Vol. 56]
1939
I
ALLEN,
Effect
ofWill onFli!?ht
Speeds
EFFECT
OF WIND
BY
FRANCIS
ON FLIGHT
H.
291
SPEEDS
ALLEN
A CONSIDERABLE
amount of data on the flight speedsof birds has been
accumulating
for someyears,especiallysincethe attentionof aviatorsand
motoristshas beendirectedto the subject,but a readingof the latest compilation of these data--that made by Miss May Thaeher Cooke of the
United StatesBiologicalSurveyand publishedas a 'Circular' of the Departmentof Agricultureunderdate of May, 1937--revealsa lack of certain
very material informationin most of the records. Few observersseemto
have considered
the importanceof recordingthe exactvelocityof the wind
and its exactdirectionin relation to the line of flight. Theseelementsare
vital factors where ground speedsare concerned,though comparatively
unimportant for air speeds. The air speedsrecordedby aviators are true
indicationsof how fast the birds were flying at the times of observation,
thoughnot of how they were progressing
on their journeys. In the ease
of birds watched from on land, whether from automobilesor otherwise, it
is necessarilythe actual distancecoveredin a certain time that is noted;
and this distancemay be very materiallymodifiedby the wind. To ascertain the air speedin sucheases--thatis, the true speedof the bird--is not a
mere matter of addingthe velocity of the wind to that of the bird going
againstit and subtractingit when the bird fliesdown wind, for that does
not take into considerationthe vastly greater number of winds that blow
from other quarters. I am speakingof birds following a definite course,
not simply working farther north or south or east or west in relation to
latitude or longitude.
It is a well-knownfact that duringstrongwindsbirds on migrationtend
to 'pile up' againstsuchbarriersas mountainrangesand the seashore.In
the latter easeseabirdsare sweptin from the seaduringstormsfrom that
quarter, while landbirdsduring high offshorewinds are held along the
shoreby their natural reluctanceto leave the land. But along the shore
and in many other placesbirds fly on definitecourseson which it is important for them to governtheir flight accordingto the wind in whichthey are
flying. It is obviousthat a slow-flyingbird will be reluctant to fly in a
strong adversewind, and even of beam winds any but the gentlest will
handicapa slowflyer seriouslyby tendingto blow the bird off its course.
Someof the effectsof windsblowingfrom other angles,however,appearto
have been very commonlyoverlooked,thoughit is all a matter governed
by the samelaw as the compositionof forces. A little experimentingwith
variousparallelograms
has produeedresultsthat have rather surprisedme,
and it may be worth while to bring them to the attention of others.
292
ALAN,
Effect
ofWindonFlight
Speeds
[Auk
[July
Let us assumethat a bird on the southwardmigrationis followinga
coastlinethat runs due north and south, and that its coursecalls for its
keepingwithin a definite distanceof shore. We may call it a Brown
Pelicanon the coastof Californiaflyingat an air speedof twenty-fivemiles
an hour. Let us assumethat a wind of ten m. p. h. arises--what the
FIC.I
Scale: I in.• 8miles.
w
•ndI0m.p.h.
•
Bird
Weather Bureau calls a 'gentlebreeze'--and blowsfrom due east. If the
pelicanwere to fly blindly by someinternal compassthat led it due south,
it would find itself after an hour at the end of a diagonalof a rectangular
parallelogramtwenty-fivemilesfarther south,but ten milesfarther west.
The bird wouldhave travelledabout twenty-sevenmilesbut wouldbe ten
milesout at sea(seeText-fig.1).
But thoughbirdsknow nothingof parallelograms
of velocities,they do
knowhowto hold a course. In this caseour pelicanwouldfly aboutsouthsoutheast to counteract the effectsof the ten-mile east wind, and an hour's
Vol. 56'1
1939
J
ALLEN',
Effect
ofWind
onFlight
Speeds
293
flightat twenty-fivem.p.h.wouldtakeit about22.9mileson its southward
course(seeText-fig.2). A shiftof the windto east-northeast
wouldspeed
our bird up to a gainof 4.4 overthe 22.9 miles. Anothershiftto northeast
rlCr. 2
Scale: lin.=Smi/es
Wind E. 1Om.p.l•,
Bird
2• m.[o.h.
