Wild Geese 9781472597427, 9781408138601, 9781408138595

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WILD GEESE

by the same author: THE WINTER BIRDS DUCKS OF BRITAIN AND EUROPE

M. A. OGILVIE

Wild Geese

Illustrations by CAROL OGIL VIE

T & A D POYSER

Berkhamsted

First published 1978 by T & AD Poyser Ltd Print-on-demand and digital editions published 2010 by T & AD Poyser, an imprint of A&C Black Publishers Ltd, 36 Soho Square, London W1D 3QY Copyright © 1978 by M. A. Ogilvie The right of M. A. Ogilvie to be identified as the author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. ISBN (print) 978-1-4081-3860-1 ISBN (epub) 978-1-4081-3861-8 ISBN (e-pdf) 978-1-4081-3859-5 A CIP catalogue record for this book is available from the British Library All rights reserved. No part of this publication may be reproduced or used in any form or by any means – photographic, electronic or mechanical, including photocopying, recording, taping or information storage or retrieval systems – without permission of the publishers.

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To Hugh Boyd

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Contents

Preface and acknowledgments

11

1

Introduction and classification

13

2

Identification

44

3

Ecology, food and feeding

79

4

Breeding

100

5

Counting, ringing and population dynamics

144

6

Distribution and status

180

7

Migration

289

8

Exploitation and conservation

309

Bibliography

331

Index

343

7

Colour plates

between pages 64 and 65 1

Heads

2& 3

Swan Goose, Pink-footed Goose, Bean Goose, Greylag Goose

4&5

White-fronted Goose, Lesser White-fronted Goose

6&7

Snow Geese

8&9

Bar-headed Goose, Emperor Goose, Red-breasted Goose

10 & 11 Canada Geese 12 & 13

Barnacle Goose, Brent Geese

14

Flying immature White-fronted Goose, Bean Goose, Lesser Snow Goose (blue phase)

15

Downy young

16

Ageing and sexing characters

List of tables

1 Percentage of blue phase Lesser Snow Geese in certain colonies

23

2 Summary of principal winter habitats and foods of geese

93

3 Latitude range of breeding distribution and normal period of egg laying

113

4 Nest sites and nest dimensions

125

5 Egg dimensions, weights and clutch sizes

126

6 I-Iatching and rearing success

133

7 Incubation and fledging periods

142

8 Census and productivity data (imaginary population)

169

9

Census and productivity data for Icelandic Pink-footed Geese

1950-76

170

10 Census and productivity data for Dark-bellied Brent Geese 1955-75 173 11 Mortality rates and life expectancies (based on Canada Geese)

178

12 Annual mortality rates of geese

179

13 Peak counts of Bean Geese wintering in Britain

184

14 Numbers of Bean Geese wintering in Netherlands

186

15 Numbers of Icelandic Pink-footed Geese wintering in Britain

193

16 Peak counts of Svalbard Pink-footed Geese wintering in Denmark

and the Netherlands, together with annual breeding success of 195 Svalbard Barnacle Geese

17 Mid-winter totals of Baltic-North Sea population of Whitefronted Geese

200

Peak counts of Greenland White-fronted Geese at Wexford Slobs, Ireland, and Islay, Inner Hebrides, Scotland

207

19 Mid-winter counts of Pacific and Interior White-fronted Geese

211

18

9

10

List of tables

20

Numbers of Icelandic Greylag Geese wintering in Britain

216

21

Numbers of pairs nesting in Lesser Snow Geese colonies

231

22

Mid-winter counts of Pacific Flyway Lesser Snow Geese

234

23

Mid-winter counts and estimated autumn totals of Central and Mississippi Flyway Lesser Snow Geese 238

24

Numbers of Greater Snow Geese counted on autumn and spring migration at Cap Tourmente, Quebec, and on wintering grounds in eastern USA 241

25

Mid-winter counts of Ross's Geese

26

Mid-winter counts of the two sub-populations of Alaskan population of Canada Geese 248

27

Mid-winter counts of Great Basin population of Canada Geese

28

Mid-winter counts of Short Grass Prairie population of Canada Geese 252

29

Mid-winter counts of Tall Grass Prairie population of Canada Geese 255

30

Mid-winter counts of Eastern Prairie population of Canada Geese

31

Mid-winter counts of Mississippi Valley population of Canada Geese 258

32

Mid-winter counts of Tennessee Valley population of Canada Geese 259

33

Mid-winter counts of Canada Geese in Atlantic Flyway and of Mid-Atlantic and North Atlantic populations 260

34

Total counts ofGreenland Barnacle Geese wintering in British Isles plus annual peak counts and breeding success figures from Islay 266

35

Numbers of Svalbard Barnacle Geese wintering in Solway

268

36

Numbers of Russian Barnacle Geese wintering in Netherlands

270

37

Mid-winter totals of Dark-bellied Brent Geese wintering in north-west Europe

272

244

251

258

38

Mid-winter counts of Atlantic Seaboard population of Lightbellied Brent Geese 276

39

Numbers of Light-bellied Brent Geese of Irish population in early winter 278

40

Mid-winter counts of Pacific Coast population of Black Brant

41

Average totals of geese in different populations during 1960s and 1970s 286

280

Preface and acknowledgments

In planning this book my original intention was to write a companion volume on geese to Ducks of Britain and Europe. However, even before I had completed the latter book I had come to the conclusion that the extra work involved in covering a wider field and producing a book dealing with the world's geese would be worthwhile. Because much of the research on geese carried out in Europe and in North America has been achieved in different circumstances and with differing objectives, bringing together information from the two continents produces more that is complementary rather than mere duplication. The North American goose biologists have one great advantage over their European counterparts, they can follow most of their geese throughout the year. In particular they can readily go to the arctic breeding grounds, and several fine long-term studies have been carried out there. Alas, for partly physical, partly financial, but mainly for political reasons, Europeans are much more restricted and far less work has been done on that crucial period of the goose's life cycle. In contrast, however, the winter studies in Europe are often more comprehensive than they are in North America. This arises because Europe is blessed with several well defined and comparatively small populations of geese which are amenable to regular and complete censusing, ringing, feeding studies, and refuge management. All of these tasks are carried out in North America, but nearly always OIl; much larger population units with the result that the information gained is unavoidably less reliable, through no fault of the researchers concerned. I believe, therefore, that I have been able to produce a better all-round picture of wild geese by bringing together information from both continents than would have been the case had I restricted myself to Europe. Little enough is known ofthe geese ofAsia but I have included them for the sake of completeness. One goose that I have not included, however, is the Hawaiian Goose. It is confined to the Hawaiian Islands and is aberrant in a number of ways when compared to other geese. Although I could have pointed out all the exceptions as I came to them, it seemed simpler to exclude the Hawaiian Goose altogether, not least because it is the subject of a forthcoming exhaustive monograph to which the enquiring reader is directed. The layout of this book follows Ducks in having some chapters in which each species, subspecies and, in this case, population, is dealt with in turn, and others of a more general character, such as that on breeding, where the complete cycle is followed through with references to the different species. I 11

12

Preface and acknowledgments

was criticised by a few reviewers of Ducks for constructing the book in this way and thus making it a little harder, but by no means impossibly so, for a reader to bring together all the facts relating to one particular species. I am completely unrepentant. I cannot conceive of anything more boring than a series of monographic chapters each dealing with a species or subspecies, which is what those reviewers seemed to be advocating. Gone would be the opportunity for comparison or contrast, for bringing out in one place the full and fascinating diversity of knowledge that there is about geese. Instead one would be forced into repetitious phrases, or even pages. One reviewer ofthis opinion was kind enough to add that there was a very full index to help the reader; I have tried to supply a similarly helpful one to this book. I wish to thank a number of people for supplying me so readily with information about geese, both published and unpublished. Chief among these are my colleagues at the Wildfowl Trust, especially George Atkinson-Willes, Dr Janet Kear, Mike Lubbock and Dr Myrfyn Owen. My former colleague, Hugh Boyd, was especially helpful with details ofnumbers and distribution of some of the North American geese. J. Chattin, E. Ferguson, Dr Jack Grieb, O. J. Merne, H. Miller, Dr Austin Reed, C. Schroeder, M. Seago,]. van Impe, and james Voelzer have been very prompt in answering my written enquiries. My publisher, Trevor Poyser, deserves grateful thanks for much hard work in unravelling my more tortuous sentences, and making sure that my meaning was clear to all and not, on occasion, to me alone. Finally, I am once again very grateful to my wife for illustrating this book, as she did Ducks.

1 Introduction and classification

The geese form one of the three familiar divisions of the family of wildfowl, or Anatidae; the others are the swans and the ducks. They are quite readily, if superficially, identifiable by their long necks and long legs, and their habits are more terrestrial and less aquatic than their relatives. The swans are long necked but short in the leg, and the ducks nearly all lack stature in each respect. The taxonomic category below the family is the tribe, of which there are ten in the Anatidae. It is at this level that the usual tripartite divisions of the wildfowl can be seen to be misleading, for it appears to give equal weight to each. However, the geese and the swans belong to the same tribe, the Anserini, while the remaining wildfowl, including the whistling ducks, sheldgeese, and all the great variety ofducks, are split between the other nine. This indicates that the swans and geese are more closely related to each other than they are to any other members of their family. The classification below the tribe is the genus, of which the Anserini contains just four: Coscoroba and Cygnus-the swans; and Anser and Branta-the geese. In Britain we have the long-standing and widely used vernacular names of 'grey' geese for Anser and 'black' geese for Branta, but these names are neither strictly accurate, nor suitable for applying to North America, where there are also white Snow Geese and dark brown Canada Geese. The separation ofthe geese into two genera is based upon a number of physical characters which differ between the species in each genus. For example, Anser species tend to have uniform plumage colouring, in greys, browns and white, while their bills and legs are pink, orange or yellow. The Branta geese have more boldly patterned black or dark brown and white plumage, and their bills and legs are always black. The plumage patterns of the downy young differ too. Anser geese have quite prominent tooth-like serrations along the cutting edge of their upper mandible which are very small or absent in the Branta species. Geese are, with the exception of introductions by man, confined to the northern hemisphere. Members of other tribes of wildfowl, including the sheldgeese and the perching ducks and geese, fill the available niche south of the equator for a large waterbird that feeds mainly by grazing on land or grubbing in marshes. Within the northern hemisphere the process of evolution, the effect ofrecent ice ages, the geography ofsuitable breeding and wintering areas, and the highly traditional nature of the use made of such areas by the geese, have combined to produce a considerable number of races 13

14

Introduction and classification

or subspecies, and discrete populations, within the relatively small number of whole species; just ten Anser and five Branta. The fossil record of birds is very fragmentary, probably because of the fragile nature of their bones. Geese are no exception and although there are some remains from the Upper Eocene Period, between 50 and 40 million years ago, which are reported to be goose-like, it was not until the Miocene Period (25 to 12 million years ago) that recognisable geese attributable to both Anser and Branta occurred. In the Pliocene Period (12 to 1 million years ago) the number of species in each genus multiplied, and in the Pleistocene Period (1 million to 10,000 years ago) most of today's species have been found. The Pleistocene Period ends with the last of the series of ice ages. These have clearly had an enormous influence on the evolution of the geese both into species and subspecies and into discrete populations. It can reasonably be assumed that the majority of the geese existing before and during the ice ages were, or became, adapted to breeding on open tundra habitat. When the ice began its long journey southwards the geese, successively, would have been pushed south before it and then allowed to spread back north in the various interglacial periods. There is also considerable evidence to show that some areas ofthe present arctic remained ice-free even during the maximum extent of the last glacial period, and that populations of birds including geese were able to survive within these refugia, as they are called, at least to breed, even if they had to migrate further south in winter, The combination of some populations ofgeese being forced to move south entirely, with others able to breed much further north, produced the necessary degree of isolation of populations which allowed evolution to take its course, leading to at first small, and then greater, differences, ending eventually in separate subspecies and finally species. Further separation ofpopulations would have occurred as tongues of ice moved south, for example, down mountain chains, thus cutting off groups of geese from each other. The principal refugia were in the region of the present Bering Sea where a land bridge joined North America with Asia, centred on Banks Island in the Canadian arctic, Peary Land in north-east Greenland, and parts of central arctic Siberia. Many smaller ice-free areas are thought to have existed but these may not have been able to support birds. When the ice retreated for the last time the geese in the refugia were able to spread east and west, while those forced to the south of the icesheets moved gradually north. Provided the separation had been for long enough, when birds from two different areas met, even though they had stemmed from the same original stock, they would have evolved sufficiently for them not to interbreed, and a new species would have been created. We can now go through the list of goose species and subspecies, and comment on their possible evolution, current status in the vexed matter of subspecies, and lesser differences such as colour phases. Evolution is not a static thing but its rate ofchange is slow compared to our span ofyears on the earth. Thus, when we wish to examine the taxonomic categories ofa group of birds such as the geese by deciding just how many species and subspecies there are, or to make judgements on their inter-relationships by placing them in a definite order, we are freezing, at a moment in time, a situation that is

Introduction and classification

15

entirely fluid and changing, albeit over a very long period. It is therefore not altogether surprising to find that people have differing views of the picture that is presented. The number ofspecies and subspecies ofgeese that exists is a matter of opinion and has fluctuated widely over the years. The classification that I follow in this book divides the geese into 15 species and is a fairly widely followed practice. However, most authorities go on to list a further 22 subspecies, plus another that is extinct, and at this point I find myself differing from them. I believe it to be farmore meaningful to deal in terms of populations of geese rather than subspecies. Most geese require active management or conservation and this only makes sense if carried out on a population basis. However, some subspecies of geese are split between several identifiably discrete populations while some populations contain birds of two or more subspecies of the same goose. In fact, the exact subspecific status of a population is of little significance, particularly as many subspecies have been described on the basis of information that can now be seen to have been completely inadequate. In the following species accounts I will point out where I differ from current taxonomic practice in the matter of subspecies and in Chapter 6 I will be discussing in depth the distribution and status of the various populations, of which over 60 have been identified, without concerning myself too much with which subspecies are involved. We start with Anser as it is considered the older more basic stock from which Branta probably evolved. It contains ten species, four of which are further subdivided into two or more subspecies.

Swan Goose Anser cygnoides This is the most primitive of the geese and although it is wrong to regard it as a direct link between the swans and the geese it does demonstrate the close relationship between the two. From this species have descended very many of our domestic goose varieties, including the familiar Chinese and African Geese.

16 Introduction and classification

MAP

1. Breeding range of subspecies of Bean Goose Anser fabalis. rossicus; §§ serrirostris; 1111 middendorffi.

fabalis and

Bean Goose Anser fabalis (Map 1) The usual division of this species is into two groups each of three subspecies, the Forest and the Tundra Bean Geese, after their apparent breeding habitat preferences. The named Forest Bean Goose subspecies are the Western A. f fabalis, Johansen's A. f johanseni, and Middendorf's A. f middendorffi, while the three Tundra Bean Geese are the Russian A. f rossicus, the Thick-billed or Eastern A. f serrirostris, and the Pink-footed Goose A. f brachyrhynchus. In the classification that I am following, however, this last goose is given full species status, and will therefore be dealt with separately. Both groups ofBean Geese show a cline of increasing size from west to east. Map 1 shows that the Bean Goose breeds right across northern Europe and Asia from Scandinavia to the Pacific coast, and it is assumed that before the last ice age there was a single species occupying this range. However, even then there was probably some differentiation into Tundra and Forest forms as, in the extremes these now take, the physiological differences would

probably have required more time to evolve than the interval since the last glaciation. The main characters are bill length and body shape, with the shorter-billed, rounder-bodied geese breeding to the north of the longerbilled, longer-bodied ones. It is an advantage to the more northerly breeders to be smaller and to have small extremities as this reduces heat loss in cold conditions.

