THE CONDOR
VOLUME

57

VARIATIONNOVEMBER-DECEMBER,

OF THE COMMON ROUGH-LEGGED
IN NORTH AMERICA
By TOMNUMBER

1955

6

HAWK

J. CADEINTRODUCTION

The Common Rough-legged Hawk (Buteo ZagopusPontoppidan) is a polytypic
speciesof panboreal distribution. During the breeding season it is restricted to the circumpolar tundra biome and to the tundra-forest ecotone, except on Kamchatka where,according to Dementiev et al. (1951:307), it also breeds in the true boreal forest.
Three or four subspeciesare currently recognized. Buteo lagopuslagopus breeds from
Scandinavia east nearly to the Yenisei River in the Soviet arctic. The Siberian form,
which has usually been called pallidus Menzbier, breeds from the Yenisei eastward to
the Anadyr Peninsula and to the shores of Bering Strait. A third described race in the
Old World, kamtschatkensis Dementiev, is said to breed on the peninsula of Kamchatka,
on Bering Island, on the western shore of the Sea of Okhotsk, and on the Kurile Islands(Dementiev et al., 1951:314). Finally, there is sanctijohannis Gmelin [for orthography, see Article 15, Intern. Rules Zool. Nomen.], the breeding form of the American
Arctic and Subarctic.
These subspecieshave been poorly characterized in the literature because they are
not defined by sharp differences. The American form is supposed to differ from B. 1.
Zagopusby its smaller average size, by its polymorphism, and by the more variegatedcolors of the dorsal plumage. B. 1. Zagopusis said to be characterized by a more uniformly brown dorsal coloration, by a 1ightLcoloredhead relative to the back, and by theabsence of a melanistic phase. The Siberian form is the most distinctive subspeciesof
the group. It differs from both lagopus and from sanctijohannis by its extremely palecoloration and, supposedly, by its larger average size. Finally, according to Dementiev( 193 1: 54-55)) who described kamtschatkensis, and Dementiev et al. ( 195 1: 3 15), who
attempt to substantiate it, the Kamchatkan form differs from the Siberian form by its
darker and more variegated colors (similar in this respect to sanctijohhanis) and by
its still larger average size.
Some Russian authors, notably Stegmann ( 193 7 : 2 16)) consider kamtschatkensis
only a variant of the Siberian form. He has shown that specimens from Kamchatka
fall within the range of variability of the Siberian population and further that threeof them that he examined are indistinguishable in color from the pale form. Also hepointed out that Dementiev’s type specimen is a juvenile and that juveniles are poorly
separable even in large series. In further support of Stegmann’s contention, I can mention three specimens examined in the present study, a nesting pair from the lower Lena
River and an adult male from Cape Bolshaja, which fit the description of kamtschatkensis but which were taken deep within the breeding range of the Siberian subspecies
as given by Dementiev et al. (1951:312-313).
.
A number of writers have noted differences between the Rough-legged Hawks ofnorthwestern Alaska and those from the region of Hudson Bay, Labrador, and Newc 313 1

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Vol. 57

foundland. Nelson (1887:142) pointed out that many Alaskan specimens are quite
light, particularly on the head, and on this basis he referred northwestern Alaskan
birds to the typical race Zagopus.He was incorrect, however, in stating that the “melanistic phase” does not occur in this region, as were Macoun and Macoun (1909:61).
Friedmann (1934: 246) later referred one of Nelson’s specimens from St. Michael’s,
Alaska, to the Siberian form pallidus and discussedseveral intergrades from that area,suggesting that in the region of intergradation between paltidus and sanctijohannis thedifference in size remains more constant than that of coloration. In 1938 (pp. 29O-291),
he discussed the birds of northwestern Alaska again, this time referring them to theform kamtschatkensis, which he considered to be distinct from pallidus. Bailey (1942:
305306) reported an additional number-of specimens from northwestern Alaska as
pallidus and gave one definite breeding record from the Seward Peninsula; later (1948:
182) he mentioned one other breeding record based on a collected specimen. He wasinclined to doubt, rightly as I hope to prove, that Asiatic and American birds could be
distinguished on the basis of size, and he also stated with clear insight that “while manyof the rough-legs of northwestern and Arctic Alaska are no doubt colored like Siberian
birds, the converse would probably be true-that many from the Asiatic side would becalled s. johunnis if compared with birds of our own continent” (lot. cit.).
In the twentieth supplement to the A.O.U. Check-list (Wetmore et al., 1945)) Buteo
lagopus pallidus (Menzbier) was recognized as an additional subspecies for North
America; in the twenty-third supplement (Wetmore et al., 1948), the name was changed
to Buteo lagopus kamtschatkensisDementiev, “because of the prior Buteo pallidus Lesson, Traite d’Orn.,livr. 2, May, 1830, p. 82, for another species.” Friedmann (1950:336)
has apparently recently followed this decision because he has placed pallidus in thesynonymy of kamtschatkensisand the breeding range given is that of the Siberian form,
including Kamchatka and northwestern Alaska.
It is apparent that there are conflicts in the names currently applied to subspeciesof Buteo lagopus. Dementiev et al. (1951:312) have also recognized the priority of
Buteo pallidus Lesson, and while retaining kamtschatkensis as a distinct subspecies,
they have renamed Buteo lagopus pallidus (Menzbier) as Buteo lagopus menzbieri.
Whether menzbieri or kamtschatkensissucceedspallidus ils the name for the pale-colored
Siberian form appears to rest on whether or not kamtschatkensis is a valid subspecies.
Field work.-During
the summer of 1952, I was one of the several persons assigned
to study the birds of the Colville River, northern Alaska, under contract between the
Office of Naval Research and the University of Alaska-Contract Nonr-768 (OO), Project NR 307 019, Dr. B’rina Kessel, principal investigator. One of the purposes of thisinvestigation was to gather critical material for taxonomic work. Field observations andcollecting were carried out from June 4 to August 19, 1952. During that time 48 nestingpairs of Rough-legged Hawks were studied, a seriesof 8 adults and’3 juveniles was collected, and a number of fledglings was banded, one of which was recovered the following
November in southern Alberta. Particular attention was paid to plumages and molt inthe living birds, which could be observed in detail at their nests.
While in the field, I became aware of the great variability of size and plumage inthis population. It was particularly striking to find extremely melanistic individuals
mated with very pale ones, and thus I began to question the correctnessof considering
the northwestern-Alaskan hawks a New World contingent of the Asiatic subspecies.The
subsequent work with museum material was, therefore, an outgrowth of impressions
received on the breeding grounds in Alaska.
Acknowledgements.-This study was carried out under the direction of Thomas R.

Nov., 1955

VARIATION

OF ROUGH-LEGGED

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315

Howell of the Department of Zoology of the University of California, Los Angeles, towhom I am much indebted for help and advice throughout the course of the work. I
would like to thank the following people for the loan of material under their care: Herbert Friedmann, United States National Museum (USNM) ; John T. Zimmer, American
Museum of Natural History (AMNH) ; W. Earl Godfrey, Canadian National Museum(CNM) ; L. L. Snyder, Royal Ontario Museum of Zoology and Paleontology (ROMZ) ;Brina Kessel, University of Alaska (U of A) ; A. L. Rand, Chicago Natural History
Museum (CNHM) ; H. K. Gloyd, Chicago Academy of Sciences (Chic. Acad. Sci.) ;J. C. Greenway, Jr., Museum of Comparative Zoology (MCZ) ; Robert W. Storer, University of Michigan Museum of Zoology (UMMZ) ; Alfred M. Bailey, Denver Museumof Natural History (DMNH) ; K. C. Parkes and W. E. Clyde Todd, Carnegie Museum(CM). Alden H. Miller very kindly provided me with study facilities at the Museumof Vertebrate Zoology (MVZ) , and Robert T. Orr gave me accessto the seriesof Buteo
lagopus from Manchuria collected by A. S. Loukashkin and now deposited in the California Academy of Sciences (CAS) . E. A. Simms of the British Museum (Natural History) was good enough to send me plumage descriptions of some specimens under his
care. I am further indebted to L. L. Snyder for a discussion of sexual dimorphism in
Buteo Zagopus.Special acknowledgement is due my wife, Renetta Mae Cade, for invaluable assistance in the preparation of figures and tables.
MATERIALAND

METHODSThis study is based on an examination of 179 specimens, of which 73 were adultmales, 65 adult females, and 41 juveniles. In so far as possible only material taken during the breeding season (May-August) has been used, the assumption being that suchspecimens were taken on, or reasonably near, their actual breeding grounds. Reilly(1954: 162) has recently offered some justifiable objections to such an assumption, but
for studies of variation requiring many specimens there seemsto be no other recourse.
The exception to this limitation is the sample from Asia, which consists of 10 summer&and 40 winter-taken individuals.
The geographic distribution of the specimens examined from North America isshown in figure 1. This distribution almost completely covers the known nearctic breeding range. Specimens were assigned to the following geographic groups for comparative
study: (1) Asia; (2) Aleutian Islands and southwestern Alaska; (3) northwestern andnorthern Alaska; (4) western Canada; and (5) eastern Canada. The latter two groups
were arbitrarily separated at 80 degreeswest longitude. These groups can be consideredmore or less natural geographic units. The Bering Sea and the Arctic Ocean provide
a hiatus in the breeding range between Asia and Alaska. In western Alaska just northof the base of the Alaska Peninsula there is the broad Yukon-Kuskokwim delta country,
which because of its low, flat terrain is little suited to the nesting requirements of theRough-legged Hawk and which, therefore, in terms of relative numbers acts as anotherhiatus between the abundant breeding populations of southwestern and northwestern
Alaska. A similar situation is believed to exist in the region of the Mackenzie River delta,
which separates the northern Alaskan hawks from those breeding east of the Mackenzie,
In the eastern Canadian arctic the range is again disrupted by Hudson Bay, the eastwest continuity of the population being maintained by only a few, scattered pairs nest&
ing on islands.
Three measurements were taken: chord of the wing, diagonal of the tarsus, and chordof the entire culmen. The instructions of Baldwin, Oberholser, and Worley (193 1: 13,76, 107) were followed. Measurements were read to the nearest millimeter only.

316

THE

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Fig. 1. Geographic distribution of specimens of Buteo Zagopus from North America.
Records include both adults and juveniles taken in the breeding season. Note peripheral nature of distribution.

The plumage traits employed in this study are those already used in descriptions of
this species (see Friedmann, 1950; Witherby et al., 1939). In addition, some other characters that proved to be important after an initial series had been examined were used.
About a dozen plumage characters have been studied; these are described in table 4.
The original sex determination of all specimens has been accepted, although in a
number of casesone may be practically certain that the specimen was incorrectly sexed.
This increases the inaccuracy of the frequency distribution of the characters and tends
to overemphasize the degree of overlap in traits between the sexes, but it has been
done to avoid the criticism of biasing the data in favor of a particular result. On theother hand, in several instances where specimensthat were recorded as adults were obviously birds of the year, a situation particularly easy to detect in males, I have felt justified in changing their status in the presentation of the data. Unsexed specimens,constituting less than 5 per cent of the total sample, were placed according to their appearance.
Formulae for all statistical calculations were taken from Simpson and Roe (1939))
except as indicated in the footnote of table 3. Chi-square values were calculated directlyfrom the raw data.
MOLT

AND AGE CHARACTERS

Rough-legged Hawks undergo a single annual molt, which takes place mainly dur.
ing the breeding season. Hence, specimens from the breeding range have only partly
acquired fresh plumage, and this is the major disadvantage in studying a sample ofbreeding birds of this species.There is no postpuvenal molt, birds of the year retaining
the feathers they acquire in the nest until the following spring, when they begin molting
into a plumage that is essentially adult in character.
Bent ( 193 7 : 2 73 ) has suggestedthat there is a second-year plumage somewhat intermediate between the juvenal and adult plumages, but I think his statements regarding
the distinctions of these three are confusing and they are at variance with my own findings. After having examined a large series, however, I am left with the impression that
there may be some progressivechangesof the plumage with age. The older males in particular appear to become less pigmented, with fewer and smaller breast markings andNov., 1955

VARIATION

OF ROUGH-LEGGED

HAWKS

317

Fig. 2. Ventral and dorsal aspectsof adult male Rough-leggedHawks. Upper row, from left
to right: MCZ 92669,Altai Mountains; DMNH 22866,Wainwright, Alaska; U of A 614,
Colville River, Alaska; AMNH 442641,Melville Peninsula,Canada.Lower row: CNHM
102133, Manchuria; CNM 16152, Labrador; USNM 299396, Izembek Bay, Alaska;
DMNH 25277,Meade River, Alaska.

