The Effect of Climate Change on Birds
by David Leech |
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6.1 Timing
of egg laying |
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| Invertebrates with herbivorous larvae, such
as moths and butterflies, must time their reproductive cycle such
that the emergence of their offspring coincides with the peak availability
of the vegetation on which they depend for food. The timing of leaf,
emergence is controlled by climatic variables, including temperature.
As the trend towards increasing global temperatures continues, the
date at which leaf emergence is initiated has advanced seasonally,
an advancement that has been tracked by herbivorous invertebrates.
Peak abundance of insect larvae, in particular caterpillars, has therefore
shifted towards an increasingly earlier date, and is likely to continue
to do so as mean annual temperatures continue to rise. |
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| There is increasing evidence to suggest that
birds are laying their eggs progressively earlier in the season in
order to ensure that the peak period of nestling energetic demand
continues to coincide with the period of maximum prey abundance: |
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- Using data collected under the BTO Nest
Record Scheme, Crick et al. (1997) identified a significant
trend towards earlier laying by a mean of 8.8 days for 20 of the
65 British bird species studied over a 25 year period, with only
one species displaying a trend towards later laying. Advancement
of laying dates was not limited to a particular taxonomic or ecological
group of species - birds laying earlier included migrant and residential
insectivores, waterbirds, corvids and seed-eaters.
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The chaffinch is one
of the species identified as having advanced its laying date over
the last 25 years in response to increasing annual temperatures.
(Graph from Crick et al., 1997).
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- An analysis by Crick and Sparks (1999), again using BTO Nest
Record Scheme data, revealed that laying date was significantly
related to temperature or rainfall trends for 31 of the 36 species
examined over a 57 year period, with 53% of these species exhibiting
a significant trend towards an advancement of laying dates. The
laying date advancement of 37% of these species could be explained
by climatic trends. Furthermore, the authors calculated that,
based on current predictions of trends in climatic temperature
over the current century, 75% of the 36 species will have advanced
their laying dates by 2080.
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| At first glance, seasonal advancement of
laying dates may seem potentially advantageous. Advanced breeding
allows adults and offspring more time to replenish their energy reserves
prior to the energetically demanding winter period (Crick et al.,
1997). In addition, earlier initiation of breeding may allow an increase
in the number of broods each individual is able to produce per season.
Przybylo et al. (2000) observed that collared flycatchers (Ficedula
albicollis) are able to alter the date of laying initiation annually
according to weather conditions, suggesting that rapid responses to
future climatic changes may potentially be possible. |
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| Several other studies, eg. Both and
Visser, (2001), have observed that chicks laid at relatively earlier
dates are more likely to fledge successfully, suggesting that birds
would benefit from advancing their laying dates still further. However,
many appear to be constrained from doing so. |
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- Egg production is energetically expensive, yet the availability
of food during the winter is low. Females may therefore require
sufficient time to recoup their energy reserves before they are
able to produce a clutch. Visser et al. (1998) observed
that, whilst great tits (Parus major) collect food for
their offspring from oak trees, females feed on birch and larch
prior to laying. Oak budding times, and thus the availability
of food for the offspring, have advanced seasonally whilst larch
and birch budding times have not, potentially reducing the period
between initiation of laying and peak food availability.
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- If increases in temperature have not been uniform across the
breeding season, the cues used by birds to predict the timing
of peak prey abundance may become increasingly unreliable. Laying
date in Dutch populations of great tits is correlated with the
temperature during the first two weeks of March (van Balen, 1973).
However, temperatures over this period have remained constant
during the period 1973-1995 whilst temperatures over the following
30 days, which determine caterpillar emergence, have increased.
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Cues used by great
tits to predict peak prey abundance may be becoming increasingly
unreliable.
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- The situation may be even more serious for migrant birds, as
cues used to initiate migration may not have varied in parallel
with changes in prey availability. Alternatively, it may simply
not be possible to advance migration dates further. Both &
Visser (2001) observed that pied flycatcher (Ficedula hypoleuca)
laying dates had advanced by 10 days over the last 20 years whilst
arrival dates on their breeding grounds had remained constant.
The decreased period between arrival and laying may reduce breeding
success or adult survival as the energy stores of the parent birds
immediately prior to breeding will become increasingly depleted.
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- Asynchrony between offspring production and prey availability
may be unavoidable if the development period of the prey species
is reduced in length at increasing ambient temperatures but the
development period of offspring cannot be. Evidence for such a
scenario was found by Buse et al. (1999) who observed that
oak winter moth caterpillars took 50 days to develop at field
temperatures, but only 20 days to develop at a constant temperature
of 15°C, whilst the period between great tit hatching and
fledging was unrelated to ambient temperature. Visser et al.
(1988) demonstrated that there was an increasing tendency for
young from early broods to be recruited into the breeding population
in the following year relative to young from later broods.
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| Lack of synchrony between chick production
and food availability may influence breeding success. Freeman et
al. (in prep) observed that brood productivity of 14 of the 26
species that they had studied decreased as temperatures during the
post-laying period increased, possibly due to the increased potential
for asynchrony between offspring demand and prey availability as temperatures
increased. |
| The predicted increase in global temperatures
over the current century may further increase the lack of synchrony
between chick production and food availability, which may have serious
consequences for the breeding success, and ultimately the abundance,
of a wide variety of British bird species. However, it is possible
that multi-brooded species may benefit from an extension of the breeding
season as temperatures rise, allowing an increase in the number of
broods produced per annum. |
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