Habitat variation and population regulation in Sparrowhawks
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1BIS 13.3 suppl. I : 76-88
Habitat variation and population regulation in Sparrowhawks
I. NEWTON
lnstitute of Terrestrial Ecology, Monks Wood Experimental Station. Abbots Ripton. Huntingdon P E I 7 2LS, U K
In a study area in south Scotland, Sparrowhawks did not occupy the available nesting places
at random, but more often used those places where breeding success was highest (here called
high-grade places). Most birds stayed on particular nesting places for only one year, but
others stayed up to 8 years. Some birds moved from low- to high-grade places as they aged.
Continued occupancy of certain places was thus produced by many different individuals
occupying such places in rapid succession, but most staying for only one breeding season.
On the most used (high-grade) nesting places pairs produced more than enough young per
breeding attempt to offset the average annual mortality, but on the less used (low-grade)
places they produced too few. Low-grade nesting places therefore acted as a sink, whose
occupancy could be maintained only by continual immigration. Over the study area as a
whole, the population was in balance, with reproduction matching mortality.
Habitat changed over periods of 15-30 years, as woodland matured. Nesting places in
young woods, with small densely-growing trees, showed the highest occupancy and nest
success. Both aspects of performance declined as the woodland aged, and trees became larger
and more widely spaced. Long-term stability in nest numbers and success in the study area as
a whole was associated with a system of rotational forest management, which ensured a
continuing availability of young woods.
It is proposed that spatial variation in habitat quality is involved in the regulation of
breeding numbers. Removal experiments confirmed the presence of non-breeders, which
could attempt to breed when high-grade nesting habitat was made available to them, but
otherwise remained as non-breeders despite the presence of vacant low-grade habitat. This
situation, involving a n interaction between habitat quality and bird quality, probably occurs
in some other raptors too.
During 1972-1988 the number of Sparrowhawks Accipiter record the breeding performance, and to ring the young and
nisus breeding in Eskdale. south Scotland, varied little. with as many breeding adults as I could catch. Further details of
no overall trend. Over the whole 17-year period, nest the study area and methods may be found elsewhere (Newton
numbers fluctuated by no more than I 5% of the mean level of 1986).
34 (Newton 1986, unpubl.). Nonetheless, over this period
the distribution of the breeding population within the study
area changed, as some nesting localities were abandoned and
NESTING HABITAT
others were taken up. These changes were associated with
changes in the distribution and structure of the woodland. In In the study area, almost all woodland was coniferous,
this paper, I examine spatial and temporal variation in mainly spruce Picea abies and P. sitchensis, with some larch
habitat use and explore the role of habitat heterogeneity in Larix spp. and pine Pinus sylvestris. It was managed for timber
population regulation. production, mostly on a 40-5 5 year rotation, depending on
In Eskdale. as elsewhere, Sparrowhawks are associated location. Every year some patches were clear-felled and re-
primarily with woodland: they nest and roost in woods and planted, while others were thinned, so that at any one time
do most of their hunting in woody habitats. However, they the area contained a mixture of tree stands of various ages,
also hunt in open country, wherever small-bird prey is and a fairly stable age structure over time. Thinning was first
available. Through suitable woodland, the nests of different done at about 16-20 years after planting, and then 3-5
pairs are spaced about 0.6 km or more apart. A pair raises no further times at 5-8 year intervals, each time removing a
more than one brood per year. Each year, in a 2 0 0 km2 study proportion of the trees and progressively opening up the
area (55"IO'N, 3Oo'W). I attempted to find all the nests, stand. Typically, after the first thinning the trees averaged1991 HABITAT VARIATION A N D POPULATION REGULATION I N SPARROWHAWKS 77
about 13.4 m in height and 3.8 m apart; after the second which nested in small woods also hunted in nearby open
thinning about 16.5 and 4.7 m, after the third about 18.5 land.
and 5.4 m, after the fourth about 20.5 and 6.4, and after the Throughout this paper. I shall base the argument on
fifth about 22.0 and 8.7 m (Table I). When the trees reached nesting places because their use was documented fully and
40-55 years of age they were usually clear-felled. Circum- consistently every year, whereas the associated hunting
stances such as location, windthrow and changing demand ranges were determined only for a small number of birds in a
for timber modified this pattern from time to time. The small number of years. A nesting place was classed as
general result, however, was that, as the woodland matured, occupied if a new nest was built, and on most of the
the trees became larger and more widely spaced, and ground ‘unoccupied’ places no signs of Sparrowhawks were found
and shrub cover became more established. The same process that year at any stage through the breeding season. From
occurred in unmanaged areas, but more slowly and in less radio-tracking, however, it seemed that every wood in the
regulated fashion, because it was not aided by periodic area was visited from time to time by several different
thinning. Sparrowhawks on hunting forays: but birds did not roost
Usually Sparrowhawks first occupied the woods for breed- regularly on ‘unoccupied’ places, nor did they defend them.