4,4
4
1.6
[rd mako• 34-mi.
onwind•frornN.N.E.
would gain it anotherfour miles. If the wind shiftedanotherpoint, to
northeast-by-north,
the bird would gain an additional1.6 miles;and a
further shift to north-northeastwould land it 1.1 miles still farther south,
makinga total of thirty-fourmileson its course. If the wind went round
294
ALLr•,
Effect
ofWind
onFlight
Speeds
[Auk
[July
to the north and blew steadilyfrom that quarter, the southingwould be
thirty-five miles;that is, the bird's twenty-fiveplusthe wind'sten. (For
convenience,
exceptin the caseof the eastwind, onlyhalf of eachparallelogram has beenshownin the diagram.)
As it may not be clear to all readersjust how the parallelograms,or
rather the semiparallelograms,
or triangles,are constructed,it may be well
to say at the outsetthat they are drawn in the mostmechanicalfashionby
empiricalmethodsemployingthe simplestinstrumentsand the most elementarymathematics.I believe,however,that the resultsare exactenough
for all practicalpurposes.Given a fixedcourse(in this casedue south),a
fixedwind speedand wind direction,and a fixedbird speed,onehasonly to
spreadthe dividersto the exactbird m.p.h. on a scale,placeone end at
the leewardend of the wind line, and then mark wherethe other end crosses
the line of the north-and-southcourse. This givesthe end of the diagonal
of the (completeor incomplete)
parallelogram,
the directionof the diagonal
being,of course,alwaysthe sameby the termsof the problem. The length
of the (vertical) diagonalindicatesthe groundspeedof the bird. It must
be emphasizedthat for the purposesof this demonstration
the bird's air
speedsare fixed. If the measureof the bird's m.p.h. is not long enoughto
reach the line of its coursefrom the leeward end of the wind line, it shows
that the bird travellingat that speedcouldnot possiblykeepits coursein
that wind, but would be blown farther and farther out to sea. As a matter
of fact, no bird'sair speedis inflexible,and unquestionably
any bird finding
itself unableto copewith the wind at its ordinary speedwould increaseits
speedas muchas necessary
and practicable.
But what we have set out to do at present is to show how the wind
affectsthe groundspeedsof birds flying at their normal air speeds. The
strongerthe wind, the moreit hindersa bird whenit blowsabeamor in that
generaldirection,and the more it helpsthe bird as it approaches
the directionof flight. With a twenty-milewind--a 'freshbreeze'as the Weather
Bureaucallsit--the neutral point comesat about one degreenorth of eastnortheastfor our twenty-five-milesouthward-flying
pelican. East of that,
the wind would hold the bird back. With the wind abeam, it could make
but fifteenmileson its course. On the otherhand,if the wind shouldswing
to northeast,the bird couldmakeabout34a/•miles;and if it shouldswing
to the north-northeast,the bird could gain nearly eight miles over that
(seeText-fig.3).
But let us increaseour wind to twenty-five miles--a 'strong breeze.'
Now our bird couldjust holdits ownin a beamwind by flyingdirectlyinto
it, but of courseno bird wouldfly in sucha wind if it felt any urgeto keepa
courseat right anglesto the wind. If, however,the windshouldswingonly
a singlepoint, 11•/• degrees,
to the north,our southward-bound
pelicanby
Vol. 56']
1939
J
ALL]•,Effect
ofWindonFlight
Speeds
FIG.
Sc•e:
295
5
lin. = 8 m•[es
Wind2.0m.p. h.
15
W•nd
IøN.•f E.r.4.E
N.E (
2.5' m.
7.8
296
[Auk
AL,•, Effect
ofWind
onFlight
Speeds
[July
flyingat normalspeedaboutten degreessouthof eastcouldgainaboutten
mileson its course,and that might be worth the effort. An east-northeast
wind would net about nineteen miles, and a northeast wind would be dis-
tinctlyfavorable,
speeding
thebirdup to 351• miles(seeText-fig.4).
FIGScale: I in. -- 8 miles
W'indfromE' 25 m.p.h.
10 m.
E.N.E.•..
lO m.
Whenwe are considering
changes
as smallas a singlepoint,we mustnot
forgetthat no strongwindeverblowssteadilyfrom onepointof the compass. No onewhohaswatcheda weather-vane
canfail to havenoticed
that suchwindsare constantlyveering,perhapsas muchas a point or even
¾ol..56]
1939 J
A•.•.E•q,
Effect
ofWind
onFlight
Speeds
297
FIG. 5
Scale:
lin. =/•raJles
30.5
m,
298
ALLEN,
Effect
ofWizut
onFlight
Speeds
[Auk
[July
more,to onesideor the otherof the meandirection.Whenwe speakof
changes
of onlyonepoint,therefore,
wemeanchanges
that average
a point
in one direction in the course of an hour.
If our strongbreezeincreases
to a 'moderategale' blowingthirty-five
m.p.h.fromthe northeast,
ourbirdby flyinga little southof eastcankeep
its course
andbetterits mileageslightly,perhaps
by twomiles. A shiftof
a singledegreeto the northwill nowspeedit up to thirty milesor more,
FIG, t5
Scale: li•.•
16
43.•,rn.
and, as the diagram(Text-fig.5) shows,a shift to northeast-by-north
will
add aboutfourteenmilesto the bird'sspeed,whilea further shift to northnortheastwill add nine milesmore,makingthe total fifty-four milesan
hour.
If the moderategaleincreases
to a 'strong'oneof fifty m.p.h.,it will
blow our bird out to seaunlessit comesfrom thirty degreesor lesseastof
north.
At about N. 30ø E. it would take it 43.3 miles on its course. One
degreemore to the north would add sevenmiles, and a wind from the
north-northeast
would carry our twenty-five-•nile
bird about sixty-two
miles(seeText-fig.6).
Vol. 56]
1939J
ALLEN,
Effect
ofWindonFlight
Speeds
299
So muchfor our comparatively
slow-flying
pelican. Now let us take a
fasterbird,saya duckwith a speedof fifty m.p.h.in a calm. With sucha
bird a ten-milewind from any directioncan make little difference. At the
worstit canreducethe speedonlyto forty andat bestit canincrease
it only
to sixty m.p.h. Ducks, however,being strong-flyingbirds, may well
FIG.
7'
Scale: I in,.- 16 miD•
Wind E.,20 m.Fk.
$8.5'm.
venture forth in strongbreezes. Our diagram (Text-fig. 7) showsthat,
with a beamwind of twentym.p.h.from the east,a southward-flying
duck
wouldmake45.8milesan hour. The beamwind wouldthushamperit, but
a northeastwindof thesamevelocitywouldspeedit up to 62.5miles. With
a wind dueeastat thirty-fivem.p.h.ourfifty-mileduckcouldholdits course
by headinga very little southof southeastand make 35.7 miles. A shift of
half a point (5• ø) to the north of east wouldadd a little overfour miles
to what the bird would make with an east wind.
A northeast wind at
300
ALLEN,
Effect
ofWind
onFlight
Speeds
[Auk
lJuly
thirty-five m.p.h. wouldenableour duck to make68.6 milesin an hour, and
a shift of onedegreeto the north wouldgive it about two-thirdsof a mile
more;andsoon (seeText-fig.8).
Wind E. ak :•m.
•.h.
FIG. 8
• caIe:!in.
16 mi/c•
V,•. ~.
ß
.80.6 m.
To seehow a fifty-milewind wouldaffecta fifty-milebird we have only
to doublethe figuresfor the twenty-five-milebird in a twenty-five-mile
wind (seeText-fig.4).
