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The Breeding of theBlackbird, Song Thrush,and Mistle Thrush in GreatBritain Part II. Clutch-SizeD. W. Snow aa Dept. of Zoological Field Studies, EdwardGrey Institute, Oxford

Version of record first published: 16 Jun2009.

To cite this article: D. W. Snow (1955): The Breeding of the Blackbird, SongThrush, and Mistle Thrush in Great Britain Part II. Clutch-Size, Bird Study,2:2, 72-84

To link to this article: http://dx.doi.org/10.1080/00063655509475814

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DISEASES IN WILD BIRDS 2(2)

aviseptica on culture and there was a history of heavy losses onthe moor _ in Scotland from which they were sent. These grousealso had heavy nematode worm burdens. Unfortunately no furthercases were sent into the laboratory.

ACKNOWLEDGEMENTS

Thanks are due to members of the British Trust for Orni-thology who provided the specimens and to the various labora-tories who collaborated in this survey. I am also grateful to MissRobb for technical assistance.

REFERENCE

JENNINGS, A. R. 1954. Diseases in wild birds. J. Comp. Path., 64:356-9.

THE BREEDING OF THE BLACKBIRD, SONG THRUSH,

AND MISTLE THRUSH IN GREAT BRITAIN

PART II. CLUTCH-SIZE

by

D. W. SNOW

(Edward Grey Institute, Dept. of Zoological Field Studies, Oxford)

Received 19 December, 1 954

(A Publication of the British Trust for Ornithology)

SUMMARY

I. The methods used in extracting clutch-size data from nest record cards ofthe Blackbird, Song Thrush and Mistle Thrush are described.

2. In the Blackbird and Song Thrush, mean clutch-size varies seasonally,reaching its highest value respectively in the third week of May and at the endof April and beginning of May. The mean clutch-size of the Song Thrush issteadier throughout the season than that of the Blackbird. There are as yettoo few cards for the Mistle Thrush for a detailed analysis of seasonal variationin clutch-size.

3. There is a tendency for the clutch-size of Blackbird and Song Thrush tovary in a parallel manner in different years. It is suggested that these annualdifferences are correlated with annual differences in temperature.

4. Period for period, clutch-size is lower in Scotland and Northern Englandthan in Southern England at the beginning of the season, and higher at the endof the season, but when clutches from the whole breeding season are takentogether there are no consistent differences between north and south. Clutch-size is rather low in Great Britain compared with the other parts of north-western Europe that have been investigated. -

5. The significance of these seasonal and geographical differences in clutch-size is discussed.

INTRODUCTION

This is the second part of an analysis of nest -records of thethree common British thrushes, of which Part I, by M. T. Myres,has already been published in this journal (antea, pp. 2-24). Thefull details for this part, as for Part I, have been deposited at theEdward Grey Institute.

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In investigating regional differences in clutch-size, the samethree main geographical divisions have been used as in Part I, butSouthern England has not been subdivided.

METHODS

Nest record cards give clutch-sizes with various degrees ofaccuracy. Three main categories need to be considered :

(I) If the nest was visited daily over the end óf the laying,period and beginning of the incubation period, clutch-size can bedetermined with certainty, disregarding some possible but unlikelysources of undetectable error.

(z)- If the nest was visited during incubation, and it is quitecertain that laying had finished (e.g. two visits showing an un-changed clutch, or one visit a sufficient number of days after anincomplete clutch had been recorded), the clutch-size so obtainedis probably accurate, but there is a possibility that one or moreeggs had been lost before the observer visited the nest, thepossibility increasing with the degree of incubation.

(3) If the nest was visited only once, but the parent wasdefinitely incubating, the clutch-size so recorded is probablyaccurate, but there is a possibility not only that an egg had beenlost previously but also that the bird may have been incubatingan incomplete clutch.

In analysing the cards, clutches of the first, most accurate typewere kept separate, while categories (2) and (3) were combined.Unfortunately there are so few cards of the first category (about5% of the total) that they cannot be treated quantitatively bythemselves. It has thus been necessary to use clutches of all threecategories. Since part of the clutch is lost not infrequently withoutthe parents deserting, and since thrushes do sit on, and partiallyincubate, incomplete clutches, the mean figures for clutch-sizegiven here must all be a little low. We would suggest that, infuture analyses of these species, clutch-sizes of category (3) shouldbe omitted,_since they are open to two sources of error and anywayare only about one-fifth as numerous as the more satisfactoryclutches of category (2).

