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The Breeding ofBlackbird, Song ThrushMistle Thrush in GreatBritain Part III. NestingSuccessD. 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 Blackbird, SongThrush Mistle Thrush in Great Britain Part III. Nesting Success, Bird Study,2:4, 169-178

To link to this article: http://dx.doi.org/10.1080/00063655509475828

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BIRD STUDYVol. 2, No. 4, December, 1955

THE BREEDING OF BLACKBIRD, SONG THRUSH AND

MISTLE THRUSH IN GREAT BRITAIN

PART III. NESTING SUCCESS

by

D. W. SNOW

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

Received 18 October, 1955

(A Publication of the British Trust for Ornithology)

SUMMARY

I. The method used for estimating hatching and fledging success from nestrecord cards is explained. (The Mistle Thrush is omitted from this part of theanalysis through lack of cards.)

a. On average, 56% of Blackbird nests and 5o% of Song Thrush nestssuccessfully reach the hatching stage. In both species, the percentage increasesthroughout the season.

3. Omitting nests that fail completely, Song Thrushes hatch a higher per-centage of their eggs than Blackbirds. The percentage hatching increases in thecourse of the season in the Blackbird, but not in the Song Thrush. In both speciesit is independent of clutch-size.

4. On average, 41% of Blackbird nests begun and 36% of Song Thrush nestssuccessfully reach the fledging stage. The rate of failure in the nestling stage in-creases in the course of the season in the Blackbird, and may do so also in theSong Thrush.

5. In every month, the Song Thrush raises on average more young from eachof its successful nests than the Blackbird. The number of young raised fromclutches of each size does not vary much in March, April and May in eitherspecies, but in June it is lower in both species.

6. The causes of failure are similar in both species. Desertion is most fre-quent at the beginning of the season.

7. The greater success of late nests more than offsets their lower clutch-size, so that the average number of young fledged for every nest started increasesthroughout the season in both species.

8. The number of young produced per pair in the course of the whole seasoncannot be estimated from the nest record cards, but a record of the nestinghistory of 38 pairs of Blackbirds over 3 years shows that each pair producedon the average 4.4 young.

INTRODUCTION

This is the third and final part of the analysis of nest recordcards of the three common British thrushes, of which Parts I andII have already been published in this journal. The title of this partis slightly misleading, as there are as yet too few cards for theMistle Thrush to make analysis of its nesting success worth while,and it has been omitted.

In analysing nesting success we have had to deal with largenumbers of cards, many of which are very incompletely filled in.

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170 BREEDING OF THRUSHES 2(4)

This has forced us to pay special attention to the sources of errorand bias which beset such an analysis, and in the end to adoptsomewhat different methods from those used in previous analysesof nest record cards. We have concentrated on estimating thefollowing: (1) the percentage of nests, of all those started, whichsuccessfully reach the hatching and fledging stages • (2) the per-centage of the eggs hatched, and the average number of youngfledged, in these successful or partially successful nests, accordingto clutch-size and month ; (3) the causes of failure in the egg andchick stages. (1) and (2) combined give a measure of the produc-tivity of nests in different months.

THE ELIMINATION OF BIAS

In estimating the percentage of nests which successfully reachthe hatching or fledging stage, any nest found at an advanced stageof the nesting cycle, if it is included, biases the percentage infavour of success, since it should be counterbalanced by all thosenests which failed to reach that stage and so were never found. Inother words, the nest record cards are a biased sample. The onlyway to eliminate this bias is to use for analysing hatching andfledging success only those nests that were found at a very earlystage. Ideally one should use the only truly unbiased sample, i.e.nests found during building. These alone are however too few, sowe have used in addition those found before the clutch was com-plete. Even this sample will therefore be biased slightly in favourof success, since some nests fail before the clutch is complete. Butthe error is probably not great, and anyway it could be argued thata nest has not properly started until the clutch has been completed.On this basis, 1,428 cards of the Blackbird and 816 of the SongThrush are available for analysis of nesting success.