(fig. 2) the ventral surface
is almost immaculate. But certainly the major change in plumage occurs with the firstmolt.
According to Bent (ZOC.cit.), the annual molt occurs between April and November.
From my sample it appears that most of the adults complete the greater part of theirmolt, especially that of the flight feathers, in June, July, August, and September. Thereis also an indication that the first annual molt begins earlier in the year than do subsequent ones; but few birds of the year survive to the time of molt, and hence there are
few immature specimens for comparison.
In so far as possible detailed records were kept on the molting of Rough-legged
Hawks observed along the Colville River in 1952. With practice it was possible to determine with considerable accuracy the condition of the remiges and rectrices of birds as
they soared about their nests. Notes were also taken on the molt of freshly collected
specimens. The primaries of this speciesare normally molted from the fourth outward,
the innermost three molting in sequence with the secondaries; and the rectrices aremolted from the central pair outward. It is therefore possible to determine the stageof molt of a particular individual by noting which feathers are missing. Table 1 summarizes data secured on the Colville River. Birds observed after July 26, the last date indi-

gradual reduction of the belly wash, until in some individuals

THE

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CONDOR

Table 1
Summary of Observations on the Molt of Buteo lagopusalong the Colville River, Alaska, in 1955
Date

Males

Females

June 17

1with right 7th primary just coming inand many new dorsal feathers.

2 with right and left 7th primaries andone central rectrix molted.

June 18

1 with right and left 7th primaries justcoming in.

1 with right 4th and 5th coming in andleft 4th coming in, left 5th complete.

June 20

2 with no visible molt.

1 with left 7th molted.
1 with right and left 7th primaries molted.

June 22

2 with right 7th primary molted.
1 with right and left 7th primaries molted.

3 with right 6th and 7th and left 7th primaries molted.
1 with right and left 7th primaries molted.
with right and left 6th primaries andone central rectrix molted.
with right and left 3rd or 4th primaries
molted ; 5-7th ones growing in.

June 25

1 with left 6th half in and right 7th half in;right central rectrix just coming in.

1 with 3rd or 4th primaries molted.

June 27

3 with no visible molt.
2 with right and left 7th primaries comingin.
1 with left 7th primary molted.
1 with right and left 3rd and 4th primaries molted (sex doubtful).
1 with 6th and 7th primaries coming in.

July 5

1 with right and left 2nd or 3rd primaries

2 with right and left 2nd or 3rd primaries
molted.
1 with left 7th molted.
2 with right and left 7th primaries molted.
1 with right and left 6th primaries molted;
7th ones coming in .
1 with 6th and 7th primaries and one central rectrix coming in.
1 with molted secondaries.

July 6

1 with right and left 3rd or 4th primaries

molted.
1 with right 5th primary half in.

molted.
July 7

1 with right and left 2nd or 3rd primaries

1 with secondaries molted on both wings.

molted.
July 8July lo

1 with right and left 2nd or 3rd primaries

1 with all fresh primaries.

molted.
1 with right and left 3rd or 4th primaries
molted.

1 with a few molted secondaries.

1 with right and left 5th primaries justcoming in.

July 26

1 with no visible molt.

1 with right and left 3rd primaries justcoming in ; one molted secondary on left
and both central rectrices coming in.
1 with all new remiges and central rectrices molted.
1 with no visible molt.

cated in the table, were so far progressedin the molt of flight feathers that determinations
were no longer possible.
In addition to showing that the main molting activity occurred in June and July inthat population that year, the data also indicate that’the females molt ahead of themales. This was even more evident when the male and female of a given pair were compred. A similar sexual difference in timing was observed for Peregrine Falcons (Falco
peregrinus) breeding in the same region, and Beebe ( 1953 : 11) has recently recorded
the same thing for this specieson the islands off the co&t of British Columbia. This may
therefore be a widespread phenomenon among falconiform species,and it could be cor-

Nov., 1955

VARIATION

OF ROUGH-LEGGED

HAWKS

319

related with the usual social arrangement among mated hawks in which the male does
most of the hunting, and hence most of the flying, during the incubating and nestling
period, while the female remains close to the nest. After the young have fledged she thenassumesa more active role as food provider.
There is no sexual dimorphism of the first-year plumage, and individual and geographic variation is much less than among adults (see table 5). Friedmann ( 1950:327)
gives a concise and accurate description of the immature plumage. I will therefore restrict my statements to a consideration of the several traits studied in detail by me as
they occur in the first-year bird.
The tail pattern is nearly always type D (table 4), occasionally type C, especiallyin the Asiatic sample, and occasionally type E in the American sample only; it is nevertype A or B. The belly pattern is most frequently type E, occasionally type D, especiallyin the Aciatic sample, and occasionally type F in the American sample only; it is nevertype A, B, or C.
The marginal coverts of the juvenal wing are most frequently type I ; occasionally,
especially in the Asiatic sample, they are type III; less frequently they are type II ;also infrequently, in the American sample only, they are type IV.
-Juvenal heads are more frequently type A than B, and the primaries are frequently
type I, II, or III. These traits occur more frequently in juveniles of both continents
than in their respective adult populations.
The shaft marks of individual feathers on juvenal breasts are usually wide andmedium brown in color; but between the Asiatic and American samples there is almost
a complete differentiation in the color of the feather margins. One hundred per cent of
the Asiatic sample has whitish or creamy margins, and over 90 per cent of the American
sample has buffy or reddish ones. The same difference holds for the feathers of the back.
The adult plumage of Buteo lugopus is extremely variable, being exceeded in thisrespect
only one other North American congener, Buteo jamaicensis. Friedmann’s(1950:3 Y S-327) description, while accurate in its terms, suffers from being a composite
of a series rather than a delineation of the range of adult variability. Again I shall discuss only those characters that I have subjected to quantitative study.
The ,adult tail pattern ranges through all types (see table 4e), although it is mostfrequently type A, B, or C, and much less frequently type D than in the case of thejuvenal tail. The adult belly pattern also ranges through all types (table 4f), but it isless frequently type E than is the juvenal belly.
The marginal coverts of the adult wing are usually type I or II; occasionally in the
Asiatic sample only they are type III, and occasionally in the American sample only
they are type IV. The adult head is less frequently type A than is the juvenal head, andthe adult primaries are less frequently type I or II than are those of the juvenal.
Adult hawks range through all the ventral and dorsal types of tables 4c and d.

INDIVIDUAL

VARI~ILITY

The variability of any sexually segregated sample of Buteo lagopus is great. For themensural characters this is best seen by examining the statistics of table 2. An idea ofthe variability in plumage can be gained by looking at figures 2 and 3. Specimens
DMNH 25277 and DMNH 22866 of figure 2 represent the observed extremes of maleplumage. The females appear to be less variable in plumage than are the males.
The basic patterns of plumage remain relatively constant from individual to individual. Most of the gross differences in plumage can be attributed to the variable

.

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CONDOR
Table 2

Measurements of Buteo Zagopus

Dimension

Wing

Population

Asia
Aleutians,
SW Alaska
NW

AlaskaW Canada
E Canada
Americanjuveniles
Asiatic
juveniles
TarsusAsia
Aleutians,
SW Alaska
NW

AlaskaW Canada
E Canada
Americanjuveniles
Asiatic
juveniles
CulmenAsia
Aleutians,
SW Alaska
NW

AlaskaW Canada
E Canada
Americanjuveniles
Asiatic
juveniles

Sex

Number of
specimens

Range

Mean withstandarderror

Standard
deviation

M
F

23
21

396-448401-456

418f3.38435k3.02

16.22
14.03

M
F
M
F
M
F
M
F

12
13
14
13
12
8
12
14

396-445
43&461
394-444403-454395-426413-459392-412416-450418k3.22445k2.52424k4.36
435f3.76
411k2.96436k4.87
406kl.71
436-tZ.59

11.16
9.11
15.29
13.6
lo.25
13.77
5.92
9.59

M
F

9
4

340-470
36a-440

422Yz8.46. ... ..
405

25.38

M
F

7
2

410-446438-452427k5.67. .. ...
445

M
F

23
20

58-72
61-75

66.0+0.70
68.6kO.89

3.36
4.02

M
F
M
F
M
F
M
F

12
12
14
13
12
7
12
14

65-71
68-78
64-73
67-77
62-72
65-75
65-71
69-75

68.3k0.55
74.9kO.80
69.820.60
71.9+0.8X
67.920.78
67.4k1.32
68.Oko.47
71.3ro.48

1.92
2.78
2.26
3.19
2.96
3.51
1.63
1.83

M
F

8
5

62-74
68-72

67.8f1.52
69.4kO.66

4.55
1.49

M
F

7
2

66-71
71-77

69.9f0.80.. .. ..
74.0

2.10
. .... .

M
F

23
21

31-40
32-38

33.7+0.40
35.6kO.37

1.94
1.69

M
F
M
F
M
F
M
F

12
12
13
12
10
7

11
14

33-38
35-39
33-38
35-39
34-3 7
35-39
32-38
34-39

34.8kO.36
36.8kO.39
35.1kO.43
36.920.38
35.1kO.27
37.0f0.53
35.OkO.49
36.8k0.39

1.25
1.37
1.55
1.19
0.84
1.41
1.65
1.46

M
F

9
5

32-38
33-38

34.120.59
35.0f0.85

I.79
1.89

M
F

5
2

34-37
37-38

35.5+0.89.... ..
37.5

2 .Ol.. ... .

Nov., 1955

VARIATION

OF ROUGH-LEGGED

HAWKS

321

amounts of melanin pigments deposited in the feather tracts. Concealed patterns areevident in even the most melanistic individuals upon close examination.
The American form of Buteo lagopus is said to be dimorphic or polymorphic. Thus
Friedmann (1950:325-327) gives separate descriptions for the “light phase” and forthe “melanistic phase,” and there are numerous other references in the literature to thetwo-phase condition of sanctijohannis. Figures 2 and 3 show a complete range of intermediate plumages between the lightest and darkest specimen. The “two phases” of theFig. 3. Ventral and dorsal aspects of adult female Rough-legged Hawks. Upper row, fromleft to right: CNHM 101222, Manchuria; USNM 230742, Alaska Peninsula; DMNH
22867, Wainwright, Alaska; CNM 20984, Baffin Land, Canada. Lower row: DMNH
22868, Wainwright, Alaska; U of A 606, Colville River, Alaska; MCZ 234081, Amak
Island, Aleutians; AMNH 269935, Port Moller, Alaska.

American Rough-legged Hawk are not discontinuous as is true of the Screech Owl(Ohs asio) and some other species,and hence it is not polymorphic in the senseof Ford
(1945), Huxley (1942:96), and others, especially Mayr (1942) and Mayr, Linsley andUsing& ( 1953 :96). The condition can best be described as a wide range of continuous
variation from a nearly uniform sooty plumage to a light brown one, the dark color
being restricted ventrally to a few small marks on an otherwise white ground and dorsally by wide whitish edges on the individual feathers. Dark birds do not occur withthe same frequency over the range of sanctijohannis, and in this respect they behave
as a polymorph variant (see Mayr and Stresemann, 1950).
The variability within any population of Buteo lagopus is so great as to preclude
accurate study of geographic variation without large series of skins. Even my samples,
which are the largest yet compared, are inadequate for anything but approximations.