There is, however, no reason why these sources of error shouldhave more effect in one part of the breeding season than another,or in one year than another, nor does it appear that they applymore to one species of thrush than to another. Thus seasonal,annual and specific differences in clutch-size should not be affected.Also, clutch-sizes obtained in this way from nest record cardsshould be free from any bias in favour of one clutch-size ratherthan another, since there is nothing, so far as is known, whichmakes it easier to find a nest with a clutch of one size rather thananother. The sorts of observational bias that affect the estimationof breeding season and nesting success do not apply here.

CLUTCH-SIZE: GENERAL

The great majority of clutches of both Blackbird and SongThrush consist of 3, 4 or 5 eggs. Clutches of 2 are not common ineither species, and it appears likely that many, perhaps the

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BREEDING OF THRUSHES 2(2)

majority, of these, as recorded from the cards, are the result ofone or more eggs being lost before discovery. The number ofclutches of 6 is even smaller (a few, which from the evidence ofthe cards are clearly clutches of 4 or 5 with one or two additionaleggs left behind from a previous clutch, have been omitted). Thegreat majority of the Mistle Thrush's clutches are also 3, 4 or 5 ;for this species also there have been a few clutches of 2, but noneof 6.

The percentage distribution of these clutch-sizes in the fouryears 1950-53, for the whole of Southern England, is shown inTable I. This table shows that, taking the breeding season as awhole, Song Thrushes lay fewer clutches of 3, but more of 5, thanBlackbirds, while the percentage of clutches of 4 is a little higher.

TABLE I

PERCENTAGE DISTRIBUTION OF CLUTCH-SIZES IN SOUTHERN ENGLAND, 1950-53

Clutch Blackbird Song Thrush Mistle Thrush6 I I a5 78 26 75

4 52 57 61

3 27 73 23z 3 3, 2

Total number of clutches 1,904 1,155 137

Mistle Thrushes lay more clutches of 4 than the other two species,but fewer large clutches, while in the percentage of clutches of 3it is intermediate between them.

SEASONAL VARIATION

It is now well known that in various species of birds clutch-size varies seasonally, rising to a maximum and then falling (seeespecially Lack, 1947). The very large number of nest record cards

•

Y •

4.5

4 -

v MAR APR MAY JUNEFigure i.—Mean clutch-size of Blackbird (dots and solid line) and Song Thrush

(crosses and broken line) in Southern England 7950-53.

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March ISong Thrush

45

4

April May June

4 5 4

3

3

1 955

BREEDING OF THRUSHES

73

for the Blackbird and Song Thrush enables this seasonal fluctua-tion to be examined in some detail. Figure I shows the meanclutch-size of these two species plotted in 5-day periods, forSouthern England in the years 1 950-53 combined. The Blackbird'smean clutch-size begins at under 3.5, rises through March, April,and early May, and reaches a peak (4.56) in the period 16-2o May,after which it falls rather sharply. (The apparent irregularity inthe latter part of June is probably due to the small number ofclutches started in that period.) The mean clutch-size of the SongThrush is steadier throughout the season. It begins much higher,at over 3.8, and rises more gradually, reaching a peak (maximum4.48), which appears to be less sharply defined than that of theBlackbird, round the end of April and the beginning of May.Thereafter it falls off, but does not sink so low at the end of theseason as does that of the Blackbird.

These seasonal changes can be examined more precisely byplotting the percentage representation of the different clutch-sizes.in each 5-day period (Figure 2). For the whole of March half, or

Blackbird

Figure 2.—Percentage distribution of clutch-sizes of Blackbird and Song Thrushin Southern England 1 950-53. Clutches of 2 and 6 are shown in black.

more, of the Blackbird's clutches consist of 3 eggs. Clutches of 5come in gradually at the end of March, but do not become prom-inent until the end of April, and reach their maximum in the period16-20 May, the period of highest mean clutch-size. The fewclutches of 6 occur in the middle of the period when clutches of 5are prominent (26 April-20 May). At the end of the season clutchesof 5 drop out entirely and clutches of 3 predominate again.