ing within a year or two after first thinning, for not until then
could they fly easily among the trees. Occasionally, however,
they occupied thicket woods at a slightly earlier stage, if a
SPATIAL VARIATION IN HABITAT QUALITY
stream or wind damage provided localized access. The birds
built their nests in the lower canopy and conducted most of In this paper I use occupancy and nest success as measures of
their activities below the canopy, rather than above it. bird performance. and ultimately as measures of habitat
quality. They are treated as separate measures of perfor-
mance, although in reality they represent different points on
the same continuum leading to production of young.
NESTING PLACES
In Eskdale. as in other areas where Sparrowhawks have
In Eskdale, as in other areas where Sparrowhawks have been studied (Newton & Marquiss I 9 76, Newton et al. I 9 79),
been studied, birds bred in the same restricted localities over not all known nesting places were occupied at any one time.
several years. They usually built a new nest each year, near In different years some 55-61 places were judged suitable
old ones, so that established breeding sites were each marked (and were used at some time), but no more than 39 were used
by a group of nests of varying ages, mostly lying within a in any one year. Moreover, some places were occupied in
circle of about 50 m radius. every year, or in most years, while others had nests only in
When occupied, such nesting places could equally be occasional years. This was not the result of independent
called ‘nesting territories’ because they were both exclusive random settling each year: over a period of years, some places
and defended. Each was occupied by only one pair at a time, were occupied more often than expected by chance at the
which aggressively prevented any other Sparrowhawks from population levels found, whereas others were used less than
breeding in the vicinity (Newton I 986). Radio-tracking work expected by chance (shown in Fig. I for periods of 5, 10 and
revealed that each nesting place lay within the hunting range I 5 years, respectively). This implied that certain nesting
of a different male (Marquiss & Newton I 982). At the start of places were preferred, while others were avoided to some
breeding, males hunted mainly near the nest, so that their extent.
hunting ranges were almost mutually exclusive but as the In Table 2 , using data for a 15-year period, nesting places
season progressed they foraged further afield, so that their are graded 1-5, according to the number of years in which
ranges came to overlap. At the same season in different years, they were occupied. Grade I places were occupied in 1-3
however, the hunting ranges associated with particular years (not necessarily successive years) in the I 5-year period,
nesting places were fairly consistent in position and extent Grade z places were occupied in 4-6 years, and so on to
(Marquiss & Newton 1982). Males which nested in large Grade 5 places which were occupied in I 3-1 5 years. Nesting
woods hunted mainly or entirely in woodland, whereas those places which were available for less than I 5 years (because of
Table I. Mean heights and spacing ojtrees (metres) in conifer woods at different growth stages
Number of thinnings received
I 2 3 4 5
Mean tree height ( fSD) 13.44 f 2 . 0 6 16.50+ 2.8 3 18.47f2.6y 2 0 . 5 0 f 2.12 22.00f2.82
Mean tree spacing ( f SD) 3.77 0.4 7 0.74 f0.59 5.42*0.62 6 . 4 0 f 0 . 3I 8.65f 1.4678 I . NEWTON IBIS 1 3 3 SIJPPL. 1
m Observed
0 Expected
20
Ic
20/
10 10
C 0
1 2 3 4 5 1 2 3 4 5 6 7 8 9 1011 1 2 1 3 1 4 1 5
Number of years in which nesting occurred
Figure I. Observed and expected occupancy of Sparrowhawk nesting places over periods of 5. t o and I 5 years in Eskdale. south Scotland. In each
period. birds occupied certain places more often than expected by chance at the population levels found, indicating preference, and other places
less often than expected, indicating avoidance. Expected frequencies were calculated assuming that, in each year, available nesting places were
occupied at random, and independently of previous history.
