’. These
indicator species have been primarily chosen for their predicted sensitivity to climatic
changes and ease of identification. Observations of these indicator species by the general
public contribute to the ClimateWatch database, which is then available to scientists for
interpretation.
The Dusky Moorhen (Gallinula tenebrosa) is one of the indicator species
identified by Climate Watch. G. tenebrosa is native to Australia and widespread through
eastern and south-western Australia (Morcombe 2004). It inhabits wetlands and brackish
estuaries. The species prefers fresh water sources with densely vegetated margins (Morcombe
2004). G. tenebrosa builds nests on floating platforms made out of reeds and other aquatic
vegetation. The diet of G.tenebrosa includes invertebrates, carrion and vegetable matter such
as the tips of grasses and seeds (Garnett 978). The most distinctive phenophase of G.
tenebrosa is their breeding period, which occurs from August through to March
(ClimateWatch 2015). During the breeding season, G. tenebrosa develop a black and blue
breeding plumage and the frontal shields on their heads turn a bright red (Ryan et al. 1996).
Shirley et al. (2003) found an increase in the mean minimum winter temperature was an
environmental cue for the development of breeding plumage in G. tenebrosa. Although this
plumage is associated with breeding, not all individuals lose their breeding colours in the
non-breeding season.
The purpose of this study was to investigate the timing of breeding events and determine if
any changes correlated to changes in climate. The approach was to analyse the nesting times
and breeding events in the ClimateWatch data and compare them to the historical data from
the Atlas of Living Australia (2015). The database of the Bureau of Meteorology (2015) was
used to examine possible links between the breeding season data and climate data. The
quality of the ClimateWatch data were examined for reliability and relevance. It is
hypothesized that G. tenebrosa will commence breeding earlier and finish later as a result of
warmer temperatures.
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2 Materials and Methods
A raw dataset for G. tenebrosa sightings was obtained from ClimateWatch (2015). The
ClimateWatch data comprised of 1217 recordings by citizen scientists over a relatively short
time period from September 2013 to May 2015. To provide perspective for this
ClimateWatch data, an extensive list of 134,251 recordings was obtained from the Atlas of
Living Australia (2015). The ALA recordings spanned from January 1770 to April 2015.
The ClimateWatch data were first filtered for potentially unreliable recordings. Inspection of
the ClimateWatch raw data points found that some were exact duplicates, with the same
recording information (that is, the same person recording, the same date, time, number of
individuals counted, latitude and longitude, and comment). These were assumed to be a
recording error, and so one of the duplicate records was removed. Data containing comments
which did not fit with the expected behaviours, such as reported groupings of 200 birds, were
eliminated. G. tenebrosa observations of ‘zero’ birds were also excluded. The photographs
taken by citizen scientists were analysed to check that the species observed was in fact G.
tenebrosa. Photos which clearly did not show the known phenotype, expected plumage
colouration, beak shape or shield colour were omitted from the data set. Notably, G.
tenebrosa was commonly confused with the Purple Swamphen, Porphyrio porphyrio.
A distribution map of the remaining ClimateWatch data set was created by uploading the
ClimateWatch data to the ALA “Mapping and Analysis’ program. The ClimateWatch and
ALA data were then analysed to determine whether G. tenebrosa has changed the timing of
breeding relative to the breeding season indicated by the historical ALA data. Although the
ClimateWatch data contains breeding behaviour observations, such as ‘courting’, ‘nest
building’ and the ‘presence of chicks’, the ALA data is not based on such phenological
observations. There are no historical records of breeding behaviour throughout the year with
which to compare the behavioural ClimateWatch data. However, ALA does contain 30
historical records of preserved egg specimens and the dates in which they were collected.
These records date from 1900 to 2006. The presence of eggs indicates that G. tenebrosa was
nesting, and thus these egg records were used as base data for the nesting times of G.
tenebrosa. As the presence of eggs was the only basis for comparison with the historical
records, the ClimateWatch data were reduced to a subset of 96 records relating to the
presence of nests. Contradictory records, which reported ‘bird on nest’ but did not indicate
the presence of a ‘nest’, were excluded.
