Breeding biology of Buzzards at Sedbergh
during 1937-67
Michael Holdsworth
INTRODUCTION
Since 1937, observations on Buzzards Buteo buteo breeding within
ten miles of Sedbergh, Yorkshire, have been recorded by the Sedgwick
Society of Sedbergh School. The present paper shows the results of
an analysis of these records, supplemented by my own observations
during 1962-66. It examines aspects of the breeding biology, in
particular territorial behaviour and breeding success in relation to
numbers of Short-tailed Voles Microtus agrestis.
The Sedbergh area is typical of much upland hill country in northwest England. Cultivated land is confined to the Lune, Rawthey,
Clough and Dee valleys and gives way, at 750-1,000 feet, to treeless
fells used for sheep grazing. The steep sides of the fells, which rise
in places to over 2,000 feet, are broken by gullies containing fastflowing streams. These gullies, together with the more substantial
crags and areas of boulder scree elsewhere on the fells, provide the
main nest sites for Buzzards, but mixed plantations high up on the
valley sides, on the boundary between the cultiveted land and the fell,
are also used.
TERRITORY AND NEST SITES
Most pairs appeared to occupy several square miles. Because of the
difficulty involved in recording territorial behaviour over such
large areas of ground, and the absence of topographical features
which might delineate boundaries on the open fell, it proved difficult
to confirm the sizes of individual territories. In the study area, however,
there were five contiguous ones on a single tract of fell about 17
square miles (4,400 hectares) in extent. If we assume that each of the
five pairs had a territory of similar size, this indicates a density of one
pair to 3.4 square miles (880 hectares).
Each of seven territories where pairs were consistently present
contained more than one nest site. In many cases these included ones
which had been in intermittent use since 1937 and even before. Some
sites are occupied more often than others in the same territories: it
appears that these must possess certain features which make them particularly attractive, the wealth of used alternatives indicating that it
cannot be a general scarcity of sites which causes this persistent
adoption of just a few. In most territories one site seems to be used
more frequently than all the others; even so it is unusual for any to
412
Buzzards at Sedbergh during 1937-67
413
Fig. 1. Breeding histories of Buzzards Buteo buteo in territories 1-3 at Sedbergh,
Yorkshire, during 1948-67, showing the locations of nest sites. Underlining of
the site in the annual key indicates that breeding was unsuccessful and a dash
that no nest was found
be occupied for more than two years in succession. In the few territories where tree sites are the rule, alternatives in crags are occasionally
occupied; similarly, the habitual crag nesters will sometimes resort
to tree sites.
Fig. 1 sets out the histories of the three best documented of the
territories with maps to illustrate the locations of nest sites. Sites A and
B in territory 1, and likewise B, C and D in territory 2, are within 100
metres of each other; the other sites appear to be on the boundaries of
the territories and are up to 1½ miles (2.4 kilometres) from the more
regularly used ones. In territories 1 and 2 these favoured areas (A/B and
B/C/D) dominate the main valley, while in territory 3 most of the
nesting is divided between two sites in a double valley system.
Territorial considerations of this rotation from site to site are briefly
discussed below, but failure of breeding also appears liable to lead to
nest change. In table 1 the data from the three territories in fig. 1 have
been presented to relate success or failure of breeding to retention or
change of nest site. These data represent 27 two-year periods of
uninterrupted observations. They suggest that, after successful
breeding, the chances of a pair of Buzzards remaining for another
season in the same nest or moving to a new one are equal. After
Bazzards at Sedbergh during 1937-67
414
Table 1, Retention or change of site in 27 nestings of Buzzards Buteo buteo in
territories 1-3 at Sedbergh, Yorkshire, during 1948-67
These data are extracted from fig, 1
After failure
TOTALS
After success
Nest retention
Nest change
8
8
2
10
9
17
failure, however, a much greater proportion moves to a different site.
Reasons for the frequent rotation after success are not clear.
A feature in many cases is the occupation of fringe sites, which are
often poor in quality and generally on the edges of the territories.
This can be seen in sites C and D of territory 1, in site A of territory 2
and in site C of territory 3. Examples of these once only or occasional
nestings in unlikely sites were also recorded in other territories less
thoroughly covered. Such sites can easily be overlooked and must often
result in a nest not being found. They are clearly exceptions to the
more usual pattern of alternation between two or three sites. An
explanation for the occupation of fringe sites is suggested by the
correlation of their use with good prey years (pages 417-419). Between
1945 and 1966 there were seven good prey years and ten bad ones.