Comparison of observed and expected frequencies: 1972-1976, xz4= 28.8. P < 0.001;1972-1 98 I , xz4= 103.6. P < 0.001.combining classes
1-4 and 8-10: 1972-1986. xzs= 198.4. P1991 H A B I T A T V A R I A T I O N A N D P O P U L A T I O N REGULATION I N S P A R R O W H A W K S 79
Table 3. Breedingperformance of Sparrowhawks according tofemale age and grade of nesting place. n = number of nests, S = number of nests which raised
young. B=mean brood size in successful nests, M=mean number of young raised per nest including failed nests
Grade of First-year females Older (2+year) females
nesting
place n S(%) B M n S(%) B M
1-2 I2 5 (42) 3.20f0.20 1.34 35 19 (54) 3.44 f 0 . 4 1 1.86
3 20 9 (45) 3.00~0.62 1.35 87 58 (67) 3.47fO.I 7 2.32
4 I5 11 (73) 2.91 f0.21 2.12 89 59 (66) 3.72fO.I 5 2.46
5 I2 9 (75) 3.67f0.33 2.75 82 66 (81) 3.72 3z 0.1 7 3.02
Significance of variation among grades of nesting place in the proportion of nests successful: first-year females. xz3= 5.56. P > 0. I , < 0 . 2 (but
Grades 1-3 versus4-5, x 2 , = 5 . 5 8 . P t 0 . 0 5 ) : olderfemales, ~ ' ~ = 9 . 1 1P.< o . o s .
Table 4. Causes of breeding failure among
Number (%) of nests which failed through Sparrowhawks on nesting places of different
Grade of Total grade
nesting number of Desertion Predation of Other
place nests Nan-laying of eggs eggs or chicks causes*
* Mostly unidentified, but also including nest collapse, egg addling and human interference.
Lack of significance in variation between grades in frequency of different classes of failure:
~ ' , ~ = 5 . 6 6n.s.
.
Table 5. Post-fledging fate of young Sparrowhawks, reared on different grades of nesting place. as indicated by ringed birds subsequently reported dead by
members ofthe public, or trappedalive while breeding in the study area. Note that data are not comparable between the sexes, because of different dispersal and
reporting frequency
Males Fern a I es
-
Grade of Number (%) Number (%) Number (%) Number (%)
Nesting Number reported found breeding Number reported found breeding
place ringed dead in study area ringed dead in study area
There was no significant variation in fate associated with grade of nesting place. Males: reported dead, xZ3= 7.75. n.s.: found breeding, xZ3 = 1.41,
n.s.: females: reported dead, xz3=o.gg. ns.: found breeding xZ3=2.42. n.s. In each analysis data from Grades 1-2 were pooled. n.s.=not
significant. However, the proportion of males reported dead from Grade 5 territories was significantly greater than the proportion recovered from
Grades 1-4 pooled (xz = 7.24. P i 0.01). It is hard to know whether to attach any biological significance to this result, as no similar differencewas
evident for females found dead, or for males or females found breeding.80 I . NEWTON IBIS 1 3 3 SIIPPL. 1
low-grade nesting places, and no one cause was pre-eminent 8
(Table 4). However, previous work had suggested that the 7
frequencies of nest failures from all such causes were b 6
increased by food-shortage, which prevented birds from 'i5
L
c
laying eggs, or caused them to abandon the eggs altogether,
'ij4
or to leave eggs and chicks unguarded in favour of hunting
& 3
(Newton 1986). Thus the greater frequency of nest failures, d
2
from whatever proximate causes, in low-grade nesting 5
2 1
habitat could be largely a manifestation of a single underly-
ing cause, namely more frequent food-shortage. 12 4 6 8 10 12 14 16 18 2 0 2 2 24 2 6 2 8 30
Once young had left the nest, there was no obvious Number of young
variation in their subsequent fate that was related to grade of Figure 3. Numbers of young produced on 51 Sparrowhawk nesting
places which remained continuously available over a Io-year period,
1977-1986.
Q nesting ptace. Thus the proportion of ringed fledglings which
0 was later trapped alive in the study area, or found dead and
reported by members of the public, showed no variation
0 0 associated with grade of natal habitat (Table 5). This was
probably because, once birds became independent, they
A 0 dispersed and mixed in the wider countryside.
As a result of differential occupancy and nest success, over
a period of years certain nesting places produced many more
young than others. This variation over a 10-year period is
illustrated in Fig. 2 for those nesting places which remained
available, and were judged fit for occupation, in all 10years.
V 3 On average, each nesting place produced 10.7young in the
- 0 10 years, but the variation was great, as 14% of places
Qo 0 produced no young in that time, while others produced up to
33 (Fig. 3). The most productive 10% of nesting places
produced 27% of all young, and the most productive 50%
produced 83% of all young. Productive and unproductive
nesting places were not concentrated in different parts of the
0 study area. but were interspersed (Fig. 2). Indeed some highly
0 A
6; productive places were close to wholly unproductive ones.
For this sample of nesting places, about 88% of the variation
in productivity over 10 years was associated with variation in
the number of years that the place was occupied (Fig. 4).
0
0" QQQ A
However, a quadratic regression model, with the intercept set
at zero, gave a better fit to the data than a linear model. This
reflected the fact that the number of young raised per nesting
attempt was also highest on the most used places (Fig. 4).
