Birdwatch UK October 2017

66 Birdwatch•October 2017 http://www.birdguides.com/birdwatch

EXPERT ADVICE BUILDING KNOWLEDGE • BUILDING SKILLS • EVENT OF THE MONTH

LOOK at the measurements

given in  eld guides and every

species will be assigned a

length, usually using the letter

L. Some species, especially the

non-passerines, will also show

the wingspan, using the letters

W or WS. The exception in the

passerines is often the crow

family.

Measuring the wingspan of a

bird is fraught with dif culties,

as it cannot be reliably recorded

in the  eld (see, for example,

Raptors of the World by James

Ferguson-Lees and David

Christie). When measuring skins

in museums, these can be

inadvertently stretched, providing

larger-than-life dimensions.

In the past many claims for

exceptionally large wingspans

were made, but these are now

recognised as erroneous and

discounted.

The usual method for

measuring a wingspan involves

placing a specimen on its

belly, and extending the wings

into a natural spread position.

The length from wing-tip to

wing-tip gives the wingspan. A

measurement of just one spread

wing, from the centre of the

body, can also be used and then

doubled. Another method, often

used for birds of prey, involves

measuring the folded wing

(carpal joint to wing-tip) and then

using a multiplier to calculate

the wingspan. For birds of prey

the multiplier is from 2.8-3.6,

for pelicans it is 4 and for large

albatrosses it is 4.5. For birds

with long folded wings, such as

Common Swift, it is 2.4.

Wingspan is a determining

factor in how large any  ying

bird can be. As a species’ size

increases, so too will the time

taken to grow new plumage.

Eventually a stage is reached

when feathers will wear out

before they can be fully regrown

and  ying with partially grown

 ight feathers is very inef cient.

Studies of feather growth

across a range of species show

that as body size increases,

At 2.5 metres, White-tailed Eagle (main photo) has the largest wingspan of any British birds of prey. This is dwarfed, though,

by that of the extinct Argentavis magnifi cens (inset), with an estimated wingspan of 5-6 metres.

primary length also grows, and

roughly doubles with each 10

times increase in mass. However,

the actual rate of feather growth

does not increase proportionally,

and so the time taken to replace

each feather increases by about

1.5 times with each 10 times

increase in mass. For a 10-

kg bird, it will take 56 days to

replace a single  ight feather.

Wing feathers are vitally

important and the process of

renewing them, by annual moult,

varies between species. Small

birds can often grow several

feathers at a time, as they

grow relatively rapidly. For large

species, such as albatrosses,

the moult of  ight feathers has to

take place over more than a year,

with complex moult procedures to

minimise any gaps in the wing.

A very large wingspan limits

a bird’s  ight. The bigger the

wingspan, the less easy it is

to  ap effectively, so gliding

becomes a way of life.

Extinct about 25 million years

ago, Pelagornis sandersi had an

estimated wingspan of 7 m (24

feet) and would glide over the

oceans like an albatross. Also

extinct, Argentavis magnifi cens is

thought to have weighed 70 kg,

with individual wing feathers of

more than a metre! It probably

moulted all of its feathers in

one go.

In Britain, White-tailed Eagles

are often referred to as ‘ ying

barn doors’ because of their

massive broad wings; they are

the largest British bird of prey.

Here are some of the largest and

smallest wingspans in the world:

• Wandering Albatross: 3.5 m
  (11.11 ft).


• Dalmatian Pelican: 3.5 m
  (11.6 ft).


• Marabou Stork: 3 m (10.5 ft).


• Andean Condor: 3 m (10.5 ft).


• White-tailed Eagle: 2.5 m (8 ft).


• Whooper Swan: 2.5 m (7 ft
  10 in).


• Goldcrest: 13.5-15.5 cm
  (5.3-6.1 in).


• Bee Hummingbird: 9 cm
  (3.5 in). ■

MAIN PHOTO: NEIL BOWMAN INSET: ARCOVENATOR (COMMONS.WIKIMEDIA.ORG)

❝

Extinct about 25 million years

ago, Pelagornis sandersi had an

estimated wingspan of 7 m

❞

Getting the measure of wingspan

BUILDING KNOWLEDGE

across a range of species show

that as body size increases,

At 2.5 metres, White-tailed Eagle (main photo) has the largest wingspan of any British birds of prey. This is dwarfed, though,

(inset), with an estimated wingspan of 5-6 metres.

A very large wingspan limits

a bird’s  ight. The bigger the

Bee Hummingbird: 9 cm

(3.5 in). ■

1710 p65-71 Expert advice FIN.indd 66 15/09/2017 10:46:07