The
Effect of Climate Change on Birds
by David Leech |
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6.2 Timing
of migration |
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| Although climate changes in temperate regions
have differed from those in the tropical in which many British bird
species winter (Houghton, 1996), the results of several studies investigating
the spring arrival times of migrant species in temperate latitudes
suggest that birds are reaching their breeding grounds progressively
earlier in the season as temperatures in these areas increase. Advancement
of arrival dates is necessary if synchrony between peak offspring
demand and peak food availability is to be maintained. |
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- Loxton & Sparks (1997) identified a trend towards earlier
migrant arrival dates in Britain, with 7 of the 27 species studied,
including wheatear (Oenanthe oenanthe), pied flycatcher,
chiffchaff, willow warbler (Phylloscopus trochilus) and
blackcap, arriving consistently earlier over the last 40 years.
Arrival dates appear to be negatively correlated to spring temperatures
for willow warblers and blackcaps, with both species arriving
earlier in warmer springs.
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Spring arrival dates
of willow warblers breeding in Britain have become progressively
earlier since the 1960s.
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- Using county record data from Leicester and Rutland and Sussex,
Sparks (1999) observed that the arrival dates of 20 of the 56
migrant species analysed had become progessively earlier since
1966, and that the arrival dates of 19 of the species were related
to temperatures in March and April.
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- Of the 32 migrant species considered by Sparks and Mason (2001),
23 significantly advanced their arrival dates between 1950 and
1998. Spring temperatures were significantly related to the arrival
dates of 10 species. Trends in departure dates were less obvious,
although 9 species did exhibit significant trends towards later
departure dates over this period, with only one departing significantly
earlier.
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- A recent study of the autumn migration of willow warblers at
Dungeness Bird Observatory (Marchant, 2002) found that dates of
passage were 5-10 days later during the period 1994-2000 than
they were during 1962-1968. Increasing temperatures may therefore
also lead to later departure dates of summer migrants.
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| Migration routes and timing may also be influenced
directly by climate change, both via changes in the strength and direction
of wind currents and by the location and quality of stop-over sites,
which may vary according to changes in global temperature regimes
(Lindstrom and Agrell, 1999). |
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There is strong evidence to suggest that
migrant bird species are advancing their arrival dates at their British
breeding grounds in response to increasing spring temperatures, with
departure dates generally remaining constant or becoming later. Increased
time spent at the breeding ground prior to migration might increase
survival probabilities of both adults and offspring, as more time
is available to recoup energy reserves necessary to fuel the long
flights to the wintering grounds.
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