Introduction and classification

17

It can be supposed that when the Bean Geese were driven south by the advancing ice they were separated into a number of units, and that some of these evolved further differences. On moving north again as the ice retreated these would have come into contact once more and whilst habitat differences may have continued to separate some ofthem in the summer, there has clearly been much overlap and mixing in winter. There is no complete agreement between authorities on the separation of Bean Geese into subspecies, and the Russians in particular have their own ideas. They consider it unrealistic to try to separate the Forest forms of johanseni and middendorffi, and instead lump them into a single subspecies sibiricus. The Russians also consider that in the west and centre of their country it is not practical to attempt to distinguish the Forest and Tundra Beans. Mixing between the two has taken place to such an extent that any differences that may once have existed have now virtually disappeared. This applies to fabalis and rossicus in western Russia, and to johanseni (or sibiricus) and rossicus in central northern Russia. In the east the Forest and Tundra forms apparently still remain more or less separate with middendorffi (or sibiricus) and serrirostris breeding as pure forms over quite large regions, although they too interbreed in the areas where they meet. There is an additional mixing factor in operation right across the U.S.S.R. Many immature non-breeding Forest Bean Geese migrate north in summer to special moulting areas on the arctic coast within the breeding range of the Tundra forms. It seems highly probable that at least some Tundra Bean Geese moult in the same localities and this could easily lead to them joining the Forest Bean Geese on their autumn migration. This would present strong opportunities for mixed pairings the following spring. To sum up the Bean Geese, it is still practical to deal in terms of shorter-billed, rounder-bodied birds breeding to the north of longer-billed, longer-bodied ones. There is additionally a cline of increasing size from west to east, and of decreasing bill length and body length from south to north. Although at the extremes of the range it is possible to find individuals that fit the descriptions of the different subspecies, there are vast areas of overlap where intergrading between two or more forms completely nullifies any attempt to ascribe precise geographical ranges for each subspecies. The races have been named by museum workers using very small samples ofskins. The more fieldwork that is done the more it becomes apparent that in any large flock of Bean Geese there are likely to be individuals close to one or more of these named subspecies, as well as every possible gradation between them. In such a situation it is clear that subspecific designations are not only not applicable but of no practical use.

Pink-footed Goose Anser brachyrhynchus As already mentioned this goose is often treated as a subspecies of the Bean Goose, from which it differs only relatively slightly, being rather smaller and rounder in its body and, most obviously, having pink legs and pink on its bill instead of orange. For such differences to have come about from what is

18

Introduction and classification

clearly its ancestor, the Bean Goose, separation must have occurred a very long time ago, almost certainly well before the last glaciation. The ranges of the Bean and the Pinkfoot are completely separate and this fact is one of the strongest arguments used by those who consider the Pinkfoot to be only a subspecies. One of the tests of a full species is that it can come into contact with its ancestral form and remain distinct, whereas two subspecies would readily interbreed. However, with the present distribution of the geese this is most unlikely to happen and one must use other criteria in coming to a conclusion. The Pinkfoot's breeding and wintering habitats, as well as size and coloration, differ from the Bean's sufficiently in some people's view, including mine, to warrant separation as a full species. There are three breeding areas of the Pinkfoot: north-east Greenland, Iceland, and Svalbard (Spitsbergen). As will be shown in more detail in Chapter 6, the birds breeding in the first two countries form one population which is completely separated both in summer and winter from the Svalbard breeders. However, the separation can only be comparatively recent in evolutionary terms as no apparent morphological differences have yet arisen. It has been claimed that the Svalbard birds are a little larger than the others but insufficient measurements have been obtained to show this conclusively. However, a few hundred or thousand more years of separation might be enough to produce something more definite.

White-fronted Goose Anser albifrons This species has an almost complete circumpolar distribution. The birds breeding in west Greenland have become the most differentiated and may well have been separated before the last ice age. They form a well-marked subspecies A. a.flavirostris, the Greenland Whitefront. Over the remainder of the range the usual practice until recently has been to name three races: the EuropeanA. a. albifrons, breeding from western arctic Russia east to about the Kolyma River at 1600E; the Pacific Whitefront A. a. frontalis, breeding in Siberia east of the Kolyma River to the Bering Strait, and in Alaska and arctic Canada; and the Tule Whitefront A. a. gambelli, a mysterious subspecies whose breeding grounds have never been found though it has been variously supposed that they must lie somewhere near the north coast ofarctic Canada, or perhaps in interior Alaska. The European and Pacific Whitefronts are very similar to each other but it has always been claimed that the Tule Whitefront is much larger and darker than the others. Very recently a further race, A. a. elgasi Elgas's Whitefront, has been described and named as being also large and dark but not so much as the Tule. With the evidence for the latter race already dubious there seems

littlejustification for naming yet another race on the basis ofa handful ofskins that appear to have been selected for the differences that they show. The fact that they might be intergrades or even just showing natural variation from normal seems not to have been given serious attention by those naming the race. Whitefronts are extremely variable in size and plumage colouring, as

Introduction and classification

19

anyone who has spent any time looking at a flock of them will readily admit. This must be taken into account in any examination of possible subspecies. The very latest proposals acknowledge this and have simplified matters considerably by designating the Whitefronts of eastern Siberia and western Alaska (those from the latter area migrating down the west coast of North America) as the Pacific Whitefront frontalis, reserving gambelli for the birds breeding in eastern Alaska and arctic Canada. These latter average darker and larger thanfrontalis and almost certainly include within their number birds which in the past have been claimed as Tule Whitefronts. The vast majority of these birds migrate to winter on the Gulf Coast but a small offshoot crosses over to winter in California within the winter range offrontalis. Here, where the larger, darker birds exist in close proximity with the smaller, paler frontalis, is supposed to be the wintering area of the Tule Whitefront. But could it not be that the contrast between the two is what is striking to the observer, over-emphasising the difference which exists? The new vernacular name ofgambelli is the Interior Whitefront, reflecting its range and migration route. The mystery races, the Tule and Elgas's Whitefronts, with their unknown breeding grounds and supposedly very tiny and threatened populations, are dismissed as invalid. This seems to make very good sense to me, though it may take a little while before those still searching for the latter's breeding grounds are persuaded that they do not exist except as part of the range of the extremely numerous Interior Whitefront. There is a strong parallel here with the so-called Giant Canada Goose (see below) where selection of extreme individuals has been used to justify the naming of a race which, however, does not stand up to close scrutiny of large numbers of birds. Returning to frontalis, it remains doubtful whether this is fully separable from albifrons. Certainly it is extremely improbable whether a Pacific Whitefront appearing in a flock ofEuropean Whitefronts, or vice versa, could be picked out, or even identified if it were shot. They intergrade where they meet in Siberia and individual variation in both races produces birds which could be mistaken for the wrong one. The bill of the Pacific Whitefront averages longer than that of the European race, but there is overlap and this seems slight evidence on which to base a subspecies. Additionally, there is evidence that as one goes from west to east across the range of the species in Russia, so the birds get slightly larger and slightly paler.

Lesser White-fronted Goose Anser erythropus Undoubtedly this small goose sprang from the same stock as the Whitefront. The two overlap slightly in range but are separated by different habitat requirements. The Lesser Whitefront breeds, like the Whitefront, right across northern Europe and Asia, but always to the south in the scrub and forest zone and hardly at all in the tundra. The pair form a good example of Bergman's Rule which states that larger forms of the same species, or the larger ofa pair of similar species, will live to the north ofthe smaller one. The explanation for this is that as body size increases the ratio of surface area to

20

Introduction and classification

volume decreases, so that a larger bird has a smaller relative body surface through which to lose heat. Being more thermally efficient therefore than a smaller bird it is able to live more satisfactorily in colder climates. It can be assumed that separation of the Lesser Whitefront from a common Whitefront-type stock took place a very long time ago and certainly well before the last glaciation. No apparent morphological differences have been found in birds from different parts of the range.

Greylag Goose Anser anser There are two described races of the Greylag, the Western A. a. anser and the EasternA. a. rubrirostris. As with the Bean Goose typical specimens ofthe two races look very different and are quite easily distinguished, but there is virtually complete gradation between them. It has been suggested that more races may be named in the future but it would seem much more satisfactory to treat the species as showing a clinal variation across its range and not try to lay down arbitrary divisions. Certainly there are separate populations with their own apparently discrete breeding and wintering ranges but they have not been separated for long enough to evolve into different races except for the two named. The differences between a typical Western and typical Eastern Greylag (with the former being smaller, darker and having an orange not pink bill) probably arose during separation at the time of the ice ages but the birds have rather spoilt things by coming together again, interbreeding and producing all shades of plumage and bill colour between the two races.

Bar-headed Goose Anser indicus This localised species is classed as an aberrant grey goose or Snow Goose. It has obvious affinities with the latter but equally any common ancestor lies a long way back in time. The relatively restricted range has inhibited the formation of any possible subspecies.

Emperor Goose Anser canagicus This species is much more similar, morphologically, to the Snow Goose than is the Barhead, and it too must have come from a common ancestor. Its range is restricted to eastern Siberia and western Alaska and, like many species ofbirds that now nest on both sides ofthe Bering Sea, this can be taken as evidence that they survived the last ice age in the Beringia refugium. This

was an ice-free area that included a land bridge joining north-east Asia with north-west North America. The lack ofspread from this area since the retreat of the ice may be due to competition with more successful species.

Introduction and classification

21

Snow Goose Anser caerulescens The Snow Goose is well differentiated into two races: the Lesser A. c. caerulescens and the Greater A. c. atlanticus. These form a convincing demonstration ofBergman's Rule with the latter breeding to the north of the former. Subspeciation is apparently complete with virtually no range overlap and no interbreeding. The Greater Snow Goose forms a single population with one wintering area and there is no suspicion offurther evolution within it. The Lesser Snow Goose, by contrast, breeds in two well separated areas, the eastern Canadian arctic, and north-east Siberia. However, some mixing of populations takes place on the wintering grounds and there is no apparent difference between birds of the two regions. The most fascinating aspect of Lesser Snow Goose evolution is the occurrence of the blue plumage phase, the birds having just a white head and neck, the rest of the body being blue-grey. These birds are usually called Blue Snow Geese, or just Blue Geese. For a great many years it was thought that they were the immature form of the Snow Goose and then, later, they were thought to be a separate species. Suspicions that they might be no more than colour phases of the same species were voiced many years ago but it was not until 1961 that it was convincingly demonstrated that this was indeed so. However, old beliefs took a long time to die and papers have been published quite recently trying to show that the two phases are different species. The evidence, though, is weak compared with the weight of data stacked against it. The two colour phases of the Lesser Snow Goose are dependent upon the operation of a single pair of genes. It has been shown that the blue phase is dominant over the white, and also that there is no sex-linkage. So, representing the two genes by the letters 'B' for the dominant blue, and 'b' for white, a male or female blue phase goose would have the genes 'BB' or 'Bb', while white phase birds would have 'bb'. If both birds of a pair are homozygotes, having the two genes the same, either 'BB' or 'bb', then all their offspring will be blue or white respectively. But ifone ofthe parents is a heterozygote, having a mixed pair of genes, 'Bb', then on mating with a homozygote, some ofthe goslings will be blue and some white, in each case a majority of the former. Although it is common to think solely in terms of the two colour phases, blue and white, there are in fact a number of intermediate forms. A total of seven have been described (numbered from 1 to 7 in increasing blueness). Numbers (1) and (2) are wholly white plumaged except for their black primaries, the only difference between them being that (2) has a certain amount ofdarkening on the alula or bastard wing, the small tuft offeathers on the outer edge of the wing near the bend. Both these forms are homozygotes with the genes 'bb'. Numbers (3), (4), (5) and (6) have dark wings, flanks and back, and decreasing amounts of white on the underparts, while in form (1) this is reduced to an occasional white feathering round the vent. Virtually all offorms (3) and (4) will be heterozygote 'BB', while forms (5), (6) and (7) will

22

Introduction and classification

contain increasing proportions ofthe homozygote 'BB' as well as some 'Bb'. The great majority if not quite all of (7) will be 'BB'. The divisions are fairly arbitrary, and in some cases the plumage differences can only be seen in the hand, but essentially (1) and (2) can be described as white phase while all of (3) to (7) are blue. The downy young Lesser Snow Geese also show polymorphism of a similar kind, but only five forms have been described of which three are genetically white, and two are blue. These, like the adult plumage, will be covered in more detail in the next chapter, on identification. Another interesting aspect of the colour phases is that pairings of two white birds or two blue birds are commoner than one white with one blue. There is in fact a strong assortive mating so that like mates with like. This is true not only for the broad groupings of white and blue, but also shows up in each of the seven colour forms mentioned. However, as well as this assortive mating there is the preference of a bird to mate with a bird that looks like its parent. Offspring of a non-assorted pair may mate with either a blue or a white goose. It has been shown with captive geese that there is a definite attraction, probably based on the initial imprinting of the gosling on its parents, for the parents' colour. To add another dimension to the study some of the goslings and some of the adults were dyed pink! The preferences still held good. One other feature is that the number ofblue phase males mated with white phase females was found to be significantly greater than the other way round. It is assumed that the male goose plays the dominant role in the selection of a mate and therefore in order to have the difference mentioned one has to have more blue males with a white parent, than white males with a blue parent. And in the study in question the former were three times as common as the latter. However, this particular colony had a predominance of white phase geese. In a colony where blue phase, geese were in the majority one would expect the ratios to be the other way round, with white goslings with a blue parent outnumbering blue goslings with a white parent. Thus, in mixed pairings those in which the male was white and the female blue would be more numerous. These pairings dependent on parent colour phases raise the whole question ofimprinting, namely the attachment ofa newly hatched gosling, or other young bird or animal, for its parent; or in abnormal circumstances for the first, usually moving, object that it becomes aware of It is not known in the case of these Snow Geese whether the young imprint on both parents or just the female, nor whether the colour of the other goslings in the brood has any significance in subsequent pairing preferences. Imprinting is certainly less than absolute as it can quite easily be changed within the first week of a gosling's life, and can still be broken at a later age.

It has been mentioned above that some colonies have more white than blue geese and vice versa. In fact there is a cline ofincreasing blueness as one moves from west to east across the Canadian arctic. Colonies in the eastern Siberian arctic have no blue phase geese at all nor do there appear to be any among the small number of Snow Geese breeding in northern Alaska. The westernmost colony in arctic Canada, at Kendall Island, has no blue geese, but the next ones

Introduction and classification

23

east, a few hundred kilometres away at Anderson River, and on Banks Island, contain a handful of blue phase birds. Then at Perry River several hundred kilometres still further east there are about 9%., increasing in the colonies around Hudson Bay to about 25 %. The colonies on Southampton Island have up to 35% blue phase birds while when one reaches Baffin Island the proportion shoots up to 41 % at Koukdjuak, 61 % at Cape Dominion, and 81 % at Bowman Bay (see Table 1). Not only is there this well-marked cline ofincreasing blue phase birds from west to east but it is in a state of change. In the 1950s and early 1960s the percentage of blue phase birds was increasing and spreading westwards at a rate of about 2 % per annum. However, this trend has now stopped and even reversed in some areas as the Table shows. The spread of the blue phase Snow Goose corresponds with a general warming of the Canadian arctic that has been taking place in the last few decades. It is of considerable interest that the apparent halt in the increase and spread has coincided with an apparent cooling of the arctic in the last five or ten years. The exact way in which the weather benefits one or other phase has been the subject of much detailed study. It appears that the blue and white phase birds are affected differently by the prevailing weather conditions on the breeding grounds, both on their arrival in spring and later in the incubation and rearing periods. Firstly, white phase geese have a marked tendency, presumably of genetic origin, to start to lay their clutches earlier than the blue phase geese in the same colony. Secondly, and in slight contradiction to this, the laying period ofall the white phase geese in a colony is actually longer than that of the blue phase birds, starting earlier and carrying on later. TABLE 1: Percentages of bluephaseLesserSnow Geesein certain colonies, showing west-east cline oJ increasing blueness and changes during the lastJorty years

Colony Wrangel Island Kendall Island Banks Island McConnell River La Perouse Bay Boas River East Bay Cape Henrietta Maria Bowman Bay Cape Dominion Koukdjuak Plain

Lat. 71 N 69N 72 N 60N 58 N 63 N 64N 55 N 65 N 66 N 66 N

Long. 179 W 135 W 125 W 94W 93W 85 W 81 W 82 W 74W 74W 73 W

19305

19405

19505

0