These nest recordings from both the ClimateWatch and historical ALA data were used to
compare the timing of breeding. The ClimateWatch data were presented as the number of
‘nest present’ recordings in each month as a percentage of the total number of recordings in
2013, 2014 and 2015. The ALA data were presented as the number of eggs in each month as
a percentage of the total number of egg records from 1900 to 2006. The use of percentages
enabled the comparison between two data sets which differed greatly in size and time frame.
81
In order to determine whether the large number of nests recorded in March, April and May
were due to increased breeding or increased activity of citizen scientists, the total number of
validated ClimateWatch recordings of any kind relating to this species were organised into a
bar graph. This was then compared with a bar graph of the ratio of nest sightings to total G.
tenebrosa observations for each month for each year, to determine whether the large number
of nest sightings in March to May, and lack of nest sightings from August to January were
partially due to variables relating to citizen scientist activity.
The ClimateWatch data were also analysed to determine the timing of breeding events
generally between 2013 and 2015, and whether the timing of these events related to the
weather patterns in those years. For this analysis, the original ClimateWatch data (with
anomalies removed) was re-filtered to isolate recordings which indicated behaviour related to
breeding, such as ‘courting or mating’, ‘nest building’, ‘bird on nest’ and ‘chicks’. This
subset of data contained 131 data points ranging from December 2013 to April 2015. The
instances of observations of these phenophases were organised into a bar graph to determine
whether there was a trend in the timing of events. To eliminate the skewing effect of the
variable citizen scientist activity, the total number of breeding observations (including
‘courting’, ‘nest-building’, ‘bird on nest’ and ‘chicks’) were graphed as a ratio of total G.
tenebrosa observations for each month (including non-breeding observations). As there is
published scientific literature on factors which may affect the timing of nest-building, an
additional graph was produced for this specific breeding behaviour.
For the purposes of determining a possible link between changes in breeding seasons and
weather patterns, data were obtained from the Australian Bureau of Meteorology (2015). The
climatic data were analysed for changes in mean minimum temperature in the months which
showed breeding outside the historical data. As the subset of ClimateWatch data with
breeding related observations mostly came from NSW, the climate of this region was of
particular interest.
3 Results
The historical ALA data for the presence of nests shows annual peaks in the nesting times of
G. tenebrosa in October (Figure 1). The historical data contained instances of nests collected
from September to February. If the historical data is assumed to be accurate, then instances of
breeding occurring outside these times could indicate a possible change in the breeding
season of G. tenebrosa.
The peak in the ClimateWatch data appears to occur in March, which is outside the times
indicated by the historical data. The season for nesting indicated by the ClimateWatch data
were from August to May. This was supported by the visual evidence provided by citizen
scientists of photographs of nests and birds taken during this time period. The ClimateWatch
data did not include any nest sightings in November or January, and very few in October,
which is within the nesting time specified by the historical records.
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Percentage of Total Records (% )
50
45
40
35
30
25
20
15
10
5
0
Jan
Feb
Mar Apr May Jun
Jul
Aug Sep
Oct Nov Dec
Months
CW (2013)
CW (2014)
CW (2015)
ALA data (1900-2006)
Figure 1 The percentage of nest observations each month. The ClimateWatch data are a
proportion of the total number of ClimateWatch records indicating nest presence
from 2013 to 2015. Similarly, the ALA data are a proportion of the total egg
related ALA dataset. The sample size from ClimateWatch comprised 96 records.
ALA data provides a total of 33 records (ClimateWatch 2015; Atlas of Living
Australia, 2015).
The historical and ClimateWatch data were consistent in June and July, when no nests were
recorded. Due to the incomplete data from 2013 and 2015, comparisons could not be made
between the years of ClimateWatch data, except to note that nests were observed in March
and April for both 2014 and 2015, which is different to the historical data. All instances of
nesting observed outside of the historical ALA breeding season occurred in New South
Wales in the March, April and May of 2014. In this region, the autumn mean minimum
temperature has been trending upwards by 0.11°C per decade since 1910 (Figure 2) and was
above the long term average of 11.3°C when citizen scientists recorded autumn nesting in
2014 and 2015.
Temperature anomaly above
long term average
Temperature anomaly below
long term average
11.3°C
Trend line
Figure 2 The mean minimum autumn temperature anomaly (from March to May) for New
South Wales and ATC from 1910-2015. In both 2014 and 2015, the autumn mean
minimum temperature has been above the long-term average of 11.3°C. This
average is based on a 30 year ‘standard’ mean from 1961-1990 (BOM 2015).
83
Although the historical ALA data shows that the breeding time of G. tenebrosa should
continue from September through to February, there was very little ClimateWatch data of any
kind collected during this time (Figure 3). Most citizen scientist data were obtained in March
to May. Despite the lesser amount of data corresponding to the August to October breeding
period, a greater proportion of the records entered by citizen scientists indicated the presence
of a nest, compared with the February to May data collection period. Figure 4 shows the
number of nests observed as a proportion of the total number of sightings for each month of
each year. All of the four data points for December 2013 indicated the presence of a nest.
This was excluded from Figure 4 in order to prevent distortion of the other data.
400
Number of Recordings
350
CW 2013
CW 2014
CW 2015
300
250
200
150
100
50
0
Jan
Feb
Mar
Apr
May
Jun
Jul Aug
Month
Sep
Oct
Nov
Dec
Ratio of Nest Sightings of Total
Recordings
Figure 3 The number of recordings from ClimateWatch from December 2013 to April
2015. There was a larger number of data points collected in March to May of
2014, compared with later in the year. Notably, very little data were collected
from August through to February, which is when the ALA data suggests G.
tenebrosa is nesting. Note that this graph shows the total number of recordings
each month, not just observations relating to nest sightings.
30
25
CW 2014
CW 2015
20
15
10
5
0
Jan
Figure 4
Feb
Mar
Apr
May
Jun
Jul
Aug
Month
Sep
Oct
Nov
Dec
The ratio of nest sightings to G. tenebrosa observations per month. There were a
higher number of nests sighted per observation for the period between August to
October than any other time of the year. All of the ClimateWatch observations for
December 2013 corresponded with a nest sighting.
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The timing of breeding behaviours does not appear to vary greatly from year to year within
the ClimateWatch data time period of December 2013 to April 2015 (Figure 5). Courting and
mating was observed in February to May 2014, August to October 2014, and March 2015.
Nest building and birds on nests were observed in 2014 from March to May and again in
August to October. Both nest building and nesting birds were also recorded in March and
April in 2015. Chicks were observed in 2014 from March to May and again in October. In
2015, Chicks were recorded in March and April.
Sightings of Phenophase
12
10
8
6
4
2
0
Dec
Jan
2013
Feb Mar Apr May Jun
Jul
Aug Sep Oct Nov Dec Jan
Months
2014
Nest Building
Bird on Nest
Courting/Mating
Chicks
Feb Mar Apr
2015
Proportion of Breeding
Observations (%)
Figure 5 Observations of behaviour relating to breeding from the available ClimateWatch
data, over the time period from December 2013 and April 2015.
50
45
40
35
30
25
20
15
10
5
0
Jan
Feb Mar Apr May Jun
Jul
2014
Figure 6
Aug Sep
Months
Oct
Nov Dec Jan
Feb Mar Apr
2015
The ratio of breeding observations to total number of records per month
(ClimateWatch 2015). The highest proportion of breeding behaviour
observations occurred in the August to October 2014 period.
Figure 6 shows the proportion of total observations of G. tenebrosa that related to breeding
for each month. The August to October 2014 period showed the highest proportion of
sightings relating to breeding behaviour. There was also a greater proportion of observed
breeding behaviour in the August to October 2014 and March to April 2015 periods
compared with March to May 2014. Figure 7 shows the number of observations of nest85
Proportion of Nest -Building
Observations (%)
building as a proportion of the total number of G. tenebrosa sightings per month. The peak in
nest building occurred in the August, September and October of 2014. Despite the large
number of nest-building observations in May (as per Figure 5 above), nest-building made up
a smaller proportion of the total records entered by citizen scientists in May, compared with
the August to October period.
35
30
25
20
15
10
5
0
Jan
Feb Mar Apr May Jun
Jul
Aug Sep Oct Nov Dec Jan Feb Mar Apr
2014
2015
Months
Figure 7
The ratio of nest-building observations to total number of records per month
(ClimateWatch 2015). Nest building activity peaked in the August to October
period.
Most of the sightings of breeding behaviour were recorded in New South Wales, although
some data points came from south Western Australia and southern Victoria (Figure 8).
