Biological J ~ u m ofthe
l
Linnean SocieQ (1992), 47: 325-335. With 2 figures
Species and pseudospecies: the structure of
crossbill populations
ALAN G. KNOX
Buckinghamshire County Museum, Tring Road, Halton, Buckinghamshire HP22 5PJ
Received 6 June 1991, accepted for publication 30 Jury 1991
The holarctic crossbills Loxia have often been regarded as one of the classic examples of avian
panmixia, despite a large number of named races, clines and other geographic variation. There are
also reports of two or more ‘subspecies’ nesting sympatrically, without interbreeding. Crossbills feed
almost exclusively on conifer seeds. Eruptions occur at times of cyclical cone crop failures; the birds
involved may then breed in new areas for one or more years. Rather than being nomadic in their
movements, to explain the clinal and/or area effects, it is suggested that erupting birds are
reasonably faithful to core breeding areas, to which some subsequently return. Genetic continuity
within a species is maintained through adjoining or overlapping core breeding areas. Although
normally connected by intermediates, some populations apparently do not interbreed when they
come together temporarily during irruptions. At such times, they behave as separate species, to
which the term pseudospecies is applied. Mechanisms promoting rapid speciation and founder
effects are discussed.
KEY WORDS-Crossbills
sympatric breeding.
- evolution - founder effects
-
Lo&
CONTENTS
Introduction . . . . . . . . . . . . .
The structure of crossbill populations . . . . . . .
Species and pseudospecies . . . . . . . . . .
The advantages ofreturning to the core breeding areas.
. .
Locally adapted populations, founder effects and rapid speciation
Acknowledgements
. . . . . . . . . . .
References
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pseudospecies - speciation
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INTRODUCTION
Strong geographic variation is usually recognized by named subspecies. It is
also frequently interpreted as an indication of reduced gene flow between
populations, and as a possible early (and still reversible) stage in the process of
allopatric speciation. Among the higher vertebrates most, if not all, speciation
takes place by this mode (Futuyma & Mayer, 1980; Findlay, 1987; also see
Payne, 1973; Grant, 1986; Grant & Grant, 1989 for discussion of possible
exceptions in birds). It, therefore, comes as a surprise to find a strongly polytypic
species, the red crossbill Loxia curvirostra, in which it is frequently claimed that
two or more ‘subspecies’ breed alongside one another in many parts of the range,
apparently without hybridizing (e.g. Griscom, 1937; Kemper, 1959; Phillips,
1974; Monson & Phillips, 1981; Peterson, 1985; Dickerman, 1986a). Such
002P4066/92/110325
+ 11 SOS.OO/O
325
0 1992 The Linnean Society of London
326
A. G . KNOX
evidence would suggest that species were involved, not subspecies (Knox, 1975,
1976, 1990; Groth, quoted by Payne, 1987; Groth, 1988).
Morphological variation within the genus Loxia is very restricted. The species
and subspecies are small-bodied, with slender, straightish bills, or large-bodied,
with heavy, steeply-curved bills, or intermediates between these extremes (Knox,
1975). Their plumages are very similar and, apart from one species which has
white wing-bars, none is completely diagnostic. The birds feed in coniferous
trees, their crossed bills being adapted for extracting seeds from fully or partly
closed cones. They eat little else. Variation in bill shape between the species and
subspecies has been correlated with the hardness of the cones of the main tree
species on which each form feeds (Lack, 1944a, b; Benkman, 1987; Massa, 1987).
A relationship in western palaearctic forms between bill curvature and the
length of cone scales on principal food trees (Massa, 1987) breaks down if the
sample is extended to include the large-billed populations which feed on the
small, hard cones of Scots pine (Knox, unpublished).
Because the seed crops of many coniferous trees vary considerably from year to
year, some crossbills undertake irruptive movements. Whole or part populations
can leave their breeding areas and fly for hundreds or even thousands of
kilometres (Reinikainen, 1937; Newton, 1970). If the birds find suitable food on
their migrations, they may settle down and nest for one or more seasons.
Sometimes new, permanent breeding groups are founded in this way but usually
these new areas are vacated after a variable length of time (Knox, 1986). Some
of the adult birds eventually return to the main breeding areas (Newton, 1970).
I n some seasons, two or more ‘subspecies’ may breed sympatrically in parts of
the crossbill’s range (refs above), or the same localities may be used for breeding
by different ‘subspecies’ in different years (e.g. Griscom, 1937; Dickerman,
1986b, 1987).
Many authors have commented on the unpredictable and nomadic
movements of these birds. As Buckley (1987) pointed out, crossbills have “often
been cited as a good example of a passerine approximation to true panmixia”.
Clearly, something is wrong with our understanding of crossbill systematics and
evolution. The degree of differentiation found between some populations seems
to be incompatible with wide gene flow and a highly dispersalist strategy.
I n this paper an unusual population structure for crossbills is suggested on the
basis of which a novel mechanism of speciation could occur.
Throughout, the name red crossbill is used when discussing the species
L. curvirostru, which has a holarctic distribution. Common crossbill refers to the
nominate subspecies L. c. curvirostra, which is widely distributed in the
palaearctic.
THE STRUCTURE OF CROSSBILL POPULATIONS
The common crossbill shows weak clinal variation in colour and size from
Europe to Siberia (Vaurie, 1959). Another cline is found between northern
Europe and the Mediterranean (Massa, 1987). Within most crossbill subspecies,
birds collected at one locality at the same time often resemble each other in
colour and/or bill shape more than they resemble specimens taken elsewhere or
at different times (Monson & Phillips, 1981; Knox, unpublished). Irrupting
common crossbills a t Fair Isle, Scotland, had thinner bills in 1953 than in 1963
SPECIES AND PSEUDOSPECIES
327
(Davis, 1964), and birds irrupting into Belgium in 1979 had deeper bills than
those in 1983 (Herremans, 1982, 1988). There were no significant differences in
wing-length at either locality. Recent work has shown that common and parrot
crossbills L. pytyopsittucus each have several distinct call-types. Although the
pattern of variation is still being investigated, only one dominant type is usually
found in any one area, and there is some form of geographic variation
throughout Europe (C. S. Adkisson; Knox unpublished). Particularly in the
U.S.A., where other subspecies occur, two or more subspecies have been
reported nesting alongside one another without interbreeding (Griscom, 1937;
Kemper, 1959; Phillips, 1974; Monson & Phillips, 1981; Peterson, 1985;
Dickerman, 1986a), and nesting birds in one area were found to belong to two
different call-types with different biometrics (Groth quoted in Payne, 1987;
Groth, 1988).
While some of these data are preliminary, it seems that clinal or area effects
(White, 1978) occur within the population structure of some crossbills, rather
than the claimed panmixia. The many ‘subspecies’ of the red crossbill (most of
whose characters are only poorly defined), and the apparent intrasubspecific
variation, both suggest that the species may be divided into populations with
broadly circumscribed core breeding ranges, from which the birds periodically
erupt. In order for the population structuring to be maintained, some of the
birds which erupt, or their offspring, may subsequently return to their original
core ranges after one or more years.
Although there is no direct evidence of this from marked birds, the existing
data are not in conflict. A large number of common crossbills were ringed in
Switzerland during irruptions in 1959 and 1963. All but one of the 17 birds
recovered within one year of ringing were found to have continued their
irruptive movements to the south or west (the other was found in Germany).
The only four recovered more than one year after ringing were all in northeastern Russia (Newton, 1970)-a relatively small area, considering the size of
the total range of the common crossbill.
For this theory to work, crossbills would probably require inherited migratory
patterns, as have been demonstrated in different populations of Sylvia warblers
(see review by Berthold, 1984). That some crossbill movements follow a pattern,
and are not just random wanderings, as has been claimed so often, is illustrated
by the ringing analysed by Newton (1970, see above), and Paevskii (1973). The
latter reported 13 recoveries of crossbills marked on irruptions at the Courland
Spit in the southern Baltic. One juvenile, found only 200 km away less than a
month after ringing, is not considered here. The only recovery from an irruption
in 1959 was made over four years later in south-east France: the movements of
this bird over the intervening years are unknown. The remaining 11 birds, from
irruptions in 1962, 1963 and 1966, all moved between 1250 and 1580 km southwest, and all were recovered within an arc spanning just 10 degrees (Paevskii,
1973). Even if the bird ringed in 1959 is included, the arc spans only 20”.
Though the origins of the birds on these different irruptions are not known, this
suggests a surprisingly narrow and straight migration route. Not all crossbills
irrupting through the southern Baltic move in the same direction, however
(MatthC, 1985).