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76 BREEDING OF THRUSHES 2(2)

The Song Thrush shows a similar pattern, but clutches of 3are much less prominent at the beginning and end of the season.Clutches of 5 occur over a more prolonged period and reach theirpeak earlier, at the end of April and the beginning of May ; thefew clutches of 6 also occur earlier and over a longer period. In bothSong Thrush and Blackbird the clutches of 2 are scattered more orless evenly throughout the season, which supports the suggestionthat many of them are due to the loss of eggs.

There are as yet too few cards for the Mistle Thrush for theseasonal change in clutch-size to be examined in detail. If clutchesare grouped in to-day periods (Table II), means for each periodvary from 3.7 to 3.9, with one exceptionally high mean (based ononly 8 clutches) of 4.6 in the last third of April ; but if mean clutch-size is calculated for each month, the figures show a slight rise,which is not however statistically significant, from 3.8 in Marchto. 4.0 in May. There are so few records for June that it , is im-possible to say whether clutch-size declines at the end of the season.

TABLE II

MEAN CLUTCH-SIZE OP THE MISTLE THRUSH, SOUTHERN ENGLAND 1950-53

Number ofclutches

Mean clutch-size

March (1) 4 —(2) 19 3.8 3.79±0.06(3) 35 3.7

April (1) 26 3.8(z) 16 3.8 3.90+0.10

(3) 8 4:6May (1) 19 3.9

(2) 5 — 4:0ó±o.13(3) 3 —

ANNUAL VARIATION

In the preceding section the cards for all the years were com-bined. But, as might be expected in species which are so influencedin their breeding by the weather, the years for which there areadequate numbers of clutches show differences between themselves.Indeed, Lack (1949), basing his analysis on broods of both speciesringed in north-western England over a number of years, hasalready shown that there are annual differences in brood-size inthe Blackbird and Song Thrush, and that the two species vary ina parallel manner in the different years : in years when the brood-size of one species was higher than average, that of the otherspecies tended to be high also. The present analysis confirmsLack's findings and suggests a relationship between annual varia-tion in clutch-size and weather.

In Table III, monthly mean clutch-sizes are shown for theyears 1950-53, together with the overall monthly means for thoseyears combined. Although some of the samples were large, in nocase did the monthly mean in a particular year differ significantlyfrom the overall mean for that month, the differences involved

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BREEDING. OF. THRUSHES

77

being all rather small. However, in 1 i out of the 14 pairs ofmonthly means that can be compared (June-July 195o and March

TABLE III

MONTHLY MEAN. CLUTCH-SIZE OF BLACKBIRD AND SONG THRUSH IN THE YEARS 1950-53,WITH THE OVERALL MEANS AND STANDARD DEVIATIONS FOR THE FOUR YEARS COMBINED.

DIFFERENCES FROM THE OVERALL MEAN ARE INCIDATED BY (+) OR (-)

No.Blackbird

meanSong Thrush

No. mean195o March 53 3.30 (-) 25 3.84 (-)

April 86 3.88 (+) 43 4.30 (+)May 68 4.26 (-) 38 4.39 (+)June July i1 3.64 (+) 7 (4.00)

1951 March 103 3.42 (+) 67 3.91 (+)April 275 3.81 (-) 153 4.11 (-)May 134 4.33 (-) 84 4.26 (-)June July 38 3.74 (+) 23 3.70 (-)

1952 March 174 3.40 (_) 131 3.86 (-)April 334 3.92 (+) 219 4.21 (+)May 133 4.32 (-) 63 4.29 (-)June July 31 3.45 (-) 35 3.74 (-)

1 953 March 67 3.48 (+) 54, 3.91 (+)April 246 3.78 (-) 1 2, , 4.08. (-)May 131 4.40 (+) 78 4.33 (+)June July 20 3.60 (-) 24 3.83 (+)

March 397 3.4o S.D. 0.52 277 3.88 S.D. 0.51AllApril 94 1 3.85 0.52 527 4.16 0.69

years May 466 4.34 0.62 263 4.30 0.70June July loo 3.61 o.6o 89 3.76 0.59

1952 are omitted), the figures for the two species vary similarly,being either both higher or both lower than the overall mean.Two of the three exceptions are June-July means, for which thesamples are much smaller than the others.

This parallel variation in mean clutch-size in the two speciescan be examined in greater detail by comparing the means for, each5-day period in the years 1951-53 (Figure 3). Ln 1951 the variationwas remarkably parallel, the outstanding feature being that meanclutch-size began high in 'both species, then dropped towards theend of March, then rose, then dropped again slightly in mid-April.It is probable that these fluctuations were correlated with tem-perature. Mean temperature was high for several days round themiddle of March, and then fell, with minima below freezing on4 out of the last 7 nights of March. There was a marked rise intemperature on I April, and this rise continued until 6 April, whentemperatures fell, remained low for about 5 days, then graduallyrose again throughout the rest of the month.

In 1952 the trend for the two species was again very similar:mean clutch-size again dropped at the end of March, reaching theexceptionally low figure of 3.08 for the Blackbird in the period 1-5April (as compared with 3.54 for the years 1950--53 combined ; thedifference is statistically significant). This was the period immedi-ately following the blizzard, whose effect on the starting of new

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78

4.5-

4

3

1951

'`..

BREEDING OF THRUSHES 2(2)

,.•''.^ ^• .^,: • ^^ ..-••. a% .:/ ^'•/

...`•^'^: •••••..i'

.-•/

1952A4-5. %; •̂,

4-7%5, ,i ^•-^/

.

=' ; ^ `,^ ',s^ ^e'\,/

3-5 ^

3

1953

. : ^ %^•

4 •_n .: ; •^'•` • `.:

^.^.

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3.5 ^

3

clutches has been described in Part I. That the drop in the clutch-size occurred in the previous period in the Song Thrush (3.65, com-pared with 3.84 for 1950-53 combined ; difference not quite sig-nificant statistically), may be due partly to the arbitrarily chosen5-day groupings, but probably also to a tendency, similar to thatmentioned in Part I in connection with the breeding season, forthe Song Thrush to respond slightly more quickly than the Black-bird to the influence of weather.

In 1953 the seasonal variation in the two species was notparallel. This was the most `typical' breeding season of the three con-sidered here, in the sense that, after a rather late start, breedingwas not seriously affected by abnormal weather. In this year theBlackbird's clutch-size showed a steady rise, similar to the rise

MAR APR MAY JUNEFigure 3.—Mean clutch-size in 5-day periods of Blackbird (solid line) and SongThrush (broken line) in Southern England 1951-53. Means have been calculatedfrom to or more clutches in each period. (Two 5-day periods have been com-

bined for the Song Thrush in May 1952 and May 1953.)

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Song Thrush NorthSouth

439 3 16 55

261 , 1 55 3 13 57 26

1955 BREEDING OF THRUSHES 79

shown when the years are combined, coming to a peak just aftermid-May. No reason can be suggested for the rather uneven courseof mean clutch-size for the Song Thrush in the early part of theseason.

Although it thus seems clear that clutch-size varies in responseto temperature, monthly mean clutch-sizes for these years showno correlation with monthly mean temperatures ; for instance, themean clutch size in April of each year is not correlated with themean April temperature for that year. This is almost certainly dueto the fact that clutches are not evenly spaced throughout themonth but tend to be more or less concentrated into limited periodswithin each month ; so that it is with the temperature during orshortly before these periods that clutch-size may be expected to becorrelated, not with the mean temperature for the whole month.

GEOGRAPHICAL VARIATION

Geographical differences in clutch-size within Great Britain arenot marked in the Blackbird and Song Thrush (there are still toofew cards for the Mistle Thrush for an examination of this point).When the whole season is taken together, and all years combined,the percentages of the different clutch-sizes in the north of GreatBritain (Northern England and Scotland) and in the south(Southern England) are as shown in Table IV. The figures show noconsistent differences.

TABLE IV

PERCENTAGES OF CLUTCHES OF DIFFERENT SIZES IN THE NORTH OF GREAT BRITAIN IN THEYEARS 1950-53

Total Clutch

Clutches 2 3 4 5 6

North 598 6Blackbird South 1,904 3

24 49 21 < I

27 52 18 I

When, however, the breeding season is divided into io-dayperiods (there are too few clutches from the north for division into5-day periods), and the mean clutch-size is calculated for eachperiod (Figure 4), it is clear that in both species clutch-size islower in the north than in the south at the beginning of the season,and higher at the end of the season. (The differences are statisticallysignificant in the second period of April and second period of Mayin the Blackbird, and in the last period of April in the SongThrush.) There is no correspondingly clear difference in the periodin which the peak clutch-size occurs, but there is a slight indica-tion in both species that it is a little later in the north than in thesouth. Thus it appears simply as if the whole breeding season has,so to speak, been shifted a little later in the north, without anyreal alteration in clutch-size.

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80

4.5

4

3.5•

4. . Song Thrush ._r

4- •.^ .-•- ,.^ ;! •---•

3.5.

3- MAR APR

,

MAY JUNE

BREEDING OF THRUSHES 2(2)

Blackbird• ; \

/,^ •^,

• ^ ^-., ^ .^:---•

^;+• •--•

• .

3 .

Figure 4.—Mean clutch-size of Blackbird and Song Thrush in Northern Englandand Scotland (broken line) and Southern England (solid line) in 1950-53.

In her earlier analysis of Song Thrush nest records, Silva ( 1 949)discussed regional variation in clutch-size in this species in somedetail. She found that average clutch-size is larger (by o.3 egg)in Holland than in Britain, and still larger (by o.5 egg) in Finland,at the same time of the year, but her data did not show consistentregional differences within Britain. Since 1949, quantitative dataon all three thrushes have been published for Belgium (Verheyen,1 953) and on the Blackbird in Shetland (Venables & Venables,1952). Verheyen's figures, which are based on an enquiry held inBelgium in 1952, can be compared directly with the 1952 figuresfor Southern England. The average clutch-size for the whole

TABLE V

PERCENTAGES OF CLUTCHES OF DIFFERENT SIZES IN BELGIUM AND SOUTHERN ENGLAND IN 1953

Total % of Clutches of Mean . .

Clutches 2 3 4 5 6 for Year

750 2 10 50 36 2 4.25

673 4 27 Si 18 <I 3.97

354 I 8 52 37 2 4.32448 3 12 59 2 5 <I 4.08

144 3 II 62 21 3 4.08

55 4 18 67 II o 3.85

Blackbird BelgiumS. England

Song Thrush BelgiumS. England

Mistle Thrush BelgiumS. England

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BREEDING OF THRUSHES $I

season was higher in all three species in Belgium than in England .

(Table V), and average clutch-size in each of the three periods intowhich Verheyen divides the breeding season was higher in Belgiumfor all three species for each period, except for the Blackbird inMay, when the averages were almost identical. It may be notedthat in England clutches of 6 formed less than o.5% of the totalfor the Blackbird and Song Thrush and were not recorded at allfor the Mistle Thrush, while in Belgium they formed 2% or 3% ofthe total for each species.

Venables & Venables (1952) give figures for the Blackbird inShetland in the years 1949-51, which indicate that mean clutch-size there too is higher in each month than it is in England andScotland (Table VI). (The difference is statistically significant onlyin June, the Shetland totals for each month being rather low.) Junewas the month with the highest mean clutch-size, not May, as it isfurther south, and it was in June that the only two clutches of 6occurred. This supports the suggestion that clutch-size reaches itspeak later in the north of England and Scotland than in SouthernEngland.

TABLE VI

MEAN CLUTCH-SIZE IN EACH MONTH FOR THE BLACKBIRD IN SHETLAND, NORTHERN ENGLAND,AND SOUTHERN ENGLAND. (SHETLAND FIGURES ARE FOR THE YEARS 1949-S I, FIGURES FOR

ENGLAND AND SCOTLAND FOR THE YEARS 1950-53)

March April May June July

Shetland Mean clutch-size — 3.9 4.5 4.7 3.5Number of clutches — (28) (44) (3,) (II)

Scotland and N. England Mean clutch-size 3.4 3.8 4.2 3.8Number of clutches (67) (292) ( 1 94) (45)

S. England Mean clutch-size 3.4 3.8 4.3 3.6Number of clutches (396) (941) (466) (IOo)

Thus the thrushes have a rather low clutch-size in Great Britaincompared with the other parts of north-western Europe that havebeen investigated. Possible reasons for this difference will be dis-cussed in the next section. Here, however, it may be pointed outthat in any one geographical area clutch-size is likely to vary withhabitat and possibly also with population density. For example,personal observations suggest that in this country the clutch-sizeof the Blackbird is greater by about half an egg in deciduous wood-land than in gardens. Hence the habitat-distribution of the clutch-size data from each country may have a marked effect on theaverages.

DISCUSSION

Lack (1947) has shown that in many European birds clutch-size varies with latitude, being higher at high latitudes than atlow latitudes, and that it varies seasonally, rising to a maximum,usually about early June, and then falling ; He has suggested thatboth these trends are correlated with day-length: when days are

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82 BREEDING OF THRUSHES 2(2)

longer, the parent bird can spend more time in finding food and socan raise more young. The essential factor is, as Lack points out,the availability of food. Theoretically the latitudinal and seasonaleffects will reach their full expression only if the availability offood remains unchanged in space and time throughout the breedingseason : if the availability of food alters, this may be expected toobscure or even to over-ride the effects due to day-length.

Seasonal variation. In Southern England the Song Thrushreaches its peak clutch-size in the period 26 April-10 May, and theBlackbird in the period 16-20 May. The young from clutchesstarted in these periods will be in the nest in the periods I I May-10 June and 1-2o June respectively, that is, before the longestday. Many of the young that are in the nest around the longest daywill be from clutches started in early June, by which time meanclutch-size has fallen markedly in both species. The total seasonalchange in clutch-size cannot therefore be adapted merely to day-length in these species, though the rise in clutch-size in the earlypart of the season may be adapted, at least in part, to the increas-ing day-length at this period.

It seems probable that there is more suitable food available fornestling thrushes in early spring than in late spring and summer.Lockie (1955) found that in two years earth-worms, which form alarge part of the food of young thrushes, were most abundantnear the surface in grassland in early spring, and least abundantin June and July ; and observations confirm that during periods ofdrought, which, so far as the breeding season is concerned, comechiefly in June and July, it becomes increasingly difficult forthrushes and Blackbirds to obtain worms, so that they are forcedto turn to other less substantial foods, and young frequently dieat such times, apparently of starvation. Thus, though preciseknowledge on this point is still far from adequate, it seems thatthe seasonal change in the abundance of food can hardly explainthe rise in clutch-size in the first half of the season, but it mightwell account for the drop in clutch-size at the end of the season.

Throughout the breeding season mean temperature rises. Thisprobably means that the young will on the average require lessbrooding, so that more time can be devoted to bringing food, andit may also be that since less food is needed to maintain body-temperature, a greater proportion can be devoted to promotinggrowth. Preliminary observations indicate that nestling Black-birds grow more quickly in May and June than in April, in spite ofan apparently less good food supply.

The provisional hypothesis may therefore be advanced that theincrease in clutch-size throughout March, April and part of Mayis adapted to the increasing warmth and increasing day-length,which over-ride any change in the abundance of food and increasethe number of young that can be successfully reared ; while the de-crease in clutch-size at the end of the season is adapted to thedecrease in the abundance of food, which at some point in May(the exact point depending on the ecology of the species) in its turnover-rides the effects of warmth and day-length.

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1955 BREEDING OF THRUSHES 83

Clutch-size does not, however, fluctuate regularly each year,but, as has been shown, there are annual differences which areprobably attributable to weather. The adaptive value of this typeof variation is not obvious, since the young from a clutch whichhas been affected in this way will not be in the nest until 17-30days later, by which time the weather is likely to be quite different.But it is at least possible, as Lack (1949) has suggested, that theweather may be one of the proximate factors determining theregular trend in clutch-size, so that temporarily abnormal weathermay, so to speak, `mislead' the birds and cause them to lay aclutch which is really adapted to the season of which such weatheris characteristic.

Regional variation. Since seasonal variation in clutch-size inthis country is evidently not adapted merely to day-length in thesespecies, we need not expect it to be greater in the north than in thesouth ; for although day-length is longer in the north, the otherfactors to which clutch-size may be adapted, such as abundanceof food and warmth, may be less favourable in the north than inthe south. Comparison of clutch-sizes in Scotland and NorthernEngland with Southern England showed that clutch-size startedlower in the north and ended higher, just as if the whole breedingseason had been shifted about two weeks later without any realalteration in clutch-size. Indeed, when the whole breeding seasonwas taken together, there were no consistent differences betweenthe percentages of clutches of different sizes in the north and inthe south. It may simply be that the equivalent conditions ofweather and food, to a combination of which it is suggested thatthe seasonal variation in clutch-size may be primarily adapted,occur about two weeks later in the north than in the south.

Geographical variation in the clutch-size of thrushes in north-western Europe as a whole has already been briefly discussed.There is evidence, in those species which have been investigated,that clutch-size is higher in Finland, Holland, Belgium and Shet-land than in Great Britain. Two sources of variation must be takeninto account in attempting to interpret these differences. First,adaptive differences may have been evolved in different areas, in re-sponce to geographical differences in availability of food, climateor other factors. Second, in any one area clutch-size may varyaccording to habitat and perhaps also, to a lesser extent, to popu-lation density. This second source of variation may be large enoughto invalidate comparisons based on differences between means ofthe order of half an egg or less (as are most of those consideredhere). Until this second source of variation has been taken intoaccount, attempts to explain the adaptive significance of theserather small regional differences in mean clutch-size would bepremature.

ACKNOWLEDGEMENTS

Thanks are due to all the observers who sent in the records onwhich this paper is based, and to Dr. D. Lack for helpful criticismsof the manuscript.

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REFERENCES

LACK, D. 1947. The significance of clutch-size. Ibis, 89:302-52.LACK, D. 1949. Family size in certain thrushes (Turdidae). Evolu-

tion, 3: 57-66.LOCKIE, J. D. 1955. The breeding and feeding of Jackdaws and

Rooks with notes on Carrion Crows and other Corvidae. Ibis,

97 34 1-69•SILVA, E. T. 1949. Nest records of the Song-Thrush. Brit. Birds,

42:97—I II .VENABLES, L. S. V. & VENABLES, U. M. 1952. The Blackbird in Shet-

land. Ibis, 94 :636-53.VERHEVEN, R. 1 953. Etude statistique relative á la biologie de nos

trois grives (Turdus sp.) indigènes, Gerfuut, 43:231-61.

ORNITHOLOGICAL SURVEYS OF NATURERESERVES, 1954

by

BRUCE CAMPBELL

Received 28 April, 1955

SUMMARY

1. Repeat surveys of the breeding bird population were made on 23 areas andnew surveys on five areas in England in the breeding season of 1954.

2. Estimates of the total population were made for Yarner Wood, pre-dominantly an oakwood, on the edge of Dartmoor, Devon, and for Moor House,a high level moorland area in Westmorland.

Ornithological surveys of certain selected nature reserves, de-clared or proposed, were carried out by the B.T.O. on behalf ofthe Nature Conservancy in 1954, on the same lines as in 1953(Campbell 1954).

Reports were received on the following areas from the observersnamed in brackets. An asterisk indicates that this was a repeatsurvey.

Devon: Braunton Burrows* (Mrs. D. K. Wilson), YarnerWood* (B. Campbell, E. A. Roberts, Miss R. J. Wilkinson) ;Devon—Dorset: part of Axmouth—Lyme Regis Landslip* (J. F.Cancellor) ; Dorset: Hod Hill* (A. J. Bull and Bryanston SchoolN.H.S.), Studland* (J. C. Follett) ; Gloucestershire: part of Bird-lip Woods* (Cheltenham Nats. Soc.) ; Hampshire: Old Winches-ter Hill (D. G. Pumfrett) ; Huntingdonshire: Holme Fen (A. E.Vine) ; Kent: part of High Halstow* (D. F. Owen) ; Lancashire:Ainsdale Dunes* (Miss M. Henderson), Roudsea Wood* (J. A. G.Barnes) ; Leicestershire: Leighfield Forest* (H. G. Cherry,R. A. O. Hickling) ; Lincolnshire: Lincolnshire Gate Scrubs*(R. A. O. Hickling) ; Norfolk: Scolt Head* (R. Chestney), WeetingHeath* (J. A. Gibb), Winterton Dunes (D. Galey) ; Northampton-shire: Castor Hanglands (N. C. Moore, R. A. O. Hickling) ;

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