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CONDORSEXUALDIMORPHISM

Sexual dimorphism is clearly evident in many characters of Buteo Zagopus,but as inevery other aspect of variation, it is not marked by sharp discontinuities, that is, allmales are not different from all females in any character studied, with one apparent
exception to be mentioned later.
Size.-Following
the rule among falconiforms, Rough-legged Hawks are sexuallydimorphic in size, the female averaging larger than the male. There is, however, a great
deal of overlap between the sexesin all mensural traits studied (table 2) except that ofbody weight. Table 3 compares the sexual dimorphism of these characters among theTable 3Comparison of Sexual Dimorphismin Mensural Characters among Sample Populationsof Bwteo lagopus
Wing

Population

Asia
Aleutian Islands and SW Alaska
Northwest AlaskaWest Canada
East Canada
C.D.

CulmenTXSUS

CD.

P.J.N.

C.D.

P.J.N.

CD.

P.J.N.

0.561
1.33
0.384

<75
<91
<fS
85
>96

0.189
1.40
0.388
0.08
1.06

<75
<92
<75
<75
>85

0.527
0.769
0.666
0.826
0.~81

<7s
>fS
<75
<80
<75

1.04

1.93

stands for coefficient of difference;
P.J.N.
stands for per cent joint nonoverlap.
Usinger, 1953:146,
for explanationand interpretationof this statistic.

See Mayr,

Linsley,

andseveral geographic samples of this study. In 9 casesout of the 15 comparisons of table 3,
the samples indicate only 75 per cent or less joint nonoverlap in size between the sexes.
Some of the differences are not even statistically significant. There is, nevertheless, someapparent geographic variation in sexual dimorphism in size that will be dealt with later.
Because the data on body weight are meager, they are summarized here. The average
weight in grams of 4 arctic Alaskan males collected on the Colville River in 1952 is 915(883-943), and the average of 4 females from the same region is 1216 (1033-1400).
Two specimenscollected by 0. J. Murie on the Old Crow River, Yukon Territory, havethe following converted metric weights: female, 1176 gm.; male, 840 gm. Dementiev
et a2. (1951:3 12) give the average of 5 males of typical Zagopusas 846 (SOO-930))
and the average of 5 females as 1151.4 (990-1280). They also give (p. 31.5) the weightof one male kamtschatkensis as 900 gm., and of 2 females as 1050 and 1209 gm. Thus,
in a total of 11 male weights and 12 female weights there is no observed overlap between the sexes,the largest male (943 gm.) and the smallest female (990 gm.), however,being separated by only 47 gm.
In view of the pronounced overlap in linear characters between the sexes,it is surprising not to find a corresponding amount of overlap in their weights. This circumstance suggeststhat none of the linear characters used in this study are accurate indicesof over-all size in Buteo lagopus.
Plumage.--An entirely unsuspected finding of this study is the fact that the sexesof Buteo lagopus are partly dimorphic in plumage. I have found no mention of this in
the literature, all authorities stating emphatically that the sexesare alike in this respect.
Plate 65 (upper left fig.) of the “British Handbook” (Witherby et al., 1939:33),
painted by the late G. E. Lodge, correctly depicts some of the differences between theadult male and female, as well as differences between them and the juvenile, but nomention is made in the text of differences between the sexes.
Among the adult plumage traits previously mentioned, the following are in varying
degrees sexually dimorphic: (1) tail pattern, (2) belly pattern, (3) marginal coverts

VARIATION

Nov., 1955

OF ROUGH-LEGGED

HAWKS

323

Table 4
Arbitrary
Character(a) Head types(crown and occiput)

Description

A

Head distinctly lighter than rest of dorsal aspect; shaft
streaks of feathers narrow.
Head not distinctly lighter than rest of dorsal aspect; shaft
streaks of feathers wide.

B(b) Outer primaries(see fig. 5)Classifications of Plumage Characters

Designation

I
II
III
IV

Outer 4 to 7 primaries with extensive (30-50 mm.) liiht area
on outer webs distal to their coverts.
Outer 3-S primaries with moderate (lo-20 mm.) light area
on outer webs distal to the ends of their coverts.
Outer l-3 primaries with faint light area on outer webs.
All primaries lacking any light area on outer webs distal tothe ends of their coverts.

(c) Breast

A
B
C
D
I
II
a
b
ab

Light brown marks on feathers.
Medium brown marks on feathers.
Dark brown marks on feathers.
Uniformly dark feathers.
Shaft marks wide.
Shaft marks narrow.
Feather edgings whitish.
Feather edgings reddish.
Feather edgings reddish and whitish.

(d) Dorsum

A
B
C
D
I

Light brown marks on feathers.
Medium brown marks on feathers.
Dark brown marks on feathers.
Uniformly dark feathers.
Shaft marks reduced; margins wide and intruding
shaft.
Shaft marks wide; margins narrow.
Feather edgings whitish.
Feather edgings reddish.
Feather edgings reddish and whitish.

II
:
ab
A

(e) Tail

B
C
D
E
(f)

Belly

(g) Marginal

towardA dark subterminal band and 3 or more complete or incomplete ones proximal to it. Example, AMNH 442641 of fig. 2.
A dark subterminal band and 1 or 2 complete or incomplete
ones proximal to it. Example, CNHM 102133 of fig. 2.
A distal “field” of brown with a faint darker subterminalband within it. Example, DMNH 23644 of fig. 6.’
A uniform distal field of brown ; no subterminal band visible.
Example, Chic. Acad. Sci. 4211 of fig. 6.
Tail uniformly dark distal to the coverts (melanistic).
Example, DMNH 25277 of fig. 2.
Immaculate to very slight barring. Example, U of A 614
of fig. 2.
Moderate barring. Example, AMNH 442641 of fig. 2.
Heavy barring. Example, CNM 16152 of fig. 2.
Lateral blotching. Example, CNM 20984 of Sg. 3.
Continuous band. Example, DMNH 22867 of fig. 3.
Uniformly dark venter. Example, MCZ 234081 of fig. 3.

coverts

I
II
III
IV

Feathers with longitudinal shaft marks.
Feathers with subterminal wedge-shaped marks.
Feathers uniformly light (creamy) colored.
Feathers uniformly dark (reddish to sooty) colored.

324

THE

CONDOR

Vol. 57of wing, (4) relative lightness of head to back, and (5) pattern of the dorsal feathers.
The dimorphism is most pronounced in the first two categories.
One finds that in the tail classesA through D of table 4e (class E is excluded be&cause of its low and nearly equal frequency in the two sexes), most males are in classes
A and B, and most females are in C and D. There is overlap into nearly all classesby
some specimens of both sexes (see table S), but when the frequencies of the sexesare
grouped into two categories, A-B and C-D, and tested by the chi-square method, thedifference between all adult males and all adult females is significant beyond the 1 percent level of confidence (table 6a). Figure 4 shows the “typical” male and female typesof tail compared with the usual juvenal (non-dimorphic) type.
Similarly, in the belly classesof table 4f (F, the melanistic class, is excluded) mostmales are in classesA through C and most females in C through E. The C class is theA

B

Fig. 4. Tail patterns of Buteo lugopus. A, juvenal (little variation)
variation ; C, adult female (much variation).

C; B, adult male (muchprincipal area of overlap between the sexes, but this is at least partly a result of thearbitrariness of the classification and of the difficulty of deciding where to place borderline specimens between type B and C and between type C and D (see table 5). A significant difference beyond the 1 per cent level is again demonstrated (table 6h).
Among the classes of the marginal coverts of the wing, a large number of males
occurs in class I and a large number of the females in class II. They are about equallydistributed in classesIII and IV. Testing male and female frequencies in the first twoclassesalso yields a significant difference beyond the 1 per cent level (table 6aa and hh) .
The ratio of type A to type B heads is greater in males than in females, and thisdifference proves to be highly significant (table 6s).
Finally, there are some subtle differences between males and females in the patterns
and colors of the dorsum, but table 6gg reveals no statistical difference between thesexes for one dorsal character, and the classification of the others did not lend itself
to statistical treatment. In general, it can be said that the male dorsum appears morevariegated than the female, with much more white, brown, red, and often slate, showing
in the individual feathers. The females appear more uniformly brown, with little whiteor reddish showing except on the margins, and seldom any slate. The individual feathersof males are frequently barred or blotched transversely in three or more rows with darkmarks interspersed with reds, browns, and grays, whereas female feathers are seldommarked in this manner but usually show wide longitudinal shaft marks of brown, occasionally with indications of marginal blotching of the same color.
It is gratifying to have independent corroboration of <thesefindings. L. L. Snyder,working with a much smaller series of Canadian specimens,but fortunately drawn from

Nov., 1955

VARIATION

OF ROUGH-LEGGED

HAWKS

325

an extremely homogeneous population, has found some of these same differences between the sexes.
A specimen that is masculine or feminine in one trait is usually masculine or feminine in the other sexually dimorphic traits also, but this is not always true. A few
specimens combine typically male and female characters. Tail and belly patterns concord most completely and are the most reliable indicators of sex.
In a total of 137 adult specimens, only about 11 per cent are sexually heterogenous
in tail and belly patterns. A closer discrimination of plumage types would probably have
reduced this figure more. The differences between tail types B and C and between belly
types C a.nd D are quite arbitrarily defined.
About 9 per cent of the adult specimenshave tail and belly types of the sex opposite
to their recorded sex. I was at first tempted to explain this solely as the result of misidentification of sex on the part of earlier collectors. Since such a small percentage ofthe specimensis involved, this is not an unreasonable assumption. One of my own specimens, however, shows a sex reversal in plumage, so while incorrect sexing probably
accounts for part of the 9 per cent, some biological factor must also be concerned.
GEOGRAPHICVARIATION

Wing length.-The mean wing length of adult males ranges from 424 mm. in northL
western Alaska to 406 mm. in eastern Canada (table 2), and statistics derived fromthese two non-adjacent samples show about 80 per cent joint nonoverlap. Between any
two adjacent samples, however, no joint nonoverlap approaches 75 per cent. There isan indication of a cline from short wings in eastern Canada to longer wings in northwestern Alaska and Asia, but the differences in the means of the several populations are
scarcely significant statistically. There is certainly no difference among the northwestern Alaskan, Aleutian-southwestern Alaskan, and Asiatic populations.
The mean wing length of adult females ranges from 435 mm. in Asia and northwestern Alaska to 445 mm. in the Aleutians and southwestern Alaska. In this case theextreme populations are adjacent, but they show less than 7.5per cent joint nonoverlap.
The Aleutian-southwestern Alaskan females do, however, show a large divergence fromthe other samples, which are remarkably similar, differing by only one mm. in theirmeans.
Diagonal of tarsus.-The mean tarsal length of adult males ranges from 66 mm. in
Asia to 69.8 mm. in northwestern Alaska, and these two adjacent populations shownearly 75 per cent joint nonoverlap. Other adjacent populations of this series are statistically indistinguishable from each other, and no cline is indicated.
The mean tarsal length of adult females ranges from 68.6 mm. in Asia to 74.9 mm.
in the Aleutians and southwestern Alaska, and these populations are separated by morethan 82 per cent joint nonoverlap. Also the eastern Canadian and western Canadian
populations show a joint nonoverlap of more than 75 per cent, and the northwestern
Alaskan and western Canadian ones almost 7.5 per cent; but the pattern of variation isirregular. Northwestern Alaskan and Aleutian-southwestern Alaskan populations are
not statistically different in this trait.
Length of c&men.-The mean culmen length of adult males ranges from 33.7 mm.
in Asia to 35.1 mm. in northwestern Alaska and western Canada, but none of the differences is statistically significant, and no particular trend is indicated.
The mean culmen length of adult females ranges from 35.6 mm. in Asia to 37 mm.
in western Canada, but this difference is not significant. The Asiatic sample shows thegreatest deviation from the others in both males and females.

