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The Changing Status of the Glossy Ibis Plegadis falcinellus in Britain

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Numbers of Glossy Ibis recorded in Britain have increased dramatically since the mid-2000s, mirroring the increase in their breeding population in southwest Europe, especially in Doñana (south Spain). Despite the increasing number of records in Britain, there are still only small numbers of Glossy Ibis present in spring and, so far, only two nesting attempts. The majority of Glossy Ibises recorded in Britain arrive in autumn, with re-sightings of colour-ringed birds indicating that most arrive during their first year. Our results indicate that, regardless of any common trend, larger numbers of Glossy Ibis tend to be recorded in Britain in years when smaller numbers have bred in Doñana. A higher proportion of Glossy Ibises then tend to be present in Britain in spring compared to the previous autumn, when temperatures are higher during the winter in between. In short, our results suggest that Glossy Ibis is more likely to breed in Britain when poor conditions for breeding in Doñana are followed by mild winters in Britain. Although we expect Glossy Ibis to begin breeding regularly in Britain eventually, there are probably very few wetlands in Britain large enough to support breeding colonies of significant size.
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SIS CONSERVATION 1 (2019) 116–121
SPECIAL ISSUE: GLOSSY IBIS ECOLOGY & CONSERVATION
116
The Changing Status of the Glossy Ibis Plegadis falcinellus in Britain
Malcolm AUSDEN1*, Graham WHITE1, Simone SANTORO2
1RSPB, The Lodge, Sandy, Beds SG19 2DL. United Kingdom
2Department of Molecular Biology and Biochemical Engineering, University Pablo de Olavide, Sevilla, Spain
*Corresponding author; e.mail: malcolm.ausden@rspb.org.uk
A R T I C L E I N F O
Article history:
Received 06 June 2018
Received in revised form 25 November 2018
Accepted 10 December 2018
K E Y W O R D S
Wetlands, Waterbirds, Glossy
Ibis, Plegadis falcinellus, Britain,
Colonisation
A B S T R A C T
Numbers of Glossy Ibis recorded in Britain have increased dramatically since
the mid-2000s, mirroring the increase in their breeding population in southwest
Europe, especially in Doñana (south Spain). Despite the increasing number of
records in Britain, there are still only small numbers of Glossy Ibis present in
spring and, so far, only two nesting attempts. The majority of Glossy Ibises
recorded in Britain arrive in autumn, with re-sightings of colour-ringed birds
indicating that most arrive during their first year. Our results indicate that,
regardless of any common trend, larger numbers of Glossy Ibis tend to be
recorded in Britain in years when smaller numbers have bred in Doñana. A
higher proportion of Glossy Ibises then tend to be present in Britain in spring
compared to the previous autumn, when temperatures are higher during the
winter in between. In short, our results suggest that Glossy Ibis is more likely to
breed in Britain when poor conditions for breeding in Doñana are followed by
mild winters in Britain. Although we expect Glossy Ibis to begin breeding
regularly in Britain eventually, there are probably very few wetlands in Britain
large enough to support breeding colonies of significant size.
Introduction
Until the early decades of the twentieth century,
Glossy Ibis Plegadis falcinellus was a fairly regular
visitor to Britain, with 340 accepted records prior to
1950 (http://www.rbbp.org.uk/). The number of
records then declined, probably mirroring the decline
in their breeding population in southeast Europe (e.g.
Doroşencu et al. 2019; Puzović et al. 2019), their
main breeding population in the region at that time.
Glossy Ibis became a very rare visitor to Britain until
the early 2000s, albeit with two long-staying
individuals present in Kent during the period between
1975 and 1992 (Brown and Grice 2005). Major
arrivals of Glossy Ibises into Britain took place in the
autumns of 1986 and 2002 (http://www.rbbp.org.uk/),
after which numbers recorded in Britain have shown
an upward trend, which we quantify and report on in
the Results.
The 1986 arrival took place before the re-
establishment of regular breeding by Glossy Ibis in
southwest Europe in 1993 (Santoro et al. 2010; Vera
et al. 2019), but the subsequent increase in numbers
of sightings in Britain has coincided with a period of
growth in their breeding population in the Iberian
Peninsula and south France. In these areas, the
breeding populations have shown a remarkable
increase, especially in Doñana (south Spain) which
nowadays hosts the main breeding and wintering
populations of Glossy Ibis in Europe (e.g. Santoro et
AUSDEN ET AL., 2019 SIS CONSERVATION 1 (2019) 116–121
117
al. 2016; Mañez et al. 2019; Champagnon et al.