First-year common crossbills as well as adults are known to take part in
eastward ‘return’ movements (P. J. Belman, personal communication; Newton,
328
A. G. KNOX
Figure 1A. Map showing the main breeding range of the red crosshill Loxiu curuiroslra in North
America (shadedj. T h e names of subspecies currently recognized are indicated, but their
distributional boundaries are only poorly known. Formal recognition of some subspecies may he
inappropriate. Thc characters of the described taxa vary from small birds in the Pacific Northwest
through larger forms in the Rocky Mountains of the western United States, to the largest in Central
America (not shown). They also become larger from west to east across the continent. T h e
distribution of the birds is highly dynamic. Crosshills erupt within and from the areas shown, and
irrupting birds have been found as far away as Texas and Florida. ( M a p based on National
Geographic Society, 1983; Dickerman, 1986a, 1987; Payne, 1987.) B. Although genetic continuity
within the main, contiguous breeding ranges would he maintained through neighbouring
populations, distant groups of crossbills may be quite distinct (as in ring-species). After irruptions,
groups of crosshills often nest for a year or two in areas far from their core ranges, before moving on
or returning to the areas from which they had departed. Sometimes, irrupting crosshills do not
SPECIES AND PSEUDOSPECIES
329
1970; Schloss, 1984). If migratory directions and distances were even partly
inherited, many young birds could return to the core areas of their own
populations. It would be possible to test migratory behaviour experimentally.
The migration pattern clearly contains much variation and flexibility.
Irrupting birds may not encounter suitable food in the direction in which they
first move. They apparently continue migrating in the same direction until they
do so, or may deviate from the original course. Some irrupting crossbills or their
offspring give rise to new populations in a dispersal strategy appropriate for birds
feeding on conifer seeds. New areas of habitat become available all the time, as
others are lost. Today, the prime causes of this are the actions of man,
particularly his forestry practices. In the past, patches of established forest will
have become unsuitable from time to time, through natural disasters such as
wind-throw, fire, avalanche and disease, only to regrow and mature again
decades later. Pollen analysis has shown how volatile the distribution of many
conifer species has been in the last millennia (Huntley & Birks, 1983; Peterson,
1983; Gear & Huntley, 1991). The crossbills are suitably adapted to take
advantage of these changing circumstances.
SPECIES AND PSEUDOSPECIES
The North American red crossbills illustrate some aspects of crossbill biology
particularly well. They have a wide distribution, their main breeding range
running down the western side of the continent from Alaska to Nicaragua, across
the boreal forest zone to Newfoundland, and along the Appalachians (Fig. IA).
During irruptions, birds have reached from Texas to Florida, and have nested
over much larger areas than are shown on the accompanying figures. Griscom
(1937) recognized no fewer than eight subspecies, and another two have been
described since (Monson & Phillips, 1981). Together, they vary from small birds
within thin bills in the Pacific Northwest, through larger forms in the Rocky
Mountains of the western United States, to the largest race, mesamericana, in
Central America. The birds also become larger eastwards across Canada and the
north-eastern United States, to Newfoundland, where a large, deep-billed race is
found.
The boundaries of the main breeding ranges of most of these subspecies are
only poorly known. As in the Old World, irrupting birds can travel thousands of
kilometres (Payne, 1987), and then nest temporarily in areas where crossbills are
normally rare or where other forms occur. While it has been claimed that some
of the races are well defined (e.g. Dickerman, 1986a), the only published
biometric study of North American crossbills suggests otherwise (Payne, 1987).
Recent reviews have reduced the number of subspecies to six or seven
(Dickerman, 1986a; Payne, 1987; Fig. lA), but there is still no consensus
interbreed with local birds when they come into contact. For example, following irruptions,
crossbills from western Canada (white arrow) and/or the Rockies (dark arrow) may not hybridize
with each other or local birds in the Great Lakes area. When they are in their core areas, the
populations to which these birds belong would normally be regarded as conspecific subspecies, or
segments of a cline. While temporarily sympatric, they may act as full species for a year or two, but
the phenomenon is only transient (pseudospecies).
330
A. G . KNOX
(Browning, 1990). Clines with superimposed area effects may be a more
appropriate description of the variation, although the clines may be sharply
stepped in places.
The manner in which crossbills have been collected for museums conceals
their real pattern of variation. I n most species where there is some form of
clinal variation, a sample from a discrete part of the cline will often appear to
belong to a well-defined form with limited variation, particularly when
compared with birds from other, discrete parts of the cline. Thus, depending on
the gradient of the cline, flocks of crossbills from areas only a short distance apart
could appear sufficiently distinct to warrant subspecific recognition. This may be
part of the reason that so many subspecies have been proposed. Compared with
most other species, however, the overall pattern of crossbill variation is obscured
by the irruptive behaviour of the birds and their propensity to breed temporarily
in areas far from the core range.
Following irruptions, there have been a number of reports of two or more
‘subspecies’ of crossbill breeding alongside one another without hybridizing (refs
above). To give one example: after a massive invasion, at least two, and possibly
three, forms of crossbill were found breeding in the same part of New York State
during the winter 1984-1985. Each type showed differences in calls, bill size and
food preferences and there was no interbreeding (Peterson, 1985).
This sort of evidence suggests that birds from widely different core populations
may not interact much while on irruption, or interbreed to any great extent
while one or both are in temporary nesting areas, perhaps because of differences
in their vocalizations and/or habitat (cone) preferences. While genetic continuity
would be maintained in the adjoining, overlapping or intergrading core
breeding areas, different populations might behave as separate species during
irruptions (Fig. 1B).
I t would seem suitable to refer to these temporarily sympatric, reproductively
isolated populations as pseudospecies*, use of the term only being appropriate
for the duration of the sympatry. T h e situation is similar to the condition found
in ring-species, but far more dynamic. The ‘ring’ only closes temporarily when
crossbills erupt from one part of their range into another and breed for a short
while without hybridizing with the local birds. The ‘ring’ reopens when the
invaders depart or die out. I a m unaware of any other bird in which this form of
population structure may exist, although the biology of some other irruptive
species might repay examination.
Crossbill populations which appear to be reproductively isolated from all
invading or sympatric populations would be better considered as separate
species. The Scottish crossbill L. scotica was recently shown to fulfil these
conditions (Knox, 1990), and is now regarded as specifically distinct from
L. curuirostra, with which it had been lumped. This may yet prove to be the
appropriate treatment for some other crossbill taxa, particularly those that are
geographically isolated. However, there is as yet no evidence that each
population of red crossbill (as described above) represents a separate species,
other than their behaviour during irruptions. Within their main ranges, they
would appear to act as one species, although detailed information is lacking.
*Dobzhansky (1972) previously applied the term pseudospecies to asexually reproducing invertebrates; there
seems little chance that its use in the present context would cause confusion.
SPECIES AND PSEUDOSPECIES
33 1
THE ADVANTAGES OF RETURNING TO THE CORE BREEDING AREAS
Two- or three-year cycles in the movements of northern seed-eating birds are
well documented in the Nearctic. They apparently result from corresponding
cycles in the seeding of trees in the boreal forests (Bock & Lepthien, 1976;
Widrlechner & Dragula, 1984; Yunick, 1984, 1988; Larson & Bock, 1986).
Though similar movements occur in the western palaearctic (Svardson, 1957;
Perrins, 1966; Newton, 1972; Burton & Holder, 1986), they are less well marked
and, consequently, rarely recognized. In northern Europe, spruce cone crops
vary considerably, often with an apparent 2-4 year cycle in abundance. A poor
year is frequently followed by a good crop next season (Reinikainen, 1937;
Svardson, 1957; Hagner, 1965; Lindgren, Ekberg & Eriksson, 1977; Fig. 2).
Crossbill migration strategy is an extreme adaptation to this variation in the
birds’ food supply. When an irruption is triggered by an oncoming poor cone
crop, some or all of the birds depart. The predictability of the cycle and its
length varies from place to place (Svardson, 1957; Hagner, 1965) but, over
much of northern Europe, it is unusual for two poor seasons to occur
consecutively. Thus, it would be advantageous for at least some of the irrupting
birds to return to the core areas after one (or two) breeding season(s) away,
particularly if the temporary area is unsuitable for a longer stay for any reason.
In this way, many of the birds could trade the cost of migration and the benefit
of possible breeding in a new area against the possibility of starvation and the
certainty of an unsuccessful breeding season in the core area. After one (or
occasionally, two) year(s) away, they are almost assured of a good cone crop in
the original core area in the following season. Further wandering in the hope of
finding a good cone crop in another new area, not already occupied by crossbills,
would likely present a higher risk. A return to the main breeding range, but not
to the original core area, might similarly present the risk of finding a cone failure
there, or another crossbill population already established. This would make
frequent mixing and interbreeding of birds from one area with those from some
120
c
,
1960
I
ro
Figure 2. The mean number of Norway Spruce Picea abies cones per tree in southern Sweden,
1909-1967. (Based on Hagner, 1965; Gotmark, 1982.)
332
A. G. KNOX
distance away unlikely, and this view is consistent with the geographic variation
found in crossbills.