Seven of the recorded occupations of fringe sites were in the good
years and only three in the bad. Precisely why a temporary unfamiliar
nest may be used more readily when prey is abundant is not easily
understood. It could be that the stronger association with the more
familiar sites is not so important when food is easily obtained, or it
could be that there is a greater risk in these circumstances of a
pair which would otherwise not have bred claiming an unused part
of an established territory. If there were such a risk, the stronghold
of the territory marked by the old regular nests (which, even if not
being used for breeding, are often still decorated with greenery)
would require less defence than the peripheral areas; thus, having the
actual nest at the edge could be a means of safeguarding the whole
territory.
B R E E D I N G BIOLOGY
Breeding season
The date on which the first egg of a clutch was laid was taken to
indicate the start of breeding. This date was calculated in the same
way as in nest record card analyses (e.g. Newton 1964), using criteria
for Buzzards from Mebs (1964). Incubation was taken to begin with
the first egg in clutches of one and two, but with the second egg in
larger clutches, and to last 33 days. A three-day interval was allowed
between the laying of the eggs and a two-day interval between the
hatching of the chicks. Three classes of nest were included: (1) those
Buzzards at Sedbergh during 1937-67
415
found with incomplete clutches, i.e. with a number of eggs later
exceeded by the number of young (three cases); (2) those containing
both young and eggs which subsequently hatched (13 cases); and (3)
those in which the ages of the young had been estimated by the observer
as less than one week (13 cases). The median laying date was found
to be 21st April, with a range from 8th April to 9th May. (Mebs's
median date for 62 nests over a five-year period in Germany was
10th April.) Only one definite replacement clutch was recorded at
Sedbergh during 1937-67.
Clutch size
Table 2 shows the percentage distribution of the various clutch sizes,
together with comparative data from Denmark, Germany and the
New Forest, Hampshire. The Sedbergh clutches are those recorded
from all nests in which young subsequently hatched. Robbed or
deserted nests have been omitted, together with a single instance of
a nest with only one egg, it being considered in all these cases that the
true clutch size had possibly been altered before the nest was examined.
Some genuine clutches of one may thus have been excluded. Single
egg loss from clutches of two or more is not infrequent and must
often pass undetected: the effect of such losses would be to make the
mean clutch size a slightly low estimate. The data on their incidence,
however, are insufficient to allow a correction factor to be applied.
Mebs was able to demonstrate a correlation between high numbers
of Microtus voles and the incidence of larger clutches of three and
four eggs; the data are insufficient to see if this also applied in the
Sedbergh area. Each of the studies in table 2 contains a long run of
figures and presumably a reasonable sample of good and bad food
years. The higher mean clutch at Sedbergh thus appears to be due to
the compounded effect of those factors known to affect the number
of eggs—altitude, latitude and density, with food the proximate
factor as in Germany. The lowness of Tubbs's (1967) figures from the
Table 2. Clutch sizes of Buzzards Buteo buteo at Sedbergh, Yorkshire, during
1937-67, compared with those in three other areas
The comparative data from Denmark, Germany and the New Forest, Hampshire,
are from Holstein (1956), Mebs (1964) and Tubbs (1967) respectively. Figures in
brackets indicate the actual numbers of clutches of one or five recorded and are not
included in the percentages or means
Total
NUMBER OF EGGS
Mean
1
2
clutches
clutch
3
4
5
Sedbergh
Denmark
Germany
Hampshire
56
—
298
33
(1)
4%
(9)
27%
18%
33%
57%
64%
62%
55%
38%
9%
0%
3.03
2.67
(1)
2.40
18%
8%
5%
(1)
0%
0%
1.84
Buzzards at Sedbergh during 1937-67
416
Table 3. Brood sizes of Buzzards Buteo buteo at Sedbergh, Yorkshire, during
1937-67
NUMBER OF YOUNG
Total
1
2
broods
4
3
5
Number of broods
Percentage of total
18
24%
27
24
36%
32%
5
7%
1
1%
75
100%
New Forest, where there is a higher density of Buzzards than in the
other areas, provides support for his view that the dependence of that
population on avian prey has failed to make good a deficiency in the
small mammalian prey usually preferred and that the clutch size has
accordingly been adjusted.
Brood size
The frequency distribution of the various brood sizes at nests where
the young were at least three weeks old is shown in table 3. Some
nests which were visited at later dates had suffered losses and these
would tend to make the mean of 2.2 a high estimate. Other studies
gave means of 1.9 (Mebs 1964) and 1.4 (Tubbs 1967).
Information is available for 40 nests which were followed through
the main part of the breeding cycle, i.e. from the start of incubation
until the young were over three weeks old. Using these data, but
excluding nests in which all the eggs failed to hatch, the success
rate was 88 young from 118 eggs laid (75%). The incidence of egg
loss from all causes was greater than the number of small young that
died, but precise data on the proportions of each are lacking. From
similar calculations, Mebs showed a population in Germany to have
an almost identical success (73%), and Tubbs's figures from the New
Forest produce a ratio of mean clutch to mean brood size of 100:78.
Although these data are not directly comparable, the success rates
appear uniform and suggest that it is the adjustment of clutch size
which determines the number of young fledging.
FOOD
Prey species
No systematic attempt has been made to record prey remains, but the
following prey species have been noted at nest sites, all the birds
being nestlings or juveniles:
Domestic Fowl Gallus gallus
Red Grouse Lagopus lagopus
Carrion Crow Corpus corone
Jackdaw Corpus maneduh
Dipper Cinclus cinclus
Ring Ousel Tardus torquatus
Meadow Pipit Anthus pratensis
Shrews Sorex spp
Mole Talpa europaea
Rabbit Oryctolagus amiculm
Short-tailed Vole Microtus agrestis
Wood Mouse Apodemus syhaticus
Stoat Mustek ertninea
Weasel Mustek nivalis
Buzzards at Sedbergh during 1937-67
417
Food supply and breeding success
Moore (1957) and Mebs (1964) both regarded the Buzzard as an
adaptable predator that takes whichever of its regular prey species are
abundant in any particular area. Dare (1957) showed that, after the
drastic reduction in Rabbit numbers as a result of myxomatosis,
Short-tailed Voles had become a significant part of the diet of this
species in Devon. Prestt (1965) and Parslow (1967) considered that
by 1955 British Buzzards had become stabilised at a lower level than
before myxomatosis. Tubbs (1967) noted that, in the absence of
mammalian prey, the New Forest population was taking large numbers
of birds, a practice not previously recorded. Table 4 shows the breeding
history of Buzzards at Sedbergh from 1947 to 1967 when observations
were most consistent. It is unfortunate that all the Sedgwick Society's
records for 1955 have been lost, but the table shows that, even if there
was a drop in population or breeding success in that year, recovery
was total by 1957. Clearly, the Buzzards at Sedbergh adapted rapidly
to the reduction in Rabbits and the resulting decrease in available
prey.
Voles and mice (mainly Common Voles Microtus arvalis) constituted
67% of the prey items found in stomach analyses of this species in
Germany (Uttendorfer 1952); Mebs converted these data into proportions by weight of 4 6 % voles and mice and 30% other mammals
(mainly Moles, Rabbits and Hares Lepus europaeus), thus confirming
that voles and mice are the main prey of German Buzzards. N o
attempts have been made to determine precisely on what food Buzzards
at Sedbergh depend. Rabbits were certainly no longer available in
such large numbers following myxomatosis, but Short-tailed Voles
remain common and Simms (1961) has shown from pellet analysis
that voles constitute up to 58% of the breeding season diet of Kestrels
Falco tinnunculus in Yorkshire. The study area supports a relatively
high density of breeding Kestrels which are primarily associated with
the upper areas of cultivated land bordering the fell.
Snow (1968) has demonstrated that there is good agreement between
the years when voles are abundant in northern England and southern
Scotland (recently 1952, 1957, 1961 and 1964) and the years when
large numbers of nestling Kestrels are ringed in these areas; when
Table 4. Bleeding success of Buzzards Buteo buteo at Sedbergh, Yorkshire,
during 1947-67
No records are available for 1955. The numbers of young given are those which
actually fledged
47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67
Pairs
Young
4 6 3 7 6 4 9
2
7 5 5 9 4 6
5
6
? 4
7
? 3 15
5 9 9 9 6
4 8 17 17 4
6 6
0
11
7 7 7
7 3 7
418
Buzzards at Sedbergh during 1937-67
plotting the relative amplitudes of the various fluctuations in the
numbers ringed, however, he stressed the importance of compensating
for the general increase in ringing activity. Similarly, when the number
of Buzzards known to have fledged each year at Sedbergh was plotted,
it was apparent that, despite the marked fluctuations, there was a
general tendency towards an increase due to a rise in observer activity;
to compensate for this, a regression line was drawn by eye and an
index calculated for each year by expressing the number actually
recorded as a percentage of the value predicted by the regression line.
This index is plotted in fig. 2 and compared with the numbers of
nestling Kestrels ringed in northern Britain expressed as percentages
of the annual totals of ringed nestlings of all species in the whole of
Britain and Ireland (following Parslow and Snow). Both indices
suggest a series of regular fluctuations, but examination shows that
Fig. 2. Upper, annual index of Buzzards Buteo buteo known to have fledged at
Sedbergh, Yorkshire, during 1948-66 (there are no data for 1955): the numbers
recorded were an unsatisfactory measure of breeding success because of a general
increase in observer activity in later years; this bias was corrected by drawing a
visual regression line on the plot of the actual numbers and then calculating the index
by expressing the total for each year as a percentage of the value predicted by the
regression line. Lower, annual numbers of nestling Kestrels Falco tinnunculus ringed
in northern Britain, expressed as percentages of the totals of ringed nestlings of all
species in the whole of Britain and Ireland, during 1947-66: a national survey
resulted in special attention being paid to Kestrels from 1963 and so the numbers
in 1963-66 were adjusted to compensate (following Parslow 1967)
Buzzards at Sedbergh during 1937-67
419
the peaks of the Buzzards are more pronounced and more similar to
those of the Kestrels after 1954-55. Parslow also noted a tendency
for Buzzard numbers to fluctuate more markedly after 1954-55.
The conclusion is that the Buzzards became more dependent on
voles after myxomatosis reduced the numbers of Rabbits.
T o examine this conclusion further, the years 1947-67 were divided
into two periods, 1947-53 and 1956-66 with 1954-55 omitted, and a
X2 test was used to establish whether the numbers of Buzzards reared
in the peak Kestrel years (1949,1952,1957,1961 and 1964) were greater
than in the other years. The test established that, whereas the Kestrel
and Sedbergh Buzzard peaks in the early period did not coincide,
their later peaks showed a significant relationship (P<0.01). Snow
considered that in 1957, 1960, 1961 and 1964, years of vole abundance,
Kestrel breeding success was correspondingly high; the present
analysis suggests that Buzzards at Sedbergh were similarly affected
and thus provides further support for the conclusion that they became
more dependent on voles.
It is likely that Buzzards took voles to some extent before
myxomatosis, and it is unfortunate that the vole data for 1947-53
are incomplete, but it appears probable that any real dependence
on them is recent. Moore, Prestt and Parslow all noted local crashes
in Buzzard populations and breeding success in 1955, and numbers
of nestling Kestrels and Barn Owls Tyto alba ringed in Britain and
Ireland in that year were unusually low (see Parslow). This indicates
that a scarcity of voles, now suggested as the major alternative prey
of Buzzards, aggravated the effects of myxomatosis in that year and
that the gradual recovery after 1955 was as much a result of increased
vole numbers as of a return by Rabbits. In addition, there is evidence
that Buzzards and Kestrels breed at Sedbergh in different habitats,
though this is probably unconnected with food supply; Lack (1946)
has suggested that predators on Microtus voles do not effectively
compete with each other, because these small mammals are superabundant for much of the time and when their populations are low
each species of predator turns to different alternative prey.
ACKNOWLEDGEMENTS
I should like to express my thanks to past members of the Sedgwick Society of
Sedbergh School, without whose field observations this paper would not have
been possible. In addition, I am specially grateful to Ian Prestt, David Bishop and
Peter Mawby for thorough criticism of earlier drafts and to Colin Bibby who,
in addition to helping with the analysis, made the statistical tests and drew the
figures.
SUMMARY
Breeding records of Buzzards Buteo buteo in the area of Sedbergh, Yorkshire, over
30 years were analysed with particular reference to territorial behaviour and to
420
Buzzards at Sedbergh during 1937-67
breeding success in relation to numbers of Short-tailed Voles Microtus agrestis.
Histories of well documented territories indicated a breeding density of one pair
to 3.4 square miles (880 hectares). In most territories there was regular alternation
between a series of nest sites, and reasons for this are suggested. The median laying
date was 21st April and the mean clutch size of 3.03 is discussed with reference
to comparative data from other published studies. The mean brood size was 2.2
and the success rate (fledged young from eggs, excluding nests in which all the
eggs were lost) was 75%. Annual totals of Buzzards fledged are considered in
relation to the food available, especially in the light of recent work on the effect
of fluctuations in vole numbers on the breeding success of Kestrels Falco tinnunculus.
The conclusion is reached that, as a result of the reduction in the numbers of
Rabbits Oryctolagus cuniculus following myxomatosis in 1954-55, the Buzzards at
Sedbergh have become increasingly dependent on voles and that for this reason
their breeding success now fluctuates in a way similar to that of Kestrels.
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Michael Holdsworth, 27 Leggfield Terrace, Warner's End, Hemei Hempstead,
Hertfordshire