Our diagramssuggestthat in a hamperingwind a bird headsdeliberately
in a very differentdirectionfrom that in whichits coursewouldleadit and
allowsthe wind to make the necessaryadjustment. It must not be under-
'Vol. 56]
1939
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ALLEN,
Effect
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Speeds
301
stoodthat this is what actually happens. We shouldnot expectto see a
bird headingstraightfor shorewhenits realintentionis to parallelthe shore.
What the bird actuallydoesis to fight every foot of the way and, however
this is done,the resultis the sameasif it steeredby compass
in the appropriate direction. It is clear, at least,that no moreeconomical
way of keeping
its coursecouldbe devisedthan by usingits full powersof flight in a direction calculatedto balanceaccuratelythe forceof the wind.
As I have said, and as everybodyknows, birds' flight speedsare not
fixedquantities. We cannotsay that thespeedof the Mallard, for instance,
is fifty m.p.h. The air-speedrecordsfor this speciesrun from forty-six
(plusor minus)to sixty. It shouldbe obviousthat eventhe air speedsof
birds are governedby conditionsother than their wing power. Meinertzhagenin an often-citedpaperin 'The Ibis' for 1921,recordedhis conclusion
that "birds have two speeds--anormal rate which is used for everyday
purposesand alsofor migration,and an accelerated
speedwhichis usedfor
protectionor pursuit, and which in somecasesnearly doublesthe normal
speed." He doesnot make it clear,however,why he limits his birdsto two
speeds. I think he was intent chiefly on showingthat birds possessed
a
powerof considerable
accelerationover their ordinary speeds,and had no
real intentionof assertingthat individualbirdswereactuallylimited to two
fixed speeds. It would seem fairly obviousthat even ordinary speeds
vary greatly. A pelicanfeedingwouldnaturally fly in muchmoreleisurely
fashionthan if it wereflying hometo its roost,and a smallbird flitting from
one tree to another could hardly get up the speedit would need for a
migrationflight. So, too, in the higherspeeds,a bird pursuedby a hawk
wouldnaturally acceleratefrom fast to fasterandfasteras the chasebecame
hotter; and wouldmake a greaterspeedin gettingaway from a fast hawk
than would be neededin escapingfrom a slowerone, becauseit would be
pressedto the limit of its strength. Perhapsfor most purposesthe most
valuablespeedrecordsare thosethat showthe very limit of the bird's
powers.
Of the 178recordsof groundspeed--notair speed--listedin Miss Cooke's
paper, only twenty-ninemake any mentionof the wind. Of these,nine
report "calm", "still air," or "no wind," and may be taken as valid records
for air speed;two say "light air"; five, "wind abeam"; one, "all types of
wind"; one, "wind behind"; one, "favoring wind"; one, "gale behind";
one,"strongheadwind"; only three giveany estimateof the velocityof the
wind in m.p.h.; and only one mentionsany relativewind directionexcept
ahead, abeam, or behind.