Table 5
Frequency Distribution of Plumage Characters among Sample Populations of Buteo ZagopuF
Web ofprimaries

PopulationA

DlXSUlll

Breast

Tail

b ab

I

II

A

B

C

D

a

b

ab

AB

0 23
020

0
0

0
1

16
19

7 10
2 14

9
6

4
1

0 17
0 15

0
0

6
6

216

3
2

6
5

3
5

1
0

3805620210
110
0 4

7

2

0’ 7

2

1 14

0

0

7 7
2 10

2
2

1
1

0
0

12 0 0 8610701700

4

011

0

1

6606601011
250701016

11
12

8
7

1
2

9 .O
9 0

2
3

110
210

B

I

II

III

IV

I

II

A

B

C

D

5

9
8

4
7

4
3

6
3

17 6 9 13
1011147

1
0

SW Alaska
Males
Females

1 12
9 4

0
0

0
0

1 12
211

10
10

NW Alaska
Males
Females

2 12
5 8

0
0

1
0

2 12
310

12 2 2 109337211110

West Canada
Males
Females111
02600170012

East Canada
Males
Females

0 12
8 5

0
0

0 12
014

Asiatic juv.
Males
Females

705020202000521600700
200200200

American juv.
Males
Females

6511356222228416421110
12 3 0 11acoverts

Belly

CDE

ABCDEFI

II

III IV

49253058

8

0042780

9

4

7

0

2 10
6 5

0
0

1
2

Asia
Males
Females

815

1s

0
0

0
1

0
0

0
1

4.6
5 5

0
0

3
5

2 10
0 10

0
0

2
5

9
7

10
2 0

1

4

4
8

4

2
0

5
0

3 1
3 10

1
0

0142320 0 1111

0 14
2 10

2
1

8
1

1
5

3
6

1
0

443211000571

2 10
5700

0

1

7 5 0
21500

0

0

354000020321

011
1 6

0
0

1
1

8
0

3 1
011

0
2

0
1

146001003812

011
5 8

0
0

1
1

0
0

0
0

2
0

4
2

0
0

000430000020

6010
1 0

1

0

0
0

0
0

212
013

0
3

0
0

7
7

2
7

1
1

011
111

343400403
200200200

4

1

1 15

0

7607520120
11 4 0 10

5

1

0 14

1

10
8 0

0
0

0
0

211
2 13

0

1
1

1
1

Nov., 1955

VARIATION

OF ROUGH-LEGGED

HAWKS

327

Fig. 5. Variation in measurementsamong sample populations of Buteo lag0pu.sexpressed
as Dice squares.A, chord of wing; B, diagonal of tarsus; C, chord of entire culmen;
seeHubbs and Perlmutter (1942) for discussionof use of Dice squares.

An important consideration emerges from a study of these measurements. Although
any given population usually shows a wide range of variability, when geographic sambpies in a sexually segregated series are compared, the samples show remarkably little
difference in their means. In other words, individual variability is great, but geographic
variation is not. In every case standard errors of the means in the male series and in thefemale series overlap with one or more others. It appears impossible, therefore, to define
geographic populations on the basis of measurements with the number of specimens
now available by employing any accepted criterion of difference used to distinguishsubspecies (Mayr, Linsley, and Usinger, 1953: 143).
Although standard deviations are influenced by variables other than true biologicalones, such as size of sample and non-random sampling, it is interesting to compare these

328

THE

CONDOR

Vol. 51

Table 6Frequency Comparisons of Plumage Traits between Samples of Buteo lugopus
Breakd;wwn&f

COIIlpXiSOIlS

TAIL

(a)

All adult males
All adult females(b) * Asiatic juveniles
American juveniles

A-B

C-D

58

10

14
2
2

59
6
25

D

57

<.Ol

5.4

<.02

(0)

(p)
(c) * All adult females
All juveniles(d) * All adult Canadianfemales
All adult Alaskan
females(e) All adult Alaskan
females
All adult Asiatic
females(f)*(g)

All adult Canadianmales
All adult Alaskanmales
All adult Alaskanmales.
All adult Asiatic
males

C

D

33
4

26
31

16

<.Ol

All adult males
All adult females( i ) * All adult Canadianmales
All adult Alaskan
males(j) All adult Alaskanmales
All adult Asiaticmales(k) * All adult Canadianfemales
All adult Alaskan
females(1) * All adult Alaskan
females
All adult Asiatic
females(m) Asiatic juveniles
American juveniles(n) All adult females
All juveniles

11

6

20

6

20

7

16

2

8

16

8

16

9

8

A

B

10

1

<.Ol

->.1

(q) * All adult Canadianfemales
All adult Alaskan
females(r) All adult Alaskan
females
All adult Asiatic
femalesP
KilUS

All others

11

E

3.7

>.05

1

>.l

14
All others

3

19

12

<.Ol

18

8

18

8

4.5

<.05

a

13

62
28
0
7
28

22

<.Ol

23
24
24
_.._
11
12
12
5
11
.._.

1.0 >.I

HEAD
1.5

8

5.44 < .02

A

All adult males
12
All adult females
39(t) * Asiatic juveniles
9
American juveniles
12(u) All adult females
39
All juveniles(v) * All Canadian males ‘-;
All Alaskan males
3(w) * All Alaskan males
3
All Asiatic males..._(x) All Canadian females 10
All Alaskan females 14(y) All Alaskan females 14
All Asiatic females
15(z) All Canadian females 10
All Asiatic females
....
(s)

4

13

4

13

3

20

1

>.l

BELLY

(h)

All adult Canadianfemales
All adult Alaskan
females
All adult Alaskan
females
All adult Asiatic
females

Chi;;gre

A-C

D-E

54
12

16
53

49

23

0

5.6

16

8

16

8

15

8

1

<.Ol

<.02

(aa)

14

1

24

1

24

6

15

D

E

4
4
24
8

5
24
29
29

2.8

1.9 >.l
2.3

2.6

<.l

<.l

1.0 >.l
2.1 <.I
3.2

>.05

<.Ol

>.1
MARGINAL

5

B

<.l

3.64 >.05

2.05 <.I
5.24 <.02

PRIMARY

COVERTSAll adult males
All adult females(bb)
All juveniles
All adult females(cc)All juveniles
All adult males(dd) * All adult Canadianmales
All adult Alaskan
males(ee) * All adult Alaskanmales
All adult Asiatic
males

I

Allothers

9
26
21
30
21
50

62
41
14
37
14
21

13

2

>.l

0

24

2

>.l

.4

22

4

22

1

>.l

5

16

5.4 <.05

Nov., 1955

Comparisons

(ff)
. ,

VARIATION
Breakdoe,“,’All Canadian6
femalesAll Alaskan
11
females
All adult Alaskan
11
females
All adult Asiatic
9
females

(gg)

II

50
All adult males
All adult females 30(ii)*
All adult Canadian22
males
All adult Alaskan
20
males
(jj)
All adult Alaskan
20
males
All adult Asiatic8males(kk) All Canadian14
femalesAll Alaskanfemales 12(11)* All adult Alaskan
12
females
All adult Asiatic
4
females(hh)III(mm) *All adult American0males
All adult Asiatic8
males(nn) All adult American
females0
All adult Asiatic
females7l

OF ROUGH-LEGGED&c&areComparisonsPvalues329

HAWKS
Breakd;zf

Chi-square P
values values

rv All others
16

2

>.l

14

2

>.l

14

1

>.I

11
All others
21
9
37
2

3

<.I

7

<.Ol

2

*7

46

0

21

4

43

0

20

I

All adult males‘ 33
All adult females 24
All adult females 24(rr)
All juveniles
22(ss)* All east Canadian
adults3
All west Canadian
adults(tt)
All Alaskan adults 1:
All Asiatic adults 35
(qq)

13
8
13

4

1

>.I

1

>.I

38
39
39
14

1

>.l

1

>.l

20

2.3 <.l

3’:
9

26

<.Ol

29

<.Ol

2

>.l

18

<.Ol

2

>.l

DORSALEDGINGS

<.Ol

6
6

(00) * All adult Americanmales
All adult Asiatic
males(pp) * All adult American
females
All adult Asiatic
females>.l

II

DORSALCOLORS
A-B

All east Canadian
adults7
All west Canadian
adults13(vv) All west Canadian
adults13
All NW Alaskan
adults22(ww) All NW Alaskan
adults22
All Aleutian-SW
Alaskan adults
9(xx) All NW Alaskan
adults22
All Asiatic adults 39

C-D

(uu)

13

2

>.I

16
All others
50

17

c.01

13
47
13

14

<.Ol

19
7
7
6
6
17
6
5

Yates adjustment applied (see, for example, Simpson and Roe, 1939:296-300).

statistics in table 2. One is impressed particularly

by the homogeneity of the eastern

Canadian but also of the western Canadian (excluding the small female sample) andof the Aleutian-southwestern Alaskan populations as compared to the extremely variable Asiatic and northwestern Alaskan ones.
It should be pointed out that the Asiatic sample cannot strictly be compared withthe other samples in this respect. It is not as seasonally homogeneous,being composedof both summer- and winter-taken specimens, and it was drawn from a much larger
geographic area than were any of the four North American samples.
Still I feel that the relatively greater variability which is indicated for the populations of the Bering Sea region, excluding the Aleutians, is biologically real, especiallysince it is associated with a correspondingly great variability of the plumage charactersin these populations.

330

THE

CONDOR

Vol. 57

Tail pattern.-In
the adult male series, type A tails range from about 66 per centin eastern Canada to less than 10 per cent in Asia. This trend is unbroken in the intermediate populations, with the sharpest change occurring between western Canada andnorthwestern Alaska. The type B tail ranges from 70 per cent in Asia to about 25 percent in eastern Canada, showing a trend opposite to that of type A tails. Again the major
disjunction occurs between western Canada and northwestern Alaska. The difference infrequency of types A and B between all Alaskan males and all Canadian males is statis
tically significant (table 6f), but it is not between Alaskan and Asiatic males (table 6g).
It will be remembered that both these types of tail pattern are masculine characters.
When they are grouped together, consistently high percentages result: 80 per cent in
Asia; 72 per cent in northwestern Alaska; 60 per cent in the Aleutians and southwestern
Alaska; 100 per cent in western Canada; 92 per cent in eastern Canada.
The low figure for the Aleutian-southwestern Alaskan males is partly the result of
the greater number of melanistic individuals in that population, but the feminine types
C and D still constitute 33 per cent, the highest figure for any male sample. The otherfigures are: 20 per cent in Asia, 28 per cent in northwestern Alaska, 0 per cent in western Canada, and 8 per cent in eastern Canada. Thus, tails of adult Canadian males
differ in two respects from those of adult Alaskan males-they have a much higher percentage of extreme masculine types and a much lower percentage of feminine types(C and D) . Alaskan males do not differ in these respects from Asiatic ones.
In the adult female series, the “juvenal” type D tail ranges from about 78 per centin the Aleutians and southwestern Alaska to 0 per cent in western Canada, but no cline
is indicated for this trait. The greatest difference in frequency occurs between western
Canada and northwestern Alaska. The “typical female” type C tail ranges from 80 per
cent in eastern Canada to about 23 per cent in the Aleutians and southwestern Alaska.
The difference in frequency of these two types between all Canadian females and all
Alaskan females is statistically significant (table 6d), but it is not between the Alaskan
and Asiatic samples (table 6e).
It is relevant to mention here that the difference in frequency of types D and C
between all adult females and all juveniles is also statistically significant at the 1 percent level of confidence (table 6c), the juveniles having a higher frequency of type D.
The difference between American and Asiatic juveniles is significant at the 2 per centlevel of confidence (table 6b), Asiatic juveniles having a higher frequency of type C.
In adult females type A and B tails range from 37 per cent in western Canada to
0 per cent in the Aleutians and southwestern Alaska and in eastern Canada. They constitute 22 per cent of the Asiatic sample, and about 16 per cent of the northwestern
Alaskan sample. The high percentage for western Canada is probably over-emphasized
by the small sample and also by the arbitrariness of the classification, since there was anumber of specimensin this sample that were intermediate between the masculine types
and type C. None of the differences in frequency of these types is significant.
Belly pattern.-In
the adult male series, the type A belly ranges from 0 per centin the Aleutians and southwestern Alaska to about 28 per cent in northwestern Alaska.
No cline is indicated. The type B belly ranges from about 41 per cent in western Canada
to only 9 per cent in the Aleutians and southwestern Alaska; again there is no cline.
The type C belly ranges from 50 per cent in eastern Canada to about 9 per cent in Asia,and in this case there is an unbroken east-west cline. The type D belly ranges from0 per cent in both Canadian samples to about 22 per cent in Asia, and there is again an
east-west trend. The type E belly ranges from 0 per cent in both Canadian samples toabout 25 per cent in the Aleutians and southwestern Alaska and 22 per cent in Asia;

Nov., 1955

VARIATIONO‘ F

ROUGH-LEGGED

HAWKS

331

the same cline is indicated for this trait as for the type D belly. The type F belly ranges
from 18 per cent in the Aleutians and southwestern Alaska to 0 per cent in Asia.
When types A through C are combined in one group, and types D and E in another,.
the difference in frequency between all Canadian males and all Alaskan males is statistically significant (table 6i), but the difference between Alaskan and Asiatic males(table 6j) is not. Thus, it can be said, first, that there are fewer extreme masculine
bellies (types A and B) in the Aleutians and southwestern Alaska than in any of theother samples, but the difference between this sample and the northwestern Alaskan
one is not statistically different. Second, there are fewer males with feminine bellies(types D and E) in the Canadian samples than in the Alaskan and Asiatic ones.
In the adult female series the type A belly does not occur. The type B belly ranges
from 0 per cent in all but two samples to about 19 per cent in Asia and about 25 percent in western Canada. The type C belly ranges from 0 per cent in northwestern Alaska
and western Canada to about 21 per cent in eastern Canada. The type D belly ranges
from about 8 per cent in the Aleutians and southwestern Alaska to 58 per cent in eastern Canada; and the type E belly ranges from about 8 per cent in eastern Canada toabout 86 per cent in Asia. There is an inverse relationship between the last two types.
The type F belly ranges from 0 per cent in Asia to about 18 per cent in the Aleutians
and southwestern Alaska.
By grouping the types into two categories, A to C and D to E, a nearly statisticallysignificant difference is again found between the frequencies of all Alaskan females andall Asiatic ones (table 61), but not between all Canadian and all Alaskan females(table 6k). This indicates that Asiatic females have a greater tendency toward mas
culine bellies (types A, B, and C) than do females from the other regions. On the other
hand, when the type E belly is compared with all other types together, one obtains astatistically significant difference (table 6g) between its frequency in Canadian andAlaskan females and between Alaskan and Asiatic females (table 6r). This results froma very high frequency of the type E belly in the Alaskan sample. The type D belly was
similarly tested (tables 60 and 6p), but no significant differences were found between
any of the adult female samples, although the difference between Alaskan and Canadiansamples gives a chi-square value near that required for P to equal .05.
Marginal coverts of z&g.-In
the adult male series,type I covert ranges from about
23 per cent in Asia to 0 per cent in both Canadian populations. The Alaskan populations
are intermediate with about 15 per cent each. Type II covert ranges from about 92 percent in both Canadian samples to 39 per cent in the Asiatic, both Alaskan samples being
again intermediate with about 78 per cent. Type I and II coverts are thus inversely
related. Type III covert constitutes about 39 per cent of the Asiatic sample; it does notoccur in any of the North American ones. Type IV covert constitutes about 8 per centof the eastern Canadian, northwestern Alaskan, and Aleutian-southwestern Alaskan
samples; it does not occur in the western Canadian or Asiatic ones.
The differences in frequency of type I between all Canadian and all Alaskan males
and between all Alaskan males and all Asiatic males are not statistically significant(tables 5dd and 6ee), but there does seem to be a trend toward higher frequency of thistype from east to west. The difference in frequency of type II between all Canadianand all Alaskan males likewise is not statistically significant (table 6ii), but the difference between all Alaskan males and all Asiatic ones is (table 6jj).
Thus, Asiatic males differ in three respects from North American males. They havea higher frequency of the feminine type I covert and a decidedly lower frequency ofthe masculine type II. In addition, they have a type III covert not found in the North

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American samples (table 6mm). In turn, North American males have a type IV covert
not found in the Asiatic sample (table 600).
In the adult female series, type I covert ranges from about 45 per cent in the Asiaticand Aleutian-southwestern Alaskan samples to about 12 per cent in western Canada.
AS in the male series, the general trend is from low frequency in the eastern samples tohigher frequencies in the western ones, but the differences between Canadian females
and Alaskan females, and between Alaskan and Asiatic females, are not statisticallysignificant (tables 6ff and 6gg). Type II covert ranges from about 74 per cent in western Canada to about 20 per cent in Asia, and again an inverse relationship is evident
between types I and II; statistically significant differences, however, are not found between the samples (tables 6kk and 611). Type III covert is not found in any of theAmerican female samples, but it comprises about 38 per cent of the Asiatic one; thisdifference is statistically significant (table 6nn). Type IV covert is found only in the* North American samples, but the difference between all North American females andall Asiatic ones is not statistically significant (table 6pp) because of its low frequency
in the American samples and the disproportionately small Asiatic sample.
Asiatic females appear to differ from American ones in exactly the same respects
that the males differ in this character.
Head.-Inthe adult male series, type A head ranges from about 37 per cent in Asia
to 0 per cent in eastern Canada, indicating a cline toward greater frequency from east
to west; but none of the American samplescontains more than 15 per cent, the greatest
disjunction being between the continental samples. Since only two types are involved
for this character, the type B frequencies are in every case inverse to the type A frequencies. There are no statistically significant differences between Canadian, Alaskan,
and Asiatic samples (tables 6v and 6w), but when all North American males arecompared with all Asiatic ones, the difference is significant at the 1 per cent level ofconfidence.
In the adult female series, the type A head ranges from about 7.5 per cent in Asia
to about 2’1 per cent in western Canada. The trend is unbroken to this point, but it isdisrupted by the eastern Canadian females, which have about 61 per cent type A. No
statistically significant differences are found between any of the samples (tables 6x,
y and z).
Thus, there appears to be a trend toward higher frequency of type A heads in bothmales and females from east to west, but the samples are not large enough to give great
confidence to this conclusion.
Outer web of primaries.----In the Asiatic sample about 74 per cent of the adult males
and about 86 per cent of the adult females fall in classesI, II, and III. In the Aleutian- southwestern Alaskan sample none of the specimens falls in classesI and II, and only
about 8 per cent of the males and 15 per cent of the females are in class III; all others
are class IV. About 7 per cent of the northwestern Alaskan males are in class II, andabout 14 per cent of the males and 23 per cent of the females are in class III; all others
are class IV. In the Canadian samples only one specimen from western Canada is inclass III; all others are class IV. The variation of this character expressesthe clearest
east-west cline to be found among the several traits studied. The greatest difference
between adjacent samples occurs between Asia and Alaska, but it is important to knowthat the northwestern Alaskan sample contains the largest number of examples that arelike the majority of Asiatic specimensthat is, with some degree of light coloring on
the outer webs of the primaries distal to the ends of the coverts. Important also is thefact that no North American specimen approaches the extreme Asiatic expression ofthis character.

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Fig. 6. Upper row, female specimens from left to right, showing, respectively, primary types
I. II. III. and IV: USNM 370449. Manchuria: CNHM 101222, Manchuria; CNHM
lbl2.25, Manchuria; DMNH 22856,‘northwestern Alaska. Lower row, females: CNHM
101223, Manchuria; DMNH 19522, northwestern Alaska; Chic. Acad. Sci. 4211, northern Alaska; DMNH 23644, northwestern Alaska; note dark feathers on backs of twocentral specimens, contrasting sharply against an extremely worn and foxed plumage;these have been called pallidus by some authorities.

This character affords the best single criterion for separating adult Asiatic Roughlegged Hawks from American ones. On this basis alone about 80 per cent of all Asiaticadults are different from about 90 per cent of all American adults, and 80 per cent of
all Asiatic adults are different from about 83 per cent of all.Alaskan ones.
The trait of light areas on the outer primaries has a higher frequency among juveniles of both continents than among adults. It occurs on all Asiatic juveniles that havebeen examined (total of 9) and on 62 per cent of the American ones, but this difference
cannot be considered significant because of the small Asiatic sample. There is a color
difference that is perhaps more important. On all Asiatic juveniles the color of the lightarea is whitish or creamy, indistinguishable from the adult color. On nearly all North
American specimens the area is buffy or reddish. Also, all the Asiatic juveniles have theextreme type I expression of this character; none of the American juveniles do.
B&t.-Frequenciesof the several traits of the breast are presented in table 5.
There is a trend toward higher frequency of the type II breast mark from east to west,
and the greatest difference between adjacent samples occurs in the Bering Sea region,
where over 50 per cent of the Asiatic females and 25 per cent of the males are type II,
whereas only about 22 per cent of the northwestern Alaskan females and about 12 percent of the males are type II. Again the northwestern Alaskan sample has the highest

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frequency of this type among North American birds. Among the color classesthere isalso a tendency for light browns to increase in frequency in a westerly direction. The
largest difference between adjacent samples is again in the Bering Sea region. Thereappear to be no pronounced differences among the samples in the edging classesa, b,
and ab; there is a slightly, but not significantly higher frequency of the a type in the
Asiatic sample, but the b and ab types are low in frequency in all the samples.
B&.-Frequenciesof the various dorsal traits are also presented in table 5. In
general these traits show the same trends as the ventral ones. Type II shows increasein frequency from east to west, and the lighter browns follow the same trend. The type amargin is much more frequent in the Asiatic sample, and the type b does not occur. The
condition is reversed in the American samples. The difference in frequency of types I
and II between all Alaskan adults and all Asiatic ones is statistically significant (table6tt), but it is not between any of the other samples (tables 6gg, rr, and ss). When thedorsal color types are placed in two groups, A-B and C-D, and their frequencies compared among the samples, a statistically significant difference is found between all
northwestern Alaskan adults and all Aleutian-southwestern Alaskan ones and between
all eastern and western Canadian adults, but not between western Canadian and northwestern Alaskan adults or between northwestern Alaskan and Asiatic ones (tables
6uu, vv, ww, xx). In the latter comparison, however, there are important differences
not brought out by this test: the D type doesnot occur at all in the Asiatic sample, and
the A type does not occur in the northwestern Alaskan sample. When the frequency of
the A type to all others is compared in the two samples, the difference is significant at
the 1 per cent level of confidence.
SEXUALDIMORPHISM

Size.-Turning
again to table 3, it appears that there are some geographic differences in degree of sexual dimorphism. The Asiatic males and females show less than75 per cent joint nonoverlap for all three measurements. The same is true of the northwestern Alaskan males and females, although their coefficients of difference average
higher than in the Asiatic sample. In the Aleutian-southwestern Alaskan population the, sexesshow more than 75 per cent joint nonoverlap for each measurement, and indeed
the two most reliable measurements, those of wing and tarsus, show more than 90 per
cent joint nonoverlap. This population has the most consistent sexual divergence in sizeof any studied. The western Canadian sexesdiverge to the extent of 85 and 80 per centjoint nonoverlap for two measurements but less than 75 per cent for the other. Also,
the eastern Canadian sexes diverge to the extent of 96 per cent joint nonoverlap forthe wing (the greatest for any measurement) and 85 per cent for the tarsus. The
cuhnen shows less than 75 per cent. The small joint nonoverlap between the sexes in
Asia and northwestern Alaska is at least in part the result of the greater variability ofthese populations, but the means of the males and females are actually closer in nearly
every case.
Plumage.-For tail pattern there is a tendency among the samples for sexual dimorphism to break down in an east-to-west direction, This results from the fact that in the
more westerly samples (Alaskan and Asiatic) both males and females have higher
frequencies of characters that are in the overall population more characteristic of the
opposite sex. Thus, in the Canadian samples more than 95 per cent of the adult males
are separable from 100 per cent of the adult females; in the Alaskan samples slightly
more than 62 per cent of the adult males are separable from slightly more than 92 percent of the females; while in the Asiatic sample about 78 per cent of the males are separ-

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able from about 74 per cent of the females. Expressing these relationships as percentages may overemphasize the actual difference between the populations, but the ‘trend is
the important thing.
A different relationship holds for belly pattern. Again the eastern Canadian sample
shows the greatest sexual divergence and the greatest homogeneity of character in thetwo sexes; this condition breaks down in a westerly direction but not so completely asin the case of tail pattern. The male trend behaves exactly as it does for tail pattern,
but the females show a slight but demonstrable reverse trend: that is, males have theirhighest frequency of masculine tails in the east and a progressively lower frequency
westward, whereas females have their highest frequency of feminine tails in the west
with progressively lower ones eastward. Thus, in eastern Canada about 9.5 per cent of
the adult males are different from 63 per cent of the adult females; in Alaska about
59 per cent of the adult males are separable from more than 84 per cent of the females;
and in Asia 65 p:r cent of the adult males are separable from more than 71 per cent of
the adult females. The actual difference between the sexesis probably greater than thesefigures suggestbecauseof the wide range of characters subsumed under the type C belly,s which is the category of major overlap, and also becauseof a number of melanistic specimens in the American samples. Again it is the trend that is important.
Sexual dimorphism of the marginal wing coverts is somewhat obscured by the factthat the extremes in variability of this character, unmarked cream and unmarked dusky,
do not appear to segregate sexually but do show a marked geographic difference: The
type III covert, unmarked cream, constitutes 38 per cent of the adult Asiatic malesample and 35 per cent of the adult female sample; the type IV covert, unmarked
dusky, constitutes 8 per cent of all North American adult males and slightly more than
8 per cent of all adult females. Both of these extremes occur on only one side of theBering Sea.
In the Canadian samples more than 91 per cent of the adult males are separablefrom more than 27 per cent of the adult females on the basis of covert types I and II;
in the Alaskan samples about 77 per cent of the adult niales are separable from 44 percent of the females; and in the Asiatic sample 38 per cent of the adult males are different
from 45 per cent of the females. This variational trend is the same as that of belly pattern.
When percentages of difference between the sexesare calculated for head types, thesame trend is again shown. In the Canadian samples 95 per cent of the males are different from 40 per cent of the females; in the Alaskan samples about 89 per cent of theadult males are separable from more than 53 per cent of the females; and in the Asiatic
sample 65 per cent of the adult males are different from 75 per cent of the adult females.
The geographic trend in sexual divergence of tail pattern is more or less concordant
with that of sexual dimorphism of size, although the latter is not as definitely clinal innature as the former because of the disruption produced by the pronounced sexual
dimorphism in size of the Aleutian-southwestern Alaskan population. In general, however, both males and females are more homogeneous for these characters in Canada,less SO in Alaska (with the exception just above), and still less so in Asia. Assuming thatthese two characters are genetically independent, the phenotypic evidence indicates twoparallel trends of gene frequency through the populations.
The close geographic concordance in the trend of sexual dimorphism of the belly
pattern, marginal wing coverts, and head types is probably not to be construed as evidence of three parallel trends of gene frequency. Since the variability of these charactersdepends upon the degree of pigmentation generally in feathers, not upon specific local
modifications, it seems most reasonable to suppose.that they are determined by the

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same genetic and physiological factors; hence, they probably indicate but a single trendof gene frequency.
MELANISM

Extreme melanism, that is, pigment saturation to the virtual obliteration of pattern, is not found in the Asiatic sample. It has already been pointed out, however, thatmelanism in Buteo Zagopusis a matter of degree and that the dark phase grades continuously into the light phase. The type C dorsum, for instance, is patterned but it isalso quite dark, and it could be considered melanistic on a relative basis. It has about a
12 per cent occurrence in the Asiatic sample; in this group only those individuals thathave light areas on their primaries can be distinguished from Canadian specimens.
The uniformly dark examples at the extreme of the color continuum occur at different frequencies over the breeding range in North America. The situation is somewhat
like that found in fulmar populations (Fisher, 1952), another species that Shows thenondisjunctive kind of polymorphism. Thus, in the Aleutian-southwestern Alaskan
sample the extreme melanistic individuals comprise about 19 per cent of the total, in
northwestern Alaska about 7 per cent, in western Canada only 3 per cent, but in eastern
Canada almost 12 per cent.
Beebe (1907: 10) observed that the melanistic type has a center of abundance in
Ungava and Labrador, and Grinnell (1909: 213), finding dark birds from Glacier Bay,
Alaska, identical to these dark eastern Canadian birds, suggested“that this ‘dark phase’is really a geographic form occurring in regions of excessivehumidity, and not identicalwith the ‘light phase’ . . .“, Disregarding any taxonomic implication of Grinnell’s statement, one does see a fairly good correspondencewith Gloger’s rule in this case.
ALLOCATIONOF INDIVIDUAL

SPECIMENS

Grinnell (1918: 505-507) was one of the earliest antagonists of the idea of applying
trinominals to individual specimens without regard to the geographic area from which
they were taken. Now, nearly forty years later, the cogency of this position is not yet
thoroughly appreciated by all ornithologists. For instance, the American Ornithologists’Union Check-list ( 193 1) and its later supplements still contain a number of extralimitalsubspeciesbased on one or a few specimens.Unless such specimenscan be demonstrated
to fall entirely outside the range of variability of conspecific American forms, I fail tosee any validity in their trinominal designation. Perhapspo one now disagreeswith thisstatement in principle, but it is certainly violated in practice.
I have personally examined nine of the Alaskan specimens of Buteo tagopus thathave been called pattidus or kamtschatkensis by one authority or another. Bailey(1948: 181-182) lists five that I have not seen: DMNH 22868, 22869, 24590, 24589,
and one in the Peet collection. In addition, there are two from Hooper Bay (Brandt,
1943) and one from St. Lawrence Island identied by Friedmann (Murie, 1936:367).
Specimen number 8 12 15 USNM is the basis for the first published record of putt&s
from Alaska (Friedmann, 1934: 246). It is recorded as a male (Bailey, 1948: 182, refers
to this bird as a female), collected by E. W. Nelson at St. Michael’s Alaska on September 16, 1879. Although not stated by Friedmann, the specimen is clearly in juvenal
plumage. It has a light head, a very large continuous belly band, a distal field of brownon the tail becoming reddish proximally, and the third, fourth, fifth, and sixth primaries(from the outside) have a light reddish area on their outer webs distal to the ends ofthe coverts. In not one character of the plumage is it like an Asiatic juvenile; it,is, infact, indistinguishable in plumage from many juveniles from the Aleutian @lands andsouthwestern Alaska. It happens to be the largest specimen of any sex that I have meas-.

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ured from North America, with a wing of 470 mm. On this basis, it is almost certainly
incorrectly sexed.
Bailey’s first breeding record of pallidus is based on DMNH 22614, an individual
taken with young at Golovin Bay, Seward Peninsula, on July 26, 1921. This specimen
has been recorded both as a male (Bailey, 1942:305-306) and as a female (Bailey,
1948: 182). I have not seen this specimen, but I have examined an unnumbered adultmale from the Denver museum taken at the same locality on July 26, 1923. This specimen bears the name pallidus on its label. It looks very much like several males in theAmerican Museum of Natural History taken on Bering Island; but it is also indistinguishable from several specimenstaken in the Canadian Arctic. It has none of the exclusively Asiatic characters such as marginal covert type III or primary type I; in fact,
its primaries are the extreme American type IV.
The second breeding record mentioned by Bailey (1948: 182) is based on Chic.
Acad. Sci. 4211, a female taken with eggsin the Romanoff (Romanzof?) Mountains ofnorthern Alaska. Friedmann identified this bird as belonging to the Asiatic subspecies(pall&s or kamtsckatkensis?). Again on the basis of plumage (see fig. 6), I would consider this bird to be a juvenile just beginning molt. The old feathers are much faded andworn and superficially resemble the light brown condition of most Asiatic birds, but the
new feathers, clearly visible in the figure, are dark like most North American examples.
The same age and plumage conditions are evident in: DMNH 19522, a female fromthe Chipp River taken on May 24, 1938; DMNH 22867, a female taken at Wainwrighton June 15,1942; UMMZ 121719, a male taken at Brownlow Point on April 10,1944;
MVZ 32253, a male taken on the Kowak (Kobuk) River on May 10, 1899-the condition of the latter specimen having already been noted by A. H. Miller (quoted by
Bailey, 1948: 181).
DMNH 23645, an adult male taken at Point Barrow on June 9, 1943, has wide
white margins on the feathers but is otherwise quite dark. It matches a few specimensfrom both Asia and Canada. DMNH 22866, an adult male from Wainwright taken on
June 15, 1942, represents the extreme male example of depigmentation (see fig. 2).
Birds approaching this extreme are more frequent in the Bering Sea regions of both
continents, but several examples were also seen from the Canadian arctic.
Thus, it would appear that the grounds for considering any American specimen of
Buteo lagopus as an example of the Asiatic subspeciesare extremely weak, being based
for the most part on an inadequate understanding of the nature of variation in thisspecies.On the other hand, the presence of characters common to populations on bothsides of the Bering Sea indicates that there is now, or has been, some genie exchange
between the continents, but this does not mean that one can equate these populations
taxonomically, or for that matter, genetically. The distribution of a character, or a groupof genetically dependent characters, can be objectively determined, but the distributionof a subspeciesis a matter of opinion and depends upon what combination and degreeof expression of characters is used to characterize it.
DISCUSSION

Wilson and Brown (1953) have recently stressedexamples to show the frailties ofthe subspeciesconcept. They particularly emphasize the lack of geographic concordance
among infraspecific populations of characters presumed to be genetically independent,
and they suggest that where populations are isolated or are undergoing secondaryintergradation, the best, but still not entirely satisfactory, way to define a subspeciesis
in terms of peaks of character concordance.

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In the case of Buteo lagbpus it is, first of all, difficult to say which of the several
characters that have been studied are genetically independent. One can presume by
extrapolation from genetic studies of other species that the mensural characters are
independent of plumage, but within these two groups it is not possible to know whether
geographically concordant characters are genetically independent or not. I am inclined
to doubt that there could be complete genetic independence of any character from another within these two groups. On the other hand, when characters are not geographically concordant, at least partial genetic independence seemsassured, assuming that thecharacters are heritable.
Size of wing concords geographically with size of tarsus in both males and females;
but the sexesare geographically discordant, that is, a population with females that possessrelatively large wings and tarsi may have maies that possessrelatively small ones.
Among plumage characters, primaries of types I and II concord geographically withdorsal and breast colors of type A and with type III marginal wing coverts; type IV
primaries concord well with dorsal and breast colors of types C and D and with marL
ginal wing coverts of types II and IV, but the correspondenceis never perfect.
A number of clines are indicated within the geographic area covered by this study.
In general they parallel each other along an east-west line. Wing length of males showsa tendency toward increasing size in a westerly direction. The females show no mensuralclines. There is, however, a tendency for the sexesto become less dimorphic in size fromeast to west, excluding the Aleutian-southwestern Alaskan population.
Nearly all the plumage characters are to some extent clinal. The light-headed individuals increase in frequency in a westerly direction; the same is true of light brown
dorsal and ventral pigments. The light patch on the outer webs of primaries, the narrowly streaked breast, and the widely edged dorsal feather all follow this trend. Of evengreater importance, however, is the fact that sexual dimorphism of plumage also follows
this cline. The greatest sexual divergence of plumage occurs in eastern Canada andbreaks down progressively toward Asia, again excluding the Aleutian-southwestern
Alaskan sample in some cases.
There is one plumage character in which the extremes are restricted to one side of
the Bering Sea or the other: the type III marginal wing covert is restricted to Asia, andthe type IV covert to America. Primaries of types I, II, and III have a major disjunction in frequency between the Asiatic and Alaskan populations and do not exist at allin the Canadian ones.
Two American samples, the Aleutian-southwestern Alaskan and the eastern Canadian, are more homogeneousin phenotype than the other two, the western Canadian andnorthwestern Alaskan. The northwestern Alaskan sample in particular is extremely
variable in both mensural and plumage characters. Some individuals of this population
are nearly identical with the most frequent Asiatic type, a much larger number are
identical with the usual Canadian examples, some are blending intermediates between
these two, while others show a non-blending combination of characters. Nearly the fullrange in variability of each character is to be found in this population.
The Asiatic population, as examined by me, is also quite variable. This is probably
the result in part of the large number of winter specimens that are included-a sample
that undoubtedly contains a mixed aggregate of breeding populations. Dementiev et al.
( 195 1: 3 14) indicate that the pale-colored Asiatic form is rather homogeneousover mostof its breeding range from the Anadyr to the Yenesei River, where it intergrades withtypical lagopus. But the nature and area1 extent of this intergradation, as well as thatbetween the pale form and the so-called kamtschatkensis, is not stated. What appear toNov., 1955