2019).
It remains to be seen whether Glossy Ibis might start
breeding regularly in Britain. Two breeding attempts
have already occurred. The first took place in 2014,
and involved a pair of birds displaying at RSPB
Frampton Marsh in Lincolnshire in eastern England,
one of which then built a nest platform (Holling et al.
2016). Then in 2016 a pair summered at RSPB Ham
Wall in Somerset in southwest England, and built a
nest platform in the old nest of a Eurasian Coot
Fulica atra. Ham Wall forms part of a large (ca 1,200
ha) complex of wetlands in Somerset known as the
Avalon Marshes. A limiting factor for the breeding of
the Glossy Ibis in Britain could be the ability of
juveniles to survive the winter and remain in the area
until they can breed. We are not aware of any
information regarding the effect of winter conditions
on the probability of ibises remaining in Britain from
autumn to spring.
In this study we aim to (i) describe the changing
status of Glossy Ibis in Britain by reporting the
variation in the yearly and monthly frequency of
sightings and their spatial distribution; (ii) test
whether numbers of Glossy Ibis recorded in Britain
are explained by the dynamics of their breeding
population in Doñana; and (iii) evaluate whether
numbers of Glossy Ibis in Britain in spring relative to
autumn might be negatively affected by winter
conditions.
Methods
Description and comparison of numbers in Britain
and Doñana
First, we summarised long-term (1950-2016) changes
in the status of Glossy Ibis in Britain using numbers
of accepted records of Glossy Ibis in Britain each
year. Before 1 January 2013 records of Glossy Ibis
were collated by the British Birds Rarities Committee
(BBRC) (https://www.bbrc.org.uk/), the official
adjudicator of rare bird records in Britain. After this
date, Glossy Ibis ceased to be classified as a rare bird,
which meant that records of them ceased to be
assessed by the BBRC. Subsequent records have
instead been assessed by county record committees,
and collated to produce an annual report on scarce
migrant birds in Britain (e.g. White and Kehoe 2017).
Both systems of assessing and collating records list
the location, and first and last dates, of each record.
This information is used to estimate the numbers of
newly arrived Glossy Ibises each year. Most
apparently recently arrived flocks of Glossy Ibises in
Britain have broken into smaller groups and dispersed
within a few days. Based on this, and on observations
of colour-ringed Glossy Ibises in Britain, the BBRC
and scarce migrants reports presume that most
subsequent records are of individuals from these
dispersed flocks (e.g. Hudson et al. 2010, 2011).
Hence the figures produced by the BBRC and scarce
migrants reports will tend to under-estimate numbers
of Glossy Ibis arriving in Britain, rather than double-
count birds.
The numbers of Glossy Ibis pairs in Doñana, their
most important breeding site in western Europe, have
been collected since 1996 by the Monitoring Team of
Natural Processes of the Biological Station of Doñana
(see Mañez et al. 2019 for details on visual count
methodology). For each population (Doñana breeding
pairs and British records), we performed a Poisson
GLM (glm function in R, R Core Team 2017) to
assess the linear trend of their annual numbers in the
period 1996–2016. Given that we were interested in
estimating the two populations’ trends if both of them
had started in 1996, we added a zero to each data set
for 1995, and for each series we ran a model without
intercept to make the two coefficients comparable.
Then we investigated whether variation in the number
of records of Glossy Ibis recorded in Britain each
year was explained by variation in the breeding
population in Doñana (see Mañez et al. 2019). The
analysis of the two time-series cross-correlation was
performed using Autobox (Version 6.0, Automatic
Forecasting Systems Inc., Hatboro, Pennsylvania,
USA). This software implements an automatic
algorithm capable of detecting, estimating and
adjusting for the presence of (i) outliers (shift-levels
or pulses), (ii) autocorrelation and (iii) non-
stationarity in the auto-regressive integrated moving-
average (ARIMA) model. The number of annual
records in Britain was set up as the dependent
variable, and the annual number of breeding pairs in
Doñana as the independent variable. Since most
SIS CONSERVATION 1 (2019) 116–121 AUSDEN ET AL., 2019
118
apparently recently arrived ibises in Britain are first
year birds, we defined the model as to allow only
immediate (no lagged) effects. The results of this
analysis indicate whether variation in numbers of
ibises breeding in Doñana explains variation in
numbers recorded in Britain during the same year, net
of any common trend between the two series.