There is however a second main source of bias, that it is mucheasier to record failure than success. Certain failure can be re-corded in as few as two early visits, but certain success can onlybe recorded when visits are paid up to a late stage (survival of nest-lings up to 9 days old has been used as evidence of successful fledg-ing unless there was evidence to the contrary). All observers are notequally persistent, and even the most persistent are not always ableto follow their nests to the end. In addition, many observers stopvisiting nests after they have ringed the young at 7 or 8 days old.Thus the sample of cards for analysis, though reasonably un-biased in itself, will not, because of the falling-off of observers,show the true nesting success ; we always have a certain number ofnests whose fate was unknown, and some allowance has to be madefor these.

In fact, instead of calculating an adjustment to be made toobserved nesting success, it is more convenient to calculate nestingsuccess afresh from a knowledge of (1) distribution of nest failuresthroughout the nesting cycle, i.e. days on which nests wereobserved to fail, and (2) the persistence of observers, i.e. times atwhich the observers themselves stopped visiting nests which theyhad not observed to fail. I am greatly indebted to Mr. C. Winsten,of the Oxford University Institute of Statistics, for making one of

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1955 BREEDING OF THRUSHES 171

these calculations, which involves a rather laborious mathematicalprocedure, for one of the Blackbird samples.. This showed approxi-mately how much adjustment needed to be made to the observedpercentages of success, and they have been compensated on thisbasis.

In this paper the results are not entirely above criticism, as inthe first place the data were not originally extracted with thiscalculation in mind, and so contain certain inconsistencies, and inthe second place the result for one sample has been applied to allthe samples, for which some of the variables are bound to be dif-ferent. It is intended to deal fully with the mathematical procedurein a forthcoming paper, since the problem is general to all analysesof nest record data.

HATCHING SUCCESS

Table I shows the monthly percentages of Blackbird and SongThrush nests in which some young successfully hatched. Bothspecies agree in showing a steady rise in hatching success through-out the season, from 5o% in March to 77% in June in the Black-bird, and from 41% in March to 7d%, in June for the Song Thrush,the Song Thrush percentage being lower than the Blackbird's inevery month. Taking the breeding season as a whole, Blackbirdshave a hatching success of 56% and Song Thrushes a hatchingsuccess of 5o%.

The above figures refer to all nests in which some younghatched, and take no account of the number of eggs hatched.Because of the rather infrequent visits paid to most nests over thehatching period, the number of eggs hatching is not always ex-actly known ; it is often not possible to tell whether the reduction innumber of young below the full clutch-size was due to the dis-appearance of an egg or of a nestling. Percentages for the numberof eggs hatching can therefore be given only between the limits ofthe maximum possible and the minimum possible, but in most casesthese limits are close together. These percentages are shown inTable II.

The figures are consistent in showing that the Song Thrushhatches a higher percentage of its eggs than the Blackbird. In theBlackbird the percentage hatched increases steadily throughout the

TABLE I

HATCHING AND FLEDGING SUCCESS OF BLACKBIRD AND SONG THRUSH NESTS. (PERCENTAGESINDICATE NESTS IN WHICH SOME YOUNG HATCHED SUCCESSFULLY AND FROM WHICH SOME

YOUNG FLEDGED SUCCESSFULLY)

N

Blackbird

hatched fledged N

Song Thrush

hatched fledgedMarch .. 276 50 39 162 41 33April .. 66o 53 39 383 48 34May .. 424 6o 44 223 54 38June .. 68 77 54 48 70 54

All months 1,428 56 41 816 5o 36

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172 BREEDING OF THRUSHES

2(4)

TABLE U

PERCENTAGES OF EGGS HATCHING, ACCORDING TO CLUTCH-SIZE AND MONTH. TOTAL FAILURES

ARE EXCLUDED)

Blackbird ..

Song Thrush

March April May June All monthsc/5 — 85-88 89-92 — 88-90c/4 86-90 89-9 1 91-95 — 89-92c/3 88 88-90 — — 88-90

All c/s 86-89 88-90 90-94 93-95 88-91

c/5 — 88-96 92-97 — 90-96c/4 93-97 91-94 90-94 — 91-95c/3 — — — — 93

All cls 93-97 91-96 91-96 91-95 91-96

season, but in the Song Thrush this is not so. In both species thepercentage of eggs hatched is independent of clutch-size.

It should be noted that these figures do not distinguish betweenthe different causes of failure, of which the chief are loss of eggs,infertility and chilling. But field observation and the evidence ofthe cards suggest that some of the difference between the twospecies may be due to what appears to be the less efficient incuba-tion behaviour of the Blackbird. Thus Blackbirds tend to lose eggsin the course of incubation more often than Song Thrushes, andthis probably accounts for part of the difference in hatching per-centages ; while the greater danger of chilling in the early part ofthe season, due to the fact that the Blackbird leaves the nest morefrequently than the Song Thrush, may be responsible for thegradual increase in hatching success in the Blackbird as the seasonadvances, compared with the steady level maintained by the SongThrush.

FLEDGING SUCCESS

The percentages of nests from which some young fledged (usingthe criterion of survival to 9 days old) are shown in Table I. It willbe noted that, as for hatching success, the percentages for the SongThrush are consistently lower than those for the Blackbird, exceptin June, for which the samples are small. In both species, fledgingsuccess, like hatching success, improves throughout the season,when measured as a percentage of all nests started. But this issimply due to the increasing hatching success. If fledging successis measured as the percentage of those nests in which younghatched, it appears to decrease somewhat in the course of theseason in the Blackbird (78%, 74%, 73% and 70% in the fourmonths), and may do so also in the Song Thrush (8o%, 71%,70% and 77%), the June figure for the Song Thrush, and perhapsalso for the Blackbird, being based on too small a sample to bereliable.

THE NUMBER OF YOUNG LEAVING NESTS

In the previous section no account was taken of the number ofyoung fledging from those nests that were successful. This is moreconveniently dealt with in a separate section. For this purposenests found at late stages can be used, as they introduce no bias,

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1955 BREEDING OF THRUSHES 173

giving altogether a total of over I,000 cards for the Blackbird andover 600 for the Song Thrush. Table III shows how the averagenumber of young leaving the nest varies throughout the season inboth species. The trends are, as would be expected, similar to thetrends in clutch-size (see Part II of this paper), which are shownin the same table. The Blackbird reaches a peak in May, and theSong Thrush rather earlier, the April figure being almost as highas the May figure. In every month, from each successful nest theBlackbird raises on the average fewer young than the Song Thrush.

TABLE III

MEAN NUMBER OF YOUNG LEAVING THE NEST, MEAN CLUTCH-SIZE, AND PARTIAL FAILUREIN EACH MONTH

March April May JuneMean no. of young leaving nest 2.9 3.2 3.5 2.9

Blackbird Mean clutch-size . . 3.4 3.8 4.3 3.7Difference (=partial failure) . . 0.5 0.6 0.8 0.8

Mean no. of young leaving nest 3.4 3.8 3.9 3.3Song Thrush Mean clutch-size .. 3.9 4.1 4.3 3.8

Difference (=partial failure) .. 0.5 0.3 0.4 0.5

The difference between the mean clutch-size for each month andthe mean number of young leaving the nest gives a convenientmeasure of the partial failure occurring at all stages after the clutchhas been laid. Table III shows that it is higher in the Blackbirdthan in the Song Thrush, and that in the Blackbird it increasesthroughout the season, while in the Song Thrush it remains moreor less steady. This is exactly the same picture as was found whencomparing partial failure in the egg stage in the two species.

Table III was based on all clutches. In Table IV the meannumber of young leaving the nest is analysed according to theclutch-size, clutches of 2 and 6 being omitted as they are too fewto give averages. This table shows that, considering only success-ful or partially successful nests, for each clutch-size the Song

TABLE IV

MEAN NUMBER OF YOUNG FLEDGED, ACCORDING TO CLUTCH-SIZE

N

45Numberfledged N

cf4Numberfledged N

cf3Numberfledged

March .. . - - 69 3.3 73 2.6April .. • 48 4.2 285 3.4 86 2.5

Blackbird May .. • 76 4.3 87 3.2 23 2.5June .. . - - 13 (3.2) 22 2.2

All months .. 132 4.2 354 3.3 204 2.5

March .. - - 59 3.5 - -

April . . • • 5 8 4.5 115 3.6 12 (2.6)Song Thrush May .. • • 45 4.4 56 3.7 - -

June .. - 3o 3.2 II (2.6)

All months .. I17 4 .5 260 3.5 38 2.7

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174 BREEDING OF THRUSHES 2(4)

Thrush produces on the average more young than the Blackbird ;that is to say, the Blackbird suffers more partial loss, clutch-sizefor clutch-size, than the Song Thrush ; and this is true not only forthe season as a whole but for each month for which the samplesare big enough for reliable means to be evaluated.

In March, April and May, the number of young fledged fromclutches of each size does not vary much in either species : forexample, clutches of 4 produce on average 3.3 young in March,3.4 in April and 3.2 in May in the Blackbird, while the equivalentfigures for the Song Thrush are 3 . 5, 3 .6 and 3.7. There appears tobe a drop, however, in the number of young fledged in June. In theSong Thrush, for clutches of 4, the drop from 3.7 in May to 3.2 inJune is statistically significant, and in the Blackbird, for clutches of3, there is a drop, which is not quite significant statistically, from2.5 in May to 2•I in June. There is thus a little evidence here thatthe reduced clutch-size at the end of the season is adapted to thesmaller number of young that the parents are able to raise. In PartII of this paper it was suggested that a decrease in the availablefood was the controlling factor.

CAUSES OF TOTAL FAILURE

In the previous sections we have considered how many younghatched and fledged from the nests that successfully reached thosestages, and have briefly discussed the causes of partial failure.Total failures are, however, numerically much more important ;some idea of the causes of total failure can be obtained from thecards, though in most cases the cause is presumed rather thancertainly known. (It should be noted that in this section, as else-where, nests are classified according to the month in which the firstegg was laid. Thus in many of them the failure under considerationoccurred in the following month.)

Losses during the egg stage have been divided broadly into'predation' and `desertion'. Destruction or disappearance of thewhole clutch, and desertion after loss of part of the clutch, have allbeen counted as predation. Failure has been attributed to deser-tion only when there was no evidence of predation ; some of thesedesertions are known to have been due to weather (especially snowstorms and gales), and others to disturbance by human beings.Losses of young have been divided broadly into `predation' and`dead in nest'. The criteria for these are the same as for predationand desertion of eggs. The death of young in the nest is attributedby observers sometimes to weather, sometimes to disturbance, andsometimes to the death of a parent bird, but often no cause isknown. Personal observations have shown that starvation is a fairlycommon cause of death of nestling Blackbirds in time of drought.

Table V summarises the causes of failure during the egg stage.It shows that in both species desertion of eggs is most frequent inMarch nests, accounting for about one half of the losses, and lessfrequent in April and May nests, accounting for about one quarterof the losses. Concomitantly, predation accounts for more of thelater than of the earlier nests.

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1955 BREEDING OF THRUSHES 175

Of the desertion of March nests, 42% and 44% were attributedby observers to the weather in the Blackbird and Song Thrush re-spectively, but of nests started in other months not more than 12%

TABLE V

CAUSES OF FAILURE IN THE EGG STAGE

% of lossCause offailure Blackbird Song ThrushPredation .. 52 48

March Desertion .. 48 51Miscellaneous — IPredation .. 77 74

April Desertion 22 23Miscellaneous 1 3

Predation .. 68 82May Desertion 31 18

Miscellaneous 1 —

Nora. Percentages based on the following totals in March, April, and May respectively:Blackbird 126, 275, 15o; Song Thrush 85, 181, 93

TABLE VI

CAUSES OF FAILURE IN CHICK STAGE

% of lossCause offailure Blackbird Song ThrushPredation .. .. 6o 44

March Dead in nest .. 36 S4Miscellaneous .. 4 2

Predation .. .. 75 75April Dead in nest .. 24 20

Miscellaneous . . 1 5Predation .. .. 84 70

May Dead in nest 13 26Miscellaneous . . 3 4Predation .. 63 62

June Dead in nest 26 38Miscellaneous . . II -

NOTE. Percentages based on the following totals in March, April, May, and June re-spectively: Blackbird 77, 174, 106, 19; Song Thrush 54, IOI, 46, 21

of desertions in any one month were attributed to the weather. TheMarch percentages are probably rather higher than usual, as theblizzard at the end of March 1952 caused extensive destruction ofearly nests in that year.

The causes of loss of young are summarised in Table VI. Thistable is based on all nests which were known to fail in the nestlingstage, since the cards used for analysis of hatching and fledgingare too few for this purpose. There should, however, be no sourceof bias in this larger sample.

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176 BREEDING OF THRUSHES 2(4)

Over the whole breeding season predation accounted for 74%of the Blackbird nestling losses and 65% of the Song Thrush losses.Analysed by months, in both Blackbird and Song Thrush preda-tion was highest in April and May nests, while the loss recordedas `dead in nest' was highest in March. The totals on which theJune percentages are based are very low, but it seems that the in-cidence of death in the nest rises again at the end of the seasonafter falling to a low level in April and May. In both species, aboutone quarter of the cases of death of young in March nests wereattributed by observers to the weather (mainly heavy rain, coldweather or snow), as were a few of those in later months. But, asmentioned above, field observation suggests that, apart from preda-tion, the most important cause of death of young in late nests, atleast in garden habitats, is food-shortage following periods of dryweather.

PRODUCTIVITY OF NESTS AND OF PAIRS

The product of fledging success (the percentage of all nestsstarted that successfully reach the fledging stage) and averagenumber of young leaving the nest gives a measure of the produc-tivity of nests, that is, the average number of young produced forevery nest started. Table VII shows the productivity of nestsstarted in each month. It is interesting that although clutch-sizerises and falls in the course of the season, presumably as anadaptation to seasonal variation in food supply and other factorsthat may affect the rearing of young (see Part II of this paper),productivity increases throughout the season in both species, thelow clutch-size at the end of the season being more than offset bythe greater success of later nests.

TABLE VII

PRODUCTIVITY OF NESTS IN EACH MONTH

% reachingfledging stage

Mean no. ofyoung

leaving nest ProductivityMarch 39 2.9 LI

Blackbird April .. 39 3.2 1.2

May .. 44 3.5 1.5June .. 54 2.9 1.6

March .. 33 3.4 1.1Song Thrush April .. 34 3.8 1.3

May .. 38 3.9 1.5June 54 3.3 1.8

For species such as these, which have several broods annually,nest record cards alone can give no information on the averagenumber of young produced by each pair per year, a more importantfigure for an understanding of the population dynamics of thespecies than the average number of young produced per nest. Thisfigure has been obtained for the Blackbird in a garden habitat at

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1 955

BREEDING OF THRUSHES

177

Oxford from personal observations in the 1953-55 breedingseasons. The nesting success of this population, which has perhapsrather few predators, is unusually high—the percentage of nestsreaching the fledging stage was 5o% in March, 44% in April, 51%in May and 64% in June, on average 8% higher than the figuresobtained from the nest record cards. The family size in the differentmonths has been similar to the figures obtained from the cards.The average number of young produced per pair in this habitat istherefore likely to be somewhat higher than for the country as awhole.

A complete nesting record was obtained for 38 pairs. These in-clude two pairs in which the male died and the female obtained anew mate for her last brood, but do not include a few pairs whosehistory was complicated by changes of partners without deaths andwhose inclusion would complicate the data without materiallyaffecting the figures. Four pairs, and possibly a fifth, had theirnesting cut short by the death of one of the partners without thesurvivor being able to nest again, an occurrence which is, of course,quite common and which should be taken into account in anyattempt to measure the productivity of a breeding population. Thenumber of young produced by these 38 pairs ranged from o (in 7cases) to io and II (one case each). 6o% of the pairs producedfrom 3 to 8 young. The mean number of young produced per pairwas 4 . 4.

COMPARISON WITH OTHER DATA

Silva (1949), Venables & Venables (1952) and Verheyen ( 1 953)have published quantitative data for the Song Thrush and Black-bird which can be compared with those presented here. All theseauthors give evidence of higher nesting success. Thus, consideringthe fate of nests, Silva calculated, from ,nest record cards, that 65%of British Song Thrush nests succeeded wholly or partially, whilefor Belgium Verheyen gives 57% for the Song Thrush and 61%for the Blackbird. These figures may be compared with 36% forthe Song Thrush and 41 % for the Blackbird, obtained from thisanalysis. Considering the percentage of eggs laid which give rise tofledged young, Silva gives 55% for the Song Thrush and Verheyen51% ; and for the Blackbird Venables, for Shetland, gives 62%,and Verheyen 58%, while Fisher (1949) gives a figure of 5o%based on nest record cards. The equivalent percentages obtainedin this analysis, derived from Tables I and II, are from 29% to 47%(according to month) for the Song Thrush and from 33% to 42 %for the Blackbird.

Clearly nesting success may vary greatly in different places,according to local conditions, and some of these differences mustbe due to this. But some of the differences must also be due to theway in which the data were collected and to the method of analysis,especially as regards making allowance for the sources of bias. Forthis reason it is worth emphasising again that the comparativeaspects of this analysis are of more value than the absolute figuresobtained.

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178 BREEDING OF THRUSHES 2(4)

CONCLUSION

This comparison between the nesting success of Blackbird andSong Thrush has revealed some similarities, and some consistentdifferences. Of all nests started, a greater proportion of those ofthe Blackbird successfully reach the hatching and fledging stages.Both species are more successful in this respect as the season ad-vances. The causes of total failure are similar for the two species.Considering only those nests which successfully reach the hatchingstage, we find that the Song Thrush consistently hatches a higherpercentage of its eggs than the Blackbird. In the Song Thrush thispercentage appears to remain steady throughout the season, but inthe Blackbird the percentage is lowest in the early part of theseason for each clutch-size.

Turning to fledging success, we find that, although the Black-bird fledges some young from a higher percentage of all nestsstarted than the Song Thrush, showing a lower percentage oftotal loss in both the egg and nestling stages, the Song Thrushfledges proportionally more young from each of its successful nests,relative to the initial clutch-size, than the Blackbird. It appearsthat in both species the number of young fledged, relative to theinitial clutch-size, remains fairly steady in March, April and May,but drops in June, and it is suggested that this may be due to foodshortage at the end of the season.

Thus at each stage the Song Thrush suffers more total loss thanthe,Blackbird, but less partial loss. The reasons for this lie outsidethe scope of the present paper. It may however be suggested, frompreliminary field observations, that its smaller size, more timid dis-position, and perhaps its tendency to choose more conspicuousnest-sites, all combine to make the Song Thrush more vulnerableto man, animal predators and other disturbing factors than thebolder and more adaptable Blackbird, and so more liable to suffercomplete loss of its nest ; while a tendency to incubate moresteadily, and perhaps greater efficiency in finding food in the laterpart of the season, enable it on average to produce more youngfrom each successful nest than the Blackbird, and to maintain asteadier level of success throughout the breeding season.

REFERENCES

FISHER, J. 1 949. The death of birds. New Naturalist Journal, no.5:36-38 .

SILVA, E. T. 1949. Nest records of the Song Thrush. Brit. Birds,42 :97-I I I.

VENABLES, L. S. V. & VENABLES, U. M. 1952. The Blackbird in Shet-land. Ibis, 94 : 636-53-

VERHEVEN, R. 1 953. Etude statistique relative a la biologie de nostrois grives (Turdus sp.) indigènes. Gerfaut, 43:231-61.

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