Figure 8
Map of the locations of breeding behaviour of G. tenebrosa, according to
ClimateWatch data from December 2013 to April 2015 (ClimateWatch 2015;
ALA 2015)
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4 Discussion
Based on the available ClimateWatch data, the hypothesis that warmer temperatures would
cause an early onset of breeding and extend the breeding season cannot be rejected. The
results were inconclusive due to the limited data. However, the analysis of the ClimateWatch
data, the historical ALA data, and the temperature data does show some interesting features.
A graphical comparison of the ClimateWatch data indicating the presence of nests and the
historical ALA data (Figure 1) suggested that in 2014 the onset of egg laying had advanced
from September to August. However, according to ClimateWatch (2015) the expected
breeding season of G. tenebrosa is between August and March. A 1989 study by Halse &
Jaensch also noted the presence of G. tenebrosa eggs in August. Therefore, breeding in
August is not substantially outside the expected breeding period. There is evidence in
scientific literature that an increase in the mean minimum winter temperature may advance
the breeding of G. tenebrosa (Shirley et al. 2003). However, according to the Bureau of
Meteorology, the mean minimum temperature anomaly in New South Wales in August 2014
was only 0.01C warmer than the historical average of 4.7C. This minor temperature
anomaly is not expected to have had a significant impact on the phenology of the Dusky
Moorhen.
The ClimateWatch data also showed evidence of nesting in March, April and May in 2014,
which is outside the nesting season indicated by the historical ALA data (Figure 1). In
autumn 2014, New South Wales experienced a mean minimum temperature 1.24C higher
than the historical autumn mean minimum temperature of 11.3C (Figure 2). However,
similar levels of nesting were observed in March and April of 2015 (Figure 1), when the
mean minimum temperature was only 0.41°C above the historical autumn average. Although
warmer spring temperatures are known to advance egg laying in birds, (Visser et al. 2009)
the effects of warmer autumn temperatures on bird phenology is considerably less
documented. It seems plausible that in a warmer autumn, environmental conditions could
remain favourable for nesting and rearing chicks, and so the nesting season of G. tenebrosa is
able to be extended. Avian reproduction has a high energy cost (Martin 1987) and so birds
such as the Dusky Moorhen attempt to coincide their main breeding season with the peak
availability of resources (Shirley et al. 2003).
This notion is supported by a study conducted by Shirley et al. (2003), which found a greater
proportion of G.tenebrosa individuals exhibited breeding plumage in populations near water
sources containing abundant free floating aquatic vegetation, which is a food source for G.
tenebrosa. This free floating vegetation included plants such as ferny azolla (Azolla pinnat),
salvinia (Salvinia molesta) and water hyacinth (Eichhornia crassipes). The distribution of
these plant species includes the regions where ClimateWatch citizen scientists have recorded
G. tenebrosa breeding activity.
Thus the ClimateWatch sightings may correspond to
ecological systems similar to that in the study by Shirley et al (2003). These aquatic plant
species also increase growth in warmer temperatures (although with an upper limit of around
45˚C) (ALA 2015). Thus if temperatures remain favourable for these plant species
87
throughout autumn, they may continue to be abundant and G. tenebrosa may respond by
extending the breeding season.
ClimateWatch citizen scientists recorded the highest proportion of nest building, and indeed
breeding behaviour, in August, September and October of 2014 (Figure 6; Figure 7). There
were fewer observations of nest building and breeding in autumn. G. tenebrosa prefer nesting
sites surrounded by dense reeds and aquatic vegetation, as this provides more protection for
G. tenebrosa offspring from predation (Shirley et al. 2003). Garnett (1978) found that near
Canberra, reed species such as Typha orientalis and T. domingensis showed the greatest rate
of growth in August and reached their maximum density in October. This reflects the timing
of the peak in nest building activity indicated by the 2014 ClimateWatch data, collected 36
years after Garnett’s 1978 study. The reeds T. orientalis, and T. domingenesis are found in all
the regions where ClimateWatch volunteers observed nest building by G. tenebrosa (ALA
2015). Avian phenology can be more sensitive to climate than plant phenology (Parmesan
2007). Thus the timing of the breeding season of G. tenebrosa could also be partly reliant on
the availability of certain plants in its habitat whose phenology may not be significantly
shifting with climate change.