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be hybrids between the pale form and the darker American form occur in eastern Asia.
Of the few breeding specimensexamined, there are two such birds from the Lena River,the most westerly locality, and four from Bering Island. A number of winter specimensfrom Manchuria were also examined. All these breeding and wintering specimens fit
Dementiev’s description of KamtschatRensis, which appears therefore to be nothing
more than one of the mixed types produced by crossingsbetween the pale form of interior Asia and the American form. The Asiatic character most often expressedin suchspecimens is the light area on the outer webs of the primaries; the American charactersmost often expressedare dark color and variegation of the back, but the variability isgreat. Most of them appear distinctly more American-like than Asiatic, which suggeststhat the determiners of the pale characters are easily dominated by corresponding
American ones.
On the basis of the samples examined by me, I believe that the entire populationof the Bering Sea coasts and the immediately adjacent Arctic Ocean will prove to bepractically indistinguishable from that of northwestern Alaska, both showing to somedegree the full range of variability of the entire Asiatic and North American populations.
Such variability indicates that the Bering Sea region (excluding the AleutianIslands) is a zone of allopatric hybridization or secondary intergradation (Mayr, Linsley, Usinger, 1953 : 101). The pale Siberian form and the darker, “polymorphic” American form, once separated by an extrinsic barrier, have since been able to disperse into
the range of one another and there freely to interbreed. That completely random breeding occurs is indicated by the observations previously mentioned along the ColvilleRiver in 1952, where melanistic. individuals were mated with very pale birds and allsorts of intermediates between these extremes occurred in the breeding population.
Although on the basis of the distribution of specific characters one can say that
there is or has recently been gene flow between the continents, nevertheless on an inferential basis there are reasons for believing that the present rate of genie exchange ismuch less than that occurring between confluent populations on the continents. In otherwords, the Bering Sea now acts as a barrier to gene flow to some extent. The northwestern Alaskan population, although showing nearly the complete range of variability of
the Asiatic and American forms, still has a preponderance of American types. The
converse appears to hold true in Asia. Why is this so?
If one looks at a small scale map of the North American and Asiatic continents, hewill see that in the region of their closest juxtaposition, namely, Bering Strait, they areopposed by a very limited coast line, which on both continents very quickly recedes
away from the zone of near contact, both to the north and to the south. Thus, from acontinental point of view, only a very small portion of the breeding ranges of Asiaticand American rough&legsare in near contact, and hence relatively few individuals are
geographically situated for dispersal across the Bering Sea. This is one possible factor
now operating to limit, but not necessarily to exclude, gene flow between the continents.
Another is the Bering Strait itself. The breeding grounds of Buteo lagopus are notcontinuous between Asia and America. They are separated by a narrow stretch of water,which again remains narrow in a very limited area only. I do not doubt that it is easily
within the flying capability of Buteo lagopus to cross Bering Strait or even probably
the widest expanse of the Bering Sea. On the other hand, Rudebeck (1950; cited byThompson, 1953: 173) has found that buteos in general show more reluctance to crossthe sound between Sweden and Denmark than some other speciesof falconiforms during the fall passage, indicating the possibility that buzzards have an aversion to flightover water. Still, the species is well distributed in the Aleutian Islands as far out as

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Umnak Island, the western limit of the distribution of voles (0. J. Murie, unpublisheddata), which are the principal prey of Buteo lagopus.
The weather of the Bering Sea could,well be an additional factor discouraging passage. Frequent low fogs reduce visibility, and high winds of gale force, or stronger, makefor difficult flying.
All these extrinsic factors probably sum up to reduce gene flow between the continents. Thus the perimetric populations of the Bering Sea should receive numericallygreater genie contributions from the populations of their respective hinterlands thanfrom each other.
On the other side of the picture, there are some intrinsic factors concerned withpopulation structure that may tend to restrict gene flow over any large, continuous rangeof the species.Miller (1947: 186-190) has presented an excellent discussion of some ofthese factors. It would be advantageous to be able to define panmictic populations of
Bzlteo lagopus in genetic terms; this, of course, is impossible. But one can make somestatements about the factors that probably influence panmixia in this species.
The breeding Rough-legged Hawk is a highly territorial individual, and pairs normally return to the same nesting area year after year; there is, therefore, little genemovement within the breeding population. Panmixia can be achieved only by the dispersal of non-territorial individuals, a situation that is widespread among avian species
as pointed out by Miller (op. cit.: 187). It becomesimportant to know two things about
the dispersal of these unattached hawks: how far do they disperse, and how many ofthem reach the status of effective breeding individuals?
One can only guessat answers to these questions. The vagility of the speciesis great.
It is not at all unlikely that a bird reared in Ungava, that migrated into the UnitedStates for the winter, could turn up the next spring in northern Alaska. It is within its
physical capacity. On the other hand, nothing is known about the “homing” inclinationof this speciesor to what extent geographic populations follow definite routes of migration. In all probability most immature birds return to the general region if not to thespecific locality where they were reared.
But immature birds and other unmated individuals returning to the breeding grounds
find a rigidly fixed population structure that nearly always occupies all the optimal
breeding niches. A few of them gain entrance into this fixed social structure throughthe death of mated birds, but most of them must either remain unmated or occupy marginal breeding sites, In either case they contribute little to the reproductive successof
the species. If Rough-legged Hawks are like most other falconiforms, these unmatedindividuals do not take up definite territories but tend instead to wander widely. Thus,
even if homing is a factor tending to insure the return of the immature bird to its native
region, once there it is more or less forced to move out. If these factors have been correctly interpreted, then the general movement of this dispersive segment of the summer
population would be centrifugal, and one would expect to find the largest number of _unmated birds and marginal pairs at the perimeter of the breeding range. There are nodata for Buteo lagopus to support or to deny such a pattern of dispersion, and it would
be difficult to show even if there were a large amount of comparative population datafrom different regions because the breeding distribution is itself essentially perimetricwith respect to the North American continent. In the case of the Peregrine Falcon, a
specieswith a much less linear breeding distribution in northern latitudes and one forwhich there are comparative population data (unpublished personal observations),
there is indeed a higher incidence of marginal nestings and unattached individuals in
the perimetric population compared to a more centrally located one. But despite theNov., 1955

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questions concerning dispersion of unattached Rough-legged Hawks, one can say thatthe distance between the site of hatching and the site of breeding of any individual
would seem to be determined largely by the essentially random occurrence of an available breeding niche into which the wandering individual could fit.
The number of birds available for dispersal is also a factor influencing the rate ofgene flow. Neither the annual mortality of the population nor its annual reproductive
successare known, but judging from the reports of Barth (1952) and Dementiev et al.,195 1) , its reproduction in Scandinavia and arctic Russia fluctuates with the basic fluctuation in the numbers of microtine rodents. I would be surprised if more than 20 per
cent of the fledged young reach breeding age on the average. It is quite likely to belower. If a Rough-legged Hawk survives its first year, it has a life expectancy quite longfor a bird-probably on the order of 10 years. When the annual decline is less than theannual increase over a number of years, there accrues in the population a segment of“reserve breeders”-highly mobile individuals capable of reproduction when the opportunity is afforded by the vacancy of a breeding niche. It is this segment of the population that is the effective mechanism of gene distribution. It probably renders the entire
North American population potentially panmictic under the presently existing environmental conditions.
HISTORICAL

INTERPRETATION

The present information about the populations of Buteo lagopus in the Bering Sea
region suggeststhat there has recently been a slow amalgamation of what were formerly
extrinsically separated populations. If this is true, then one should expect that formerly
there existed a barrier to the dispersal of populations. The history of events in the
Bering Sea region during the Pleistocene glaciations provides such a situation.
Current geological theory (see Flint, 1947, for review) supposes that during the
Pleistocene much of the Bering Sea basin and the immediately contiguous basin on the
Arctic Ocean alternated between dry land status, correlated with periods of maximal
glaciation when sea levels were low, and inundation during interglacial periods when
sea levels were high. The former concept of the Bering Strait “land-bridge” has thus
gradually metamorphosed into the idea of the “Bering Sea-Yukon refugium,” an essentially unglaciated region of large extent roughly following the continental shelves, confluent with the lower river valleys, and well isolated from the rest of the coeval North
American biota by several thousand miles of ice. It was not, however, isolated from thelarge unglaciated region of Siberia, which is inferred to have existed during the maximum of the Wisconsin (last) glaciation (see plate 3 of Flint, 1947). Other refugia of
the northern biota are inferred on the basis of both geological and biogeographicalevidence to have existed as isolated entities to the southwest and to the southeast of thegreat continental ice sheets.
The theory of biotic refugia within the glaciated regions was first clearly stated byHulten ( 193 7)) primarily based on biogeographical data, and it was used by him andlater by Raup (1946) to explain the phytogeographic patterns of present boreal floras.
It has also provided a basis for cogent interpretations of certain trends in speciation
among northern groups of birds (Rand, 1948; Pitelka, 1950) and mammals (Rand,
1954). Drury (1953) has shown a correspondencebetween the geographic affinities ofplants in the St. Elias region of Yukon Territory and the geographic affinities of thebirds there, based on Hulten’s original scheme.
In all likelihood the Yukon-Bering Sea refugium provided suitable conditions for abreeding population of Rough-legged Hawks. Such a group of birds must have been
completely cut off from the rest of the North American populations, which then resided

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in refugia far to the south OPtheir present breeding ranges. This Alaskan population,
however, would have been continuous with the Asiatic one at this time, presumably
sharing the characters now most typical of the pale Siberian form.
During the retreat of the last glaciation, one or more biotic corridors opened up
gradually between the unglaciated southern regions of North America and the unglaciated region of Alaska and the Bering Sea. At the same time, the rising sea began toinvade the basins that are now covered by the northern part of the Bering Sea and thecontiguous Arctic Ocean. The first effect of this deglaciation allowed an avenue for dispersal of both Alaskan and North American Rough-legged Hawks. The latter, being alarger population, probably effected a faster and numerically greater dispersal than did
the peripherically situated Alaskan hawks, which for a time may have maintained migratory affinities with the rest of the Asiatic population by crossing over the invading
Bering Sea.
As the melting of the glaciers continued, the Bering Sea and the Arctic Ocean began
more and more to separate the two continents, making it increasingly less likely for birds
wintering in Asia to cross into Alaska. More and more voids were thus left in the ecologic niche occupied by Buteo lagopm in Alaska, and these were probably taken up by
dispersing individuals from the North American population. But this displacement probably did not take place rapidly, and there was plenty of time for the two continental
populations to mix their genes thoroughly in the zone of contact in Alaska. Some of thismixture was carried over to the Asiatic side of the Bering Sea before the continents
were completely isolated by water, and as already indicated there has probably continb
ued to be some admixture of Asiatic and North American genes right up to the present.
The result is now a thoroughly hybridized population on both sides of Bering Strait, the“foreign” genes gradually being swamped out in the more interior segments of thecontinental populations.
The Aleutian Islands and southwestern Alaska were probably repopulated by Roughlegged Hawks following the retreat of the glaciers from a population that had been
isolated in the refugium to the southwest of the ice sheets.These birds may have undergone change in the direction of greater saturation of pigment in the refugium, or they
may not have done so until after establishment in the Aleutian Islands. In either case,on the basis of their characters, one can say that they do not appear to have been derived from the same recent stocks as either the northwestern Alaskan or the eastern
Canadian populations.
The breeding populations of Canada were probably derived from birds residing ina refugium to the southeast of the ice sheets. These Rough-legged Hawks may haveundergone a somewhat parallel change in the direction of pigment saturation, or again
they may not have been modified until after their dispersal into Canada following theretreat of the glaciers. Alternatively, saturated pigmentation may have been common
in all populations of Buteo Zagopusprior to their isolation in refugia during the Pleistocene and then have been virtually lost in some populations during isolation.
While this suggestedpicture of the history of the Rough-legged Hawk populationsin North America during late glacial and postglacial times is consistent with what is
known about glaciation and the present patterns of geographic variation in the species,
it must be considered a highly tentative model. Any attempt at a reconstruction of thehistory by indirect methods is subject to great and indeterminate error.
TAXONOMIC