Monthly frequency and spatial distribution
We investigated changes in the monthly abundance
and spatial distribution of Glossy Ibis in Britain since
the start of recent influxes in 2002. To do this, we
calculated the number of Glossy Ibis ‘bird-site-days’
per month. For each accepted record, we multiplied
the number of ibises recorded at a site by the number
of days between the first and last date they were
recorded there. To investigate changes in the
abundance of ibises, we then summed the number of
ibis ‘bird-site-days’ in each month, and divided this
by the total number of days in the month. This
provided an estimate of the mean number of Glossy
Ibises present in Britain per day during each month.
We investigated the geographical distribution of
ibises by summing the number of ibis ‘bird-site-days’
in each bird recording area in Britain.
Winter conditions and numbers of ibises present in
spring compared to the previous autumn
We also investigated the relationship between the
abundance of Glossy Ibises in Britain in spring
compared to in the previous autumn, and the mean
temperature of the winter in between. We ran a
Spearman correlation test (cor.test function in R)
between the (i) ratio of ‘bird-site-days’ in April and
May and ‘bird-site-days’ during the previous
September and October and the (ii) mean UK
temperature anomaly during December to February
inclusive. Temperature data were from the UK Met
Office
(https://www.metoffice.gov.uk/climate/uk/summaries
/anomalygraphs). For this analysis we only used data
collated in the period 2009-2015, since very few
ibises were present in Britain in autumn and/or spring
before then.
Results and Discussion
Description and comparison of numbers in Britain
and Doñana
After the long period between 1950 and 1986 when
the species was almost absent in Britain, numbers of
Glossy Ibis recorded in Britain have increased
dramatically, particularly since 1996 when the
Doñana colony became established (Figures 1 and 2).
The Doñana yearly rate of increase has been 1.92
times greater than that in Britain (on the log-scale,
Doñana: β = 0.427, SE = 0.00027, p < 0.001; Britain:
β = 0.222, SE = 0.00209, p < 0.001). This is not
surprising, given that the growth rate in Doñana is
determined by the population’s high breeding
productivity (Santoro et al. 2016), whereas the
population in Britain comprises birds that have
dispersed from other areas.
Figure 1. Numbers of accepted records of Glossy ibis in
Britain between 1950 and 2016
AUSDEN ET AL., 2019 SIS CONSERVATION 1 (2019) 116–121
119
Figure 2. Numbers of breeding pairs of Glossy Ibis in
Doñana and numbers of accepted records of Glossy Ibis
in Britain during the period 1996 2016. Red squares
and dashed line indicate, respectively, the number of
breeding pairs in Doñana and the relative smoothed
trend. Black circles and dashed line the number of
accepted records in Britain and the relative smoothed
trend
According to the final model selected by Autobox,
which accounted for the statistically significant
causes of non-stationarity (autoregressive factor AR1,
coeff. = 0.9, E = 0.103, p < 0.001; pulse at time 17,
coeff. =37.5, SE = 17.3, p = 0.044; pulse at time 18,
coeff. = 99.9, SE = 16.4, p < 0.001), the number of
ibises recorded in Britain was negatively affected by
the dynamics of the population in Doñana (Intercept =
0.709, SE = 5.36, p = 0.203; slope = - 0.00581, SE =
0.00163, p = 0.002). This indicates that, regardless of
any common trend, in years when there are smaller
numbers of breeding pairs in Doñana, larger numbers
tend to be recorded in Britain. This might be because
poor conditions for breeding in Doñana also tend to
result in higher rates of dispersal of birds towards
other areas such as Britain after the breeding season.
A previous study (Santoro et al. 2013) demonstrated
that when breeding in Doñana was prevented by dry
years, the probability of dispersal towards other
regions increased between 2.5 and 4 times depending
on the individual’s previous fidelity in the area.
