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221

the nesting season, making competition higher anddecreasing the number of drinks per bird per day.
How the finches first began hovering for the nectar
One assumes theremains an unsolved question.
finches would ordinarily
drink by perching rather
than hovering.
Unlike hummingbirds,
they are notwell equipped for hovering, as they can get support
only on the downstroke.
While finches were completely intolerant of other species of birds in the feeder
tree, they never showed any aggression toward hummingbirds. These were common in the area and drankregularly at the feeder, alternating drinks with thefinches. The initial gradual increase in nectar consumption before the study began indicates that thefinches learned to drink the nectar from the feeder
and had ample opportunity to observe the hummingbirds daily. It is interesting to speculate that thefinches may have learned to hover by imitating thehummingbirds.
Whether this hovering behavior is a local adaptation is not known. To the author’s knowledge, hovering
behavior on the part of finches has not been reported
in other parts of California.
SUMMARYFIGURE
3. A male House Finch completing
tiple drink on a single hover.

a mul-A total period of at least 458 hr was spent recording
observations. During 205 of these hours, a count ofdrinks taken totaled 8971, or an overall average of 44
drinks per hr. The peak of the drinking activity occurred at the onset of nesting (mid-March)when thenumber of drinks per finch per day was 140. The
finches at this time were ingesting an average of 12%of their body weight in sugar. The total average
number of birds drinking per day increased during

ZONE-TAILEDHAWK ANDVULTURE:MIMICRYOR
AERODYNAMICS?
HELMUT

TURKEY

C. MUELLER

Department of Zoology
University of North Carolina
Chapel Hill, North Carolina 27514

Willis (Condor 65:313, 1963) has shown that theZone-tailed Hawk (Buteo ~Zbonotatus) differs from
related hawks and closely resembles the Turkey Vulture (Cathades aura) in color, shape, and mannerof soaring. Willis suggests that B. albonotutus is anaggressive mimic of Cuthartes and that this mimicry
may permit the hawk to approach closely potential
prey which have become habituated to the vulture.
\Villis mentions that the resemblance in wing shapeand dihedral might be due to similarities in the mannerof soaring; I believe this is the major reason for theresemblance. The best aerodynamic “design” for abird that habitually soars near the substrate includes

This is a report of a behavioral study on House Finches(Carpoducus mexicanus) in the Palos Verdes Peninsula area of California. The finches were quite normal
in courting and nesting pattern, but regularly fed byhovering at a hummingbird
nectar bottle as a chiefsource of their nourishment.
The unusual featureof the study is that each finch actually had to learn
how to hover in order to obtain the liquid. The learning process was observed for several finches and it
followed an extremely consistent pattern. The hovering performance is very proficient and can be sustained virtually motionless for a maximum of 5 sec.
This behavior can be learned by adults of both sexes
and by the immature offspring. The learning process
occurs over a 2-6-week period. The study was conducted during the daylight hours from February toJuly 1969, and observations were recorded during a
total of at least 458 hr. During 205 of these hours,
8971 drinks, by House Finches, were noted.
Accepted for publication

4 August 1971.

rather long and narrow wings that are held at apronounced dihedral. To my knowledge, all raptorial
birds that habitually soar near the substrate share thisdesign, including C. aura, B. ulbonotutus, and theharriers ( Circus sp. ).
A bird that holds its wings horizontal is quite unstable in soaring flight; if the lift on one wing exceedsthat on the other, the bird will begin to roll. Since
the lift generated by a wing is perpendicular to thewing axis, the vertical lift component will decrease
and a lateral component will develop. This lateralcomponent will pull the bird to the side and the lossof vertical lift will cause the bird to lose altitude; thecombination results in a side-slip. Thus a bird thatsoars with the wings held horizontal is very unstable
and will roll, lose altitude, change direction, and evencompletely lose control unless immediatechangesare made in the wings to adjust for the differences inlift. Such a bird soaring near the substrate will encounter frequent fluctuations in wind velocity andwill have to adjust its wings almost constantly. Abird soaring with its wings held at a dihedral is considerably more stable. When a roll occurs, the vertical- .

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component of the lift of the upward wing decreases
and that of the downward
wing increases, automatically correcting for the roll.
Birds that habitually soar at higher altitudes presumably do not have a high dihedral because fluctuations in wind velocity are less frequent and because
there is a cost for maintaining a high dihedral.
Ina bird soaring with horizontally held wings, all of thelift can be resolved into two components: one vertical and overcoming gravity and the other directed
forward and overcoming drag. If the bird adopts ahigh dihedral. there is a third comuonent to the liftof each wing, a component which does not contributeto either overcoming gravity or drag, and hence is“wasted” insofar as to its contribution to maintaining
flight.
This third component is directed laterally

toward the bird and tends to roll the bird. The rollcomponents of the two wings oppose each other, providing stability.
The effective lift lost by utilizing
a high dihedral is presumably offset by the increasein stability and the elimination of the necessity foralmost constant adjustment of the wings.
Thus the Zone-tailed
Hawk
may resemble theTurkey Vulture (and the harriers) in shape and manner of soaring because of aerodynamics and not because of aggressive mimicry. The color pattern of thehawk may be the result of mimicry, which developed
after the bird had developed a resemblance to thevulture in the manner of flight.
I thank Vance Tucker and Edwin Willis for fruitful
discussions of the ideas contained in this note.

