Page 70 - BBC Wildlife Volume 36 #04

TIT RESEARCH

Decades-worth of data are also being used to answer
more applied questions. The Wytham tit study was the SPOTTHE DIFFERENCE
first to show the impact of climate change on great tit
breeding, something that Lack and his students could GREAT AND BLUE TIT EGGS
hardly have expected in the 1940s. Just as Britain’s army
Great tits usually lay in the region of 5–11
of citizen scientists is reporting wildflowers blooming
eggs per clutch, and blue tits lay 6–12, though
earlier and the dates when we notice trees blossoming
the latter are known to produce up to 16. The
and coming into leaf have moved forwards, so the tits
female lays one egg each morning, and on the
have adjusted when they lay their eggs.
final day starts incubating her clutch – this
Today, great tits lay two to three weeks earlier on
process typically takes 12–14 days.
average than in the 1960s. “When I began working at
Tit eggs are cream with brown speckles. BLUE TIT
Wytham Woods, I would start checking the nestboxes
Historically, it was thought that the speckles
on 20 April,” says Andy. “Now, if I started checking
helped with camouflage. But Andy Gosler
then all the birds would be incubating already. I’d
realised that speckling wasn’t uniform: the
have completely missed the laying period.”
first egg in a clutch tends to be less marked
than the last, and some females had far more
GROWING UP FAST
speckled eggs than others.
Deciding when to start breeding every year is crucial
In fact, in great tits the speckles seem to
for the tits. They rely on the superabundance of winter
have a structural function, as they occur
moth caterpillars, Operophtera brumata, which are
where the shell is thinner. Females nesting on
available for a couple of weeks each spring when the
low-calcium soils laid thinner-shelled, more
oak trees are budding. Ella tells me that in the space
speckled eggs than females on high-calcium
of a fortnight parent tits could bring about 10,000
soils, and females may become calcium-
caterpillars back to their naked and blind chicks to imagebroker/Alamy
deficient by the time they lay their last eggs. GREAT TIT
transform them into fully formed birds capable of
flight. “This is the equivalent of a human couple
raising 10 newborn babies into 60kg teenagers in two
weeks – it’s hard not to be in awe.” Below: bat project females nesting near late-budding trees. Second, the birds
It seems that the tits can ‘predict’ caterpillar co-ordinator Dani can control when and how much they incubate their eggs,
Linton shows a
availability, since larger clutches are laid in years helping the hatch date coincide with caterpillar availability.
group of young
where there will be lots of moth caterpillars. Just how On warm days, when caterpillars are growing more quickly,
naturalists a bat.
they estimate this, and then adjust the timing of their The nestboxes females spend more time on the eggs to speed up hatching.
broods, is a question that puzzled Lack. But while the at Wytham are The opposite is seen on cooler days. It’s a web of connection
precise mechanisms are still a mystery, it now appears used by bats after between oaks, caterpillars and tits, and remarkable to realise
the young tits
that the Wytham tits are using a couple of cues. that the birds are responding to climate change.
have fledged.
First, the tits pay attention to the developmental stage
of oaks near their nestbox. Females nesting near early DEDICATED TEAM
budding trees start laying earlier in the season than “So what’s next?” I ask Ben. “How long have you got?” he
replies. He’s got a point. An incredible amount of influential
THE BIRDS CAN CONTROLHOWTHEY research has been done in 70 years, but that has no bearing
on the ultimate potential of this system. In fact, the longer
INCUBUATETHEIR EGGSTO COINCIDE the Wytham tit study continues the greater its value to
WITH CATERPILLAR AVAILABILITY. science, because the evolving insights from decades of
observations enable researchers to formulate new and
exciting questions that wouldn’t otherwise be possible.
I’m struck by the sheer enthusiasm of the researchers and
their love of being in these woods. Andy tells me that he
came to feel so connected to ‘his’ patch of woodland that it
felt like a bereavement when he stopped working there. But
surely you get fed up of hearing the great tit’s repetitive
‘teacher, teacher’ song? Ella laughs. “Never! Although I also
don’t think I’ll ever be able to hear that song without
thinking about work. In 50 years’ time, when I hear a
great tit singing in spring, I’ll probably still think I need
to get up early to catch birds…”
JO WIMPENNY + FIND OUT MORE
studied zoology at Discovermoreat www.
the University of wythamwoods.ox.ac.uk.
Bristol and is co-author of Wytham Woods features in
a David Attenborough Natural
10,000 Birds: Ornithology
World programme about
since Darwin (Princeton
birds’ eggs on BBC Two.
University Press, £37.95).
April 2018