The extent of the local core area may vary considerably, depending on the
predictability of local cone crops. Where the food supply is very reliable, the
birds will be almost sedentary (e.g. in Cyprus: the endemic subspecies
L. c. guillemardi is restricted to an area of about 30 km2 in the Troodos
Mountains). Where food is less predictable, the core areas are likely to be much
larger.
Less is known about the two-barred (or white-winged) crossbill L. leucoptera
than the other species of Loxia. Only one subspecies is found across the boreal
forests of the Nearctic, another has a wide distribution in the palearctic taiga
and a third is sedentary on the island of Hispaniola. Core areas in the first two
subspecies may be very large, for the birds appear to show less geographic
variation than is found in L. curvirostra.
LOCALLY ADAPTED POPULATIONS, FOUNDER EFFECTS AND RAPID SPECIATION
The strategy just described would permit the development of locally adapted
populations, or clines, in response to local conditions, although frequent
irruptions would presumably promote a degree of variability, Were, for example,
an irruption of common crossbills to occur into a pine area, the birds with
slightly heavier bills would presumably be a t an advantage over those with
smaller bills (Benkman, 1987). Assuming that components of bill size/shape are
inherited in crossbills (as they are in other species, e.g. Grant, 1986), offspring of
the irrupting birds could be expected to have heavier bills, on average, than the
adults. Some or all of the larger-billed birds (adults and/or first-years) might
then settle and create a new population (or reinforce an old one) with selfselected founder effects. Significant numbers of birds remaining selectively in
areas where their bills were most suited to the cones available could have
profound effects on the origin and maintenance of geographic variation.
However, if the remainder of the population left the new area, it would consist of
smaller-billed birds, on average, than the ones which originally erupted. O n the
other hand, if all the population moved on or returned to the core area, they
would have larger bills, on average, than the birds which erupted. The reverse
would occur in the event of irruptions into areas with soft-coned conifers.
Stabilizing selection in the core area would tend to reduce these effects over
subsequent breeding seasons. Variation within and between irruptions would
facilitate founder effects in new populations.
A clinal chain or network of core breeding areas, each genetically connected
to its neighbours but reproductively isolated through ecology or behaviour from
the more distant populations, would lend itself to mechanisms of rapid
speciation. Changes in climate, loss of habitat, or any factor leading to local
extinctions could remove intermediate populations and leave genetic breaks in
former clines. If an extinction occurred quickly, speciation could take place with
great speed. The populations on either side of the break could then come
together as full species. This is similar to speciation in other birds, but differs in
tempo and relies on reproductive isolation of non-adjacent sections of the cline.
The mechanisms discussed depend on the differences in crossbill body size, bill
size and bill shape being mainly inherited. Benkman (1988) and Benkman &
SPECIES AND PSEUDOSPECIES
333
Lindholm (1991) have demonstrated the extent to which crossbill bill length
could be influenced by different food types and wear, but these factors do not
appear to affect the critical parameters of the radius of the culmen, bill height or
bill width (Knox, unpublished). Studies on other seed-eating birds have
emphasized the genetic components of bill shape (e.g. Grant, 1986). Recent
work has shown that some biometric variation in birds is environmental (James,
1983; Boag, 1987;Jehl, Francine & Bond, 1990), but a great deal is known to be
genetic (e.g. Alatalo & Gustafsson, 1988).
While much of this paper is necessarily speculative, it offers a model of
crossbill population structure, many parts of which are amenable to
investigation. It also explains some of the otherwise anomalous features in
crossbill biology.
ACKNOWLEDGEMENTS
This paper is dedicated to Professor Vero Wynne-Edwards, on the occasion of
his 85th birthday. Professor Wynne-Edwards first introduced me to some of the
questions surrounding crossbills.
I have benefited from practical help from, discussion or correspondence with a
number of people, including Professors A. J. Cain, F. W. Robertson and V. C.
Wynne-Edwards, Curt Adkisson, Sam Alexander, Jeff Groth, Jon Hardey,
Pekka Helle, Marc Herremans, Brian Huntley, Roxie Laybourne, Bruno Massa,
the late Desmond Nethersole-Thompson, Ian Newton, Tim Parmenter, Nick
Picozzi, Robert Rae, Lars Svensson and Adam Watson. P. H. Greenwood,
J. J. D. Greenwood, I. Newton, A. M. Reynolds, N. Picozzi, D. W. Snow and
an anonymous reviewer read and commented on the manuscript.
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