Furthermore, the immediate effect (in the same year)
of the Doñana dynamics on numbers of ibises
recorded in Britain, is supported by the evidence that
most ibises arrive in Britain during their first year. Of
the 135 Glossy Ibises thought to have arrived in
Britain in autumn during 2009-16 (i.e. since the large
increase in numbers of records), 62 were juveniles,
four were adults, with the age of the remaining 69 not
reported. Evidence that the majority of Glossy Ibises
arriving in Britain are first year birds is further
supported by the results of colour ringing. There
have, so far, been sightings of 33 colour-ringed
Glossy Ibises in Britain, of which 30 were ringed as
nestlings in Doñana in southwest Spain, and three as
nestlings in the Petite Camargue in the south of
France. The majority of these individuals have been
recorded in Britain during their first year (21 out of
30 birds ringed in Doñana; all three of the birds
ringed in the Petite Camargue). Many of these birds
have dispersed quickly from their breeding areas,
with eight of the colour-ringed Glossy Ibises having
been recorded in Britain within three months of being
ringed as nestlings in Doñana.
Monthly frequency and spatial distribution
The peak of records of the species in Britain tends to
be in autumn, but this pattern has not been consistent
over time (see Figure 3). The majority of assumed
arrivals of Glossy Ibises in Britain have been in
coastal counties of southwest and southeast England.
Both of these regions are also favoured by Glossy
Ibises during the rest of the year (Figure 4).
Figure 3. Mean numbers of Glossy Ibises recorded in
Britain since the start of recent influxes in 2002
SIS CONSERVATION 1 (2019) 116–121 AUSDEN ET AL., 2019
120
Figure 4. The abundance of Glossy Ibis in different bird
recording areas in Britain between 2002 and 2016
Winter conditions and numbers of ibises present in
spring compared to the previous autumn
Because the majority of Glossy Ibises arriving in
Britain are first year birds, most would have to
survive at least one winter in Britain before breeding.
There has been considerable variation in the
abundance of Glossy Ibises in Britain in spring,
compared to in the previous autumn (Figure 5).
Figure 5. The relationship between the abundance of
Glossy Ibises in Britain in spring compared to the
previous autumn, and the mean UK temperature
anomaly during the winter in between. Temperature is
expressed as the mean difference from the 1961-90
average, with negative values in the x-axis indicating
that the winter was colder than the 1961-90 average.
The dashed line indicates the regression line of the ratio
between spring and previous autumn records on the
temperature anomaly
This variation appears to be largely related to the
severity of the winter, with a higher proportion of
birds being present in spring compared to in the
previous autumn, when temperatures are higher
during the winter in between (Spearman correlation
coeff. = 0.86, p = 0.024). There is no evidence that
Glossy Ibises show a more southerly, or south-
westerly, distribution in Britain in winter than during
the rest of the year (Figure 4). This is perhaps
surprising, given the milder winter temperatures in
southwest England compared to further north and east
in Britain.
Our results therefore suggest that Glossy Ibises are
more likely to breed in Britain when poor conditions
for breeding in Doñana result in birds dispersing
north in autumn, and these are followed by mild
conditions in Britain that enable birds to remain there
through the winter. However, it is unclear whether
Britain could support significant-sized breeding
colonies of Glossy Ibis, even as the climate continues
to warm. In particular, there are probably very few
wetlands in Britain large enough to support
significant-sized breeding colonies of colonial
waterbirds (Ausden et al. 2014). The most likely area
in Britain for Glossy Ibis to begin regularly breeding
is probably the Avalon Marshes. This large complex
of wetlands is in southwest England, a region with a
AUSDEN ET AL., 2019 SIS CONSERVATION 1 (2019) 116–121
121
large number of records of Glossy Ibis, and there has
already been one nesting attempt in the Avalon
Marshes, as described earlier. The Avalon Marshes is
playing an important role in the colonisation of
Britain by several other southerly-distributed
waterbird species (Ausden et al. 2014; Hughes 2018).
Acknowledgements
We are grateful to the BBRC and to Steve White for
providing records of Glossy Ibis, and for their help in
producing this paper. Mark Gurney helped produce
the distribution maps. We also thank the BTO for
providing details of sightings of ringed Glossy Ibises
in Britain. The BTO Ringing Scheme is funded by a
partnership of the British Trust for Ornithology, the
Joint Nature Conservation Committee (on behalf of:
Natural England, Natural Resources Wales and
Scottish Natural Heritage and the Department of the
Environment Northern Ireland), The National Parks
and Wildlife Service (Ireland) and the ringers
themselves. We are also grateful to David Reilly for
his kind and thorough assistance in time series
analyses, and to the ICTS-RBD and the Ringing
Office of the Estación Biológica de Doñana for data
collection in Spain.
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