The last-mentionedrecord,which is alsoone of the three giving figures
for the velocity of the wind, is that of Brown Pelicansby Carl R. Smith
publishedin 'The Gull,' monthly bulletin of the AudubonAssociationof
302
ALLEN,
Effect
ofWind
onFlight
Speeds
[Auk
[July
the Pacific,in September,1924. This singlerecordis praiseworthyas an
attempt to giveexactdata, but I must call attentionto the fact that it is
opento seriousquestion. The timingof the pelicanswasdoneat El Pismo
Beachnear San Luis Obispo,California,when the birdswereflying "south,
keepingcloseto shore." We are informedthat the prevailingwind on this
stretch of the coastis from the northeast,but no statementis made as to
its exact directionat this particularpoint and time. Mr. Smith goeson
to say, "Their speed,in practicallycalm air, reaches14 milesper hour; in
a half-gale,it risesto 16 milesper hour;with the wind blowingat fifty miles
per hour, the birdstravel at the rate of 22 milesper hour." Now fourteen
m.p.h. in a calmseemsvery slowfor a BrownPelican. Longstreet's
record
citedby Miss Cookefor four birdswith wind abeam(not a favoringwind)
is twenty-sixm.p.h., and Melnertzhagencitesa recordfor anotherspecies
of pelicanof fifty-onem.p.h.with a "sidewind" of fifteenm.p.h.,whichat
the very leastwouldmeanan air speedof fully thirty-sixm.p.h. Moreover
it is not statedhow Smith'swind speedof fifty m.p.h. was ascertainedor
just what is meant by "half-gale." It is hard to see,at any rate, how his
birds could possiblyhave flown in a fifty-mile gale from the northeast
without beingblownout to seaeven if they couldincreasetheir speedto
thirty-five miles. A more favorablewind at anything approachingthat
speedwouldhave taken them muchmorethan twenty-twomileson their
course. I am at a lossto accountfor Mr. Smith'sfiguresand am sorrynot
to be able to acceptMiss Cooke'sexplanation,whichseemsto be founded
on the now-discredited
theory that a wind blowinghorizontallyhas some
mysteriouslifting powerof its own that makesit easierfor a bird to fly
againstit than with it. It is only fair to quoteMiss Cooke'sexactwords
for the benefitof readerswho may not haveher paperat hand. Sheseems
to haveregardedthesestrongwindsfrom the northeastasfavoringwinds-whereasit is clear that, blowingat suchspeedsthey must have beenseriously hampering,to say the least--and, to accountfor the comparatively
slight gainsin flight speedafter so great increases
of what might appear
to be a favoringwind, shesays: "The explanationis that in order for a
bird to remain aloft it must rest on a current of air againstthe under side
of its wings,and the currentmustcomefrom the front, not from behind,the
backwardtilt of the wingscatchingthiswind. Thesefactsare well recognizedin modernaeronautics.Both birds and airplanesmusttake off and
land into the wind. Therefore,if the wind is directly behind,the bird must
move its wingsfaster in order to get the necessary
upbearingcurrentor
elsebe blownalongby the wind without beingable to guideits course."
This overlooksthe fact that the bird, by its progressthrough the air,
creates
the currenton whichit mustrest. Otherwiseit wouldbe impossible
for a bird to fly in a calm. As a matterof fact, for supporting
a bird in the
Vol.
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ALLEN,
Effect
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Speeds
303
air, exceptfor the variationsin forceand directionof the wind, oneway of
the wind is as goodas anotherand one velocityof the wind is as goodas
another. A balloonfloatsupright in a steadywind as in a calm, and a bird
makesthe sameair speedin a steadywind, whetherflyingwith it, against
it, or acrossit. Whicheverway the bird goes,it getsits supportfrom the
currentof air that it meetsand getsno moresupportfrom a headwind than
from a stern wind, because,beingbornealongwith the wind, the air in
whichit fliesis stationarysofar as the bird's movementswithin it are concerned. Only in landingand in taking off is it important for a bird, as for
an airplane,to headinto the wind, becausethen the flier passesfrom one
dementto another. (Thereis alsothe exceptionof the hoveringbird, which
headsinto the wind to maintainits positionover a particularspotof land
or water.) The dislikethat birds seemto have for flying with the wind
applies,I think--after the first inertia is overcome--onlyto strongwinds,
whichare accompanied
by frequentannoyingpuffsthat rufflethe feathers
when they strike them from the rear.
I hopeI have shownthe importanceof fuller and moreprecisedata on
wind directionand wind speedin recordsof the groundspeedsof birds.
When we considerthat a differenceof a very few degreesin the directionof
a wind may make all the differencebetween completefrustration and
substantialprogress
for the bird, and that apparentlyneutral windsbecome
increasingly
unfavorableas they increasein strength,we canappreciatethe
seriousness
of the gapsin the publisheddata.
Professor
FrederickA. Saunders,of the PhysicsDepartmentof Harvard
University,hasbeenkind enoughto readthispaperandhashelpedme with
valuablesuggestions.
215 La GrangeSt.
WestRoxbury,Massachusetts