RECOVERY
OF A PENNSYLVANIABANDED BLUE-GRAY
GNATCATCHER
IN WESTERNMCXICO

According to Larry L. Hood (in litt. ) of the Bird
Banding Laboratory, USFWS,
there had been only
two recoveries of banded birds of this species throughearly 1968, and the data on one of these were suspect.
The other was a bird both banded and recovered inCalifornia.
h4iller et al. (Pac. Coast Avifauna 33:202, 1957)
give no records of the eastern race of the Blue-gray
Gnatcatcher, P. c. caerulea, from the Pacific slope ofMexico north of Huehuetan,
in southern Chiapas.
Their westernmost
records were from Patzcuaro,
Michoacan, and Iguala, Guerrero (well north of theSierra Madre de1 Sur).
The Patzcuaro record wasbased on a specimen taken by Lea and Edwards on
18 March 1946 (Condor 52:267, 1950).
Accordingto these authors, “the specimen closely matches thenominate race in lightness and blueness of the crownand back, and is too large for deppei. We hesitate
to place this specimen with caerulea, however, sincetransients from the eastern United States are so unexpected, and since intermediates between deppei andumoenissima might be expected to resemble cuerulea
in some instances.”
The capture of an undoubted example of eastern
P. c. cueruleain the Pacific coastal lowlands of Jalisco
strongly suggests that a re-examination of gnatcatcher
specimens from western and southern Mexico, atleast as far north as Jalisco, is in order; it appears
quite likely to us that identification of some museum
specimens may well have been made on the basis ofgeographic probability.
We have found one unreported specimen of cueruleu in the Carnegie Museum,collected by P. Shufeldt at La Barca, easternmostJalisco, 16 November 1899; although on the Central
Plateau rather than on the Pacific slope, this locality
is well west of those listed for cueruleu by Miller
et al. ( Zoc. cit.).
We are indebted to Earl B. Baysinger, (then) Chiefof the Bird Banding Laboratory, USFWS, for sending
us a copy of the letter reporting this recovery; toLarry L. Hood of the same Laboratory for checking
records of gnatcatcher recoveries; to Allan R. Phillips
for information on the distribution of Polioptilu cuemlea in Mexico and for meticulous translation of
Spanish-language correspondence; and especially toSr. David Sahag&n V. for reporting the finding of theband and for the additional information he so kindly
sent us.

KENNETH

C. PARKES

AND

MARY

HEIMERDINGER

CLENCH

Carnegie Museum
Pittsburgh,

Pennsylvania

15213

The characters and distribution of the subspecies ofthe Blue-gray Gnatcatcher(Polioptila caerulea) inMexico are badly in need of revision (Allan R. Phillips,
in litt. ) . The problem is complicated by the abundanceof migrant
and wintering
birds from populations
breeding in the United States. That no easy assumptions can be made about geographic origins of such
migrants is additionallyemphasized by the recent
recovery of a banded gnatcatcher.
At Powdermill Nature Reserve, Carnegie Museum’s
field station in the Ligonier Valley (3 miles S ofRector),
Westmoreland
County, Pennsylvania,
145
Blue-gray
Gnatcatchers
were banded from1961
through 1969. Although we have had a few returnsof our local breeding birds, no recoveries from elsewhere had been reported until 1970.
On 1 November 1970, a bird bearing band no. 1172.5569 was killed by a boy with a slingshot in Llano
Grande, Jalisco, Mexico. Noticing the band, the boy
brought it to a neighbor, Sr. David Sahaglin V.,
who had sent reports of other recovered bands to theU. S. Fish and Wildlife Service. The band had beenplaced on an adult ( AHY) female Blue-gray Gnatcatcher at Powdermill Nature Reserve by Robert C.
Leberman on 9 May 1969. The senior author wroteto Sr. Sahaglin for verification of the species identityof the banded bird and received from him an excellent
description of a gnatcatcher, together with the information that this kind of bird, locally called “pisito,”
was well known to the inhabitants of Llano Grande.
Sr. Sahagun also supplied details on the exact locationof his village, which is in the Municipio de Tomatlan,Jalisco. It is shown on the Guadalajara sheet (NF-13)of the American Geographic Society “Millionth Map,”
and lies approximately 72 km SE of Puerto Vallarta.
This is not only the first recovery of a banded Bluegray Gnatcatcher south of the United States but oneof very few recoveries of the species from anywhere.

Accepted for publication

Accepted for publication

12 May1971.

20 August1971.