RECOMMENDATIONS

It should now be abundantly clear that attempts to describe subspeciesof Buteo
lagopus based on a few specimens, with no consideration of variation over the wholeNov., 1955,

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343

range of the species,are misleading. Indeed, the patterns of variation manifested by thevarious characters of Buteo Zagopusare ill-suited to the trinominal system because theso-called peaks of concordance are not conspicuous and because the variability of thecharacters at any given geographic locus is great. Clear-cut geographic differences simply
do not exist except, perhaps, on a continental basis. I therefore suggest the following
modifications of the existing taxonomic treatment of this species.The name, sanctijoha&s, should be restricted to the breeding populations of Canada east of the Mackenzie
River. Possibly the Aleutian&southwestern Alaskan population might be included tomake a polytopic subspecies,but this I feel is not meaningful since the variation of thispopulation suggeststhat it has undergone independent though somewhat parallel evolution at least since late Wisconsin time. Its distinctness is not, however, sufficient for
subspecific recognition. The regions contiguous to the Bering Sea (excluding the Aleutians) and the adjacent Arctic Ocean should be considered a zone of hybridization, andbirds from these areas should not be trinominally designated. The population of Kamchatka and adjacent regions falls in this same category, and thus kamtschatkensisin thesenseof Dementiev should not be considered a valid subspecies.The name, kamtschutkensis Dementiev, therefore, becomes the first available synonym to replace pallidus
Menzbier, as already recognized in the Twenty-third Supplement to the A.O.U. Checklist (Zoc. cit.), and the name, menzbieri Dementiev, newly proposed, should become asynonym of kamtschatkensis.Admittance of the Siberian subspeciesto the North American list (Wetmore et al., 1945) should be revoked since there is no unambiguous demonstration that examples unquestionably from Asia have occurred in Alaska.
SUMMARYAND

CONCLUSIONSThe Common Rough-legged Hawk (Buteo Zagopus) is a polytypic species,three orfour subspeciescurrently being recognized. The present study was undertaken to determine the nature of the variation of this speciesin North America. A sample of Asiaticbirds was also employed for comparative purposes.
Molt occurs from April until late fall, but is accomplished mainly during the breeding months of June, July, August, and early September. Males tend to molt later thanfemales, and it is suggestedthat this is an adaptation to the primary role that the maleassumesas food provider for the female and young during the first half of the breeding
season.
Juveniles are best distinguished from adults by differences of pattern and color on
the tail and belly. Adult males are more distinct from juveniles than are adult females.
There is no clearly defined second-year plumage, but there may be some minor changesof plumage correlated with age.
The so-called polymorphism of the Common Rough-legged Hawk is nondisjunctive
in nature, and the darkest extremes grade in unbroken series to the lightest extremes.
The individual variability of Buteo lagopusis so great as to preclude definitive studyof variation without large series of skins.
Female Rough-legged Hawks are larger than males, but there is much overlap between the sexesfor all measurements studied except that of grossbody weight, for which
no overlap is indicated. The adult plumage is also sexually dimorphic, a condition thathas not heretofore been noted. Tail and belly patterns are the best indicators of sex, buthead type, marginal wing coverts, and colors and patterns of the dorsum are also tosome extent different.
There is remarkably slight geographic variation in mensural characters. No populations are clearly distinguishable on the basis of measurements used in this study. Geo-

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graphic variation of plumage characters tends to show an east-west cline. Most characters parallel each other, but some are inversely related. The greatest disjunctions in thevariation of characters usually occurs between Alaska and Asia.
Sexual dimorphism of size varies geographically, the most dimorphic population
being in the Aleutians and southwestern Alaska, but the general trend seems to be
toward lessersexual dimorphism from eastern Canada west into Asia. Sexual dimorphismof plumage also varies geographically and shows an even greater tendency to break
down in a westerly direction. The east Canadian population is the most sexually divergent in plumage.
Extreme melanistic examples appear to be more frequent in regions of high humidity, that is, in the Aleutians and southwestern Alaska and in Ungava and Labrador,
but they occur at a low frequency throughout the breeding range in North America.
There is little geographic concordance of characters presumed to be genetically independent, and peaks of character concordance are not therefore definable.
Populations inhabiting regions adjacent to the Bering Sea show nearly the full range
in variability of each character of both the North American and Asiatic populations,a condition that indicates a zone of hybridization. Even so, the continental configurations of Asia and North America, the interposition of the Bering Sea, and perhaps
weather still appear to act in combination as an extrinsic barrier against free population
dispersal. Structure of the breeding population may also limit dispersion in continuous
continental populations, but the presence of a small, highly mobile reserve of potential
breeders probably renders continental populations essentially panmictic.
During the Wisconsin glaciation, “Alaskan” Rough-legged Hawks inhabiting the
Yukon-Bering Sea refugium were probably panmictic with Siberian ones. Amalgamationof this isolated population with more southern North American ones probably took placeslowly as they came into contact through corridors which opened up with the retreat of
the ice caps. It appears that the North American population by virtue of its greaternumbers has contributed the largest amount of material to the gene pool in the Alaskanzone of hybridization.
The name sanctijohannis should be restricted to the breeding population of Canada
east of the Mackenzie River. Specimens from regions contiguous to the Bering Sea andthe adjacent Arctic Ocean should be considered intergrades and should not be trinominally designated. The form kamtschatkensis Dementiev, sensu stricto, is not distinguishable from the Siberian form and this name is therefore the first available to replace
pallidus (Menzbier) for the Siberian subspecies. The Siberian subspecies should bedeleted from the North American list since there is no unequivocal demonstration of
its occurrence in that region.
LITERATURE

CITED

American Ornithologists’ Union Committee.
1931. Check-list of North American birds. Fourth Edition (published by the Union, Lancaster,Pa.).
Bailey, A. M.
1942. Siberian rough-legged hawk in northwestern Alaska. Auk, S9:305.
1948. Birds of arctic Alaska. The Colorado Museum of Natural History, Popular series, No. 8.
Baldwin, S. P., Oberholser, H. C., and Worley, L. G.
1931. Measurements of birds. Sci. Pub]. Cleveland Mus. of Nat. Hist., Vol. 2.
Barth, E. K.
1952. Breeding biology of the rough-legged buzzard and variations in the population in 19381952. Papers on Game Research, 8:127-138.

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345

HAWKS

Beebe, F. L.
1953. The Peregrines of the northwest coast. Falconry News and Notes, l(1) :5-11.
Beebe, W.
1907. Geographic variation in birds with especial reference to the effects of humidity. Zoologica,
l:l-41.
Bent, A. C.
1937. Life histories of North American birds of prey. Part I. U. S. Nat. Mus., Bull. 167.
Brandt, H.
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Cleveland).

Dementiev, G. P.
1931. Buteo lagopus kamtschatkensis, subsp. nova. Ornith. Monatsb., 34:54-55.
Dementiev, G. P., Gladkov, E. C., Ptushenko, E. C., Spangenberg, E. P., and Sulilovokaya, A. M.
1951. Birds of the Soviet Union. Vol. I (Government Publ. Soviet Sci., Moscow; in Russian).
Drury, W. H., Jr.
1953. Birds of the Saint Elias quadrangle in southwestern Yukon Territory.
Nat., 67: 103-128.
Fisher, J.
1952. The fulmar (Collins, London).

Canadian Field-

Flint, R. F.
1947. Glacial geology and the Pleistocene Epoch (John Wiley and Sons, New York).
Ford, E. B.
1945. Polymorphism. Biol. Rev., 20:73-88.
Friedmann, H.
1934. The Siberian rough-legged hawk in Alaska. Condor, 36:246.
1938. Stegmann on the Falconiformes of the U.S. S. R. Auk, 55:29&291.
1950. Birds of North and Middle America. Part 11. II. S. Nat. Mus., Bull. 50.
Grinnell, J.
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Publ. Zool., 5:171-264.
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Hubbs, C. L., and Perlmutter, A.
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Hulten, E.
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Period (Bokforlags Aktie-

Huxley, J.
1942. Evolution, the modern synthesis (Harper and Brothers, New York).
Macoun, J., and Macoun, J. M.
1909. Catalogue of Canadian birds (Canadian Geological Survey, Ottawa).
Mayr, E.
1942. Systematics and the origin of species (Columbia University Press, New York).
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1953. Methods and principles of systematic zoology
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(MacGraw-Hill

Book Company,

New

Mayr, E., and Stresemann, E.
1950. Polymorphism in the chat genus Oenanthe (Aves) . Evolution, 4:291-300.
Miller, A. H.
1947. Panmixia and population size with reference to birds. Evolution,

1:186-190.

Murie, 0. J.
1936. The birds of St. Lawrence Island, Alaska. Appendix V in: Archaeological excavations at
Kukulik, St. Lawrence Island, Alaska, by 0. W. Geist and F. G. Rainey. University of
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Nelson, E. W.
1887. Report upon natural history collectionsmade in Alaska between the years 1877 and 1881.
Government Printing Office, Washington, D.C.
Pitelka, F. A.
1950. Geographicvariation and the speciesproblem in the shorebirdgenusLimnodromus. Univ.
Calif. Publ. Zool., 50: I-108.
Rand, A. L.
1948. Glaciation, an isolating factor in speciation.Evolution, 2:314-322.
1954. The ice age and mammal speciationin North America. Arctic, 7:31-35.
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1946. Phytogeographic studies in the Athabaska-Great Slave Lake region, II. Jour. Arnold
Arboretum, 27:1-85.
Reilly, E. M., Jr.
1954. Bird recordsand the A.O.U. Check-list ranges.Auk, 71:156-163.
Simpson,G. G., and Roe, A.
1939. Quantitative zoology (McGraw-Hill Book Company, New York).
Stegmann,B.
1937. Fauns de l’U.S.S.R. sous la direction de S. A. Sernov de 1’Academie redige par A. A.
Stackelberg.Oiseaux,vol. 1, no. 5. Falconiformes.Moscow. (In Russian.)
Thompson,A. L.
1953. The study of the visible migration of birds: an introductory review. Ibis, 95:1&S-180.
Wetmore, A., et ol.
1945. Twentieth supplementto the A.O.U. Check-list. Auk, 62:43&449.
1948. Twenty-third supplementto the A.O.U. Check-list. Auk, 65:438-443.
Wilson, E. O., and Brown, W. L., Jr.
1953. The subspeciesconcept and its taxonomic application. SystematicZoology, 2 :97-111.
Witherby, H. F., Jourdain, F. C. R., Ticehurst, N. F., and Tucker, B. W.
1939. The handbookof British birds. Vol. 3 (H. F. and K. Witherby, London).

Department of Zoology, University of California, Los Angeles, California, March
15, 1955.