The limitations of the ClimateWatch data set included the variability of the citizen scientist
activity throughout the year. For example, the highest percentage of nests was observed in
2014 from March to May. However, this may be partially explained by the high level of
citizen scientist activity as this period also corresponded with the peak in the total number of
G. tenebrosa sightings (including non-breeding related observations) (Figure 3). Nests are
more likely to be observed when there is increased activity of ClimateWatch volunteers. In
contrast, although there were fewer data points collected in total in the August to October
period in 2014 (Figure 3), a higher proportion of the observations recorded during this time
related to nest observations (Figure 4). Furthermore, despite the ALA data indicating that
egg-laying should continue from September through to February, there was very limited
ClimateWatch data of any kind collected in this period (Figure 3). However, the lack of nest
observations during these times does not necessarily indicate that the Dusky Moorhen is not
breeding. There is a strong correlation between periods when observations of G. tenebrosa
are recorded and the semester times of universities known to participate in this citizen science
program (ClimateWatch 2015). This may explain the patterns in citizen scientist activity. A
similar distorting effect occurred with the general breeding observations of ‘courting’, ‘nestbuilding’ ‘nesting’ and ‘chicks’ (Figure 5). To encourage a more even spread of data points
throughout the year and to lessen the impact of university semesters, ClimateWatch could
increase public awareness of the program to gain wider participation.
Observations by citizen scientists are less reliable than data collected by professional
scientists as citizen scientists vary in skills and attitude. Even with the critical interrogation
and filtering of the ClimateWatch data, there may still be erroneous information. The data set
may still contain observations by citizen scientists that mistook the Dusky Moorhen for other
species (for example, the Purple Swamphen, Porphyrio porphyrio) but did not include visual
88
evidence, and otherwise made observations which conformed to the expected behaviour and
distribution of the Dusky Moorhen.
The data from ClimateWatch is not sufficient to determine a trend for changing breeding
times from the breeding season evident in historical ALA data. Trends in phenology can only
confidently be determined from data collected frequently over a long time period.
ClimateWatch is a relativley recent program and the ClimateWatch data spans a very limited
time period from September 2013 to May 2015. There is only one complete calendar year of
data, in 2014. A single year is not sufficient because the period of favourable conditions for
breeding varies between years and so the timing of breeding will vary slightly every year
(McNamara et al. 2008). Furthermore, during this time, the data were not collected at evenly
timed intervals and so the effect of the different seasons was not properly evident. G.
tenebrosa may be stopping or slowing breeding in the warmer months to avoid heat stress.
The lack of ClimateWatch data from November to February means such occurances are not
being documented. Thus the relationship between the changing climate and the phenology of
the Dusky Moorhen warrants further investigation. If citizen scientists continue to contribute
to ClimateWatch for a longer period of time, and make observations regularly throughout the
year, eventually a more accurate and definite trend may emerge.
To further investigate the impact of climate change on the phenology of G. tenebrosa, it
would be useful to determine the specific impacts of climate change on the phenology of the
other species on which G. tenebrosa depends. The effects of climate change on the primary
producers on which the G. tenebrosa depends are mostly unknown. These species may be
increasing in abundance earlier in the year or continuing to be abundant for longer. They may
not be shifting their phenology at all. If the primary producers on which the G. tenebrosa
relies react differently to changes in temperature, precipitation or photoperiod, then there may
be a mismatch between the timing of seasonal acitivites which have evolved to occur
simultaneously (Visser & Both 2005). In the summer months, the high temperatures and lack
of precipitation may be causing plants to slow or cease growth, affecting the food availability.
Understanding any peceived changes in the timing of seasonal events involves knowledge of
the ecological context of those changes.Analysis of the ClimateWatch data does show that
warmer temperatures could cause the breeding season of G.tenebrosa to start earlier and
conclude later. However, due to the limitations of the ClimateWatch data set, the effects of a
changing climate on the timing of breeding cannot be definitively determined.
Acknowledgements
We would like to thank Climate Watch, the Atlas of Living Australia and the Bureau
of Meteorology for providing the data which was analysed in this paper. We would also like to acknowledge the
help of Blair Bentley and Nicola Mitchell who provided suggestions and guidance during the writing of this
article.
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