Q*
-5Km-
TURNOVER A N D MOVEMENTS OF
Figure 2. Distribution and productivity of 5 I Sparrowhawk nesting
places which remained available continuously over a lo-year period
BREEDING BIRDS
(1977-1986). Shading within each circle depicts the number of This section is concerned with the residence periods of
young produced over the 10years as a proportion of the maximum individuals on nesting places, and with the year-to-year
possible (60).
movements of individuals between places. As the grading of
The distribution of nesting places is constrained by the distribution
nesting places depended partly on these same features, grades
of woodland (not shown), and the map excludes all places which
were available for only part of period concerned (because of were re-allocated for each bird from a period of years (out of
woodland growth and felling).Note that nesting places varied greatly the full 15) which excluded the residence or movement
in their productivity, and that productive and unproductive places periods of the individual concerned. This procedure gave
were interspersed through the area. some degree of statistical independence to the measures being1991 H A B I T A T V A R I A T I O N A N D P O P U L A T I O N REGIJLATION I N S P A R R O W H A W K S 81
35
0
Figure 4. Number of young raised at
30 different Sparrowhawk nesting places over
a 10-year period in relation to occupancy.
The total numbers of young produced at
25 each nesting place ( T ) depended largely on
0 the numbers of years that each nesting
place was occupied (N), and partly on the
-0 mean numbers of young produced per
a 20 0
nesting attempt ( Y ).
.-v)
E Statistical details: on a linear regression
0 model, with the intercept set at zero. 84.5%
5 15 of the observed variation in T-values could
? be explained in terms of N. On a quadratic
relationship (T= 0 . 3 8 2 N + 0.203NZ).some
87.8% of the variation in T-values could be
10
explained in terms of N. The quadratic term
0 0 0
thus explained nearly 21% of the
outstanding I 5.5% of the variation after
5 fitting the linear model. This reflected the
m e fact that the mean number of young
I m o m
produced per breeding attempt ( Y )was
higher on the most used territories. On a
0 1 linear regression, Y=o.918 +o.13oN. with
1 5 3 4 5 6 f 8 9 'O N explaining I 3.4% of the observed
Years occupied variation in Y-values (P82 I. N E W T O N IBIS 1.3.3 SIIPPL. 1
or movement to another place. Most birds which changed Table 8. Mean contribution to future nesting population made by
nesting places moved less than 5 km, but a few moved further Sparrowhawk breeding attempts in different grades of nesting habitat
(Newton 1986).
Year-to-year fidelity to particular nesting places could be
Mean number Balance between
examined from individuals trapped in successive years. As of young Mean number mean annual
expected from the preceding analysis, the tendency to stay on Grade of females that survive production
the same nesting place from one year to the next increased nesting raised to nest and mean
with grade of nesting place (Table 7). Moreover, of birds place per nest’ themselves’ annual mortality3
which changed nesting place, about 70% moved to a place
classed from other years as being as high or higher in grade 0.595 0.179 Negative
than their previous place (Table 7). This was a significantly 0.785 0.2 36 Negative
larger proportion than expected if birds had resettled on 0.800 0.240 Negative
nesting places a t random (xZ4=14.6, P1991 H A B I T A T V A R I A T I O N A N D P O P U L A T I O N KEGXJLATION I N S P A K K O W H A W K S 83
breeding beyond the first year had started earlier, a bigger would normally occupy high-grade habitat (fitting the
pool of recruits would have been available. Thus for every ten supposition that they came from a non-breeding pool), and
females which delayed breeding to the third year, there must, that breeding performance depends on some interaction
on average mortality rates, have been around 14 present as between habitat quality and bird quality. On such small
non-breeders in their second year, and these in turn would samples caution should be attached to the occurrence of
have been derived from at least 2 0 one-year-olds. Likewise. replacements only on high-grade nesting places, but the
many males did not breed until after their first year, although findings were consistent with the view that some (mostly
the mean age of first breeding may have been lower than in young) birds were able to breed only on good nesting places
females. In other words, rapid growth in the breeding and not if the only places available to them are poor. This
population could have occurred at any time if some males would explain why some (by definition low-grade) nesting
and females had bred at an earlier age. That they did not do places remained vacant every year, despite the presence in
so. in the presence of vacancies among potential nesting the general area ofpotential breeders in their first 1-3 years of
places, implies some form of constraint, which resisted life.
growth in breeding numbers.
Almost certainly this constraint was imposed by the
habitat, or by some interaction between the bird and the
TEMPORAL VARIATION IN HABITAT
habitat. Assume that the birds themselves varied in quality
QUALITY
(or hunting skill) as well as the habitat, and that some poor
quality (largely young) birds could breed only in good habitat The sections above were concerned with spatial variation in
but not in poor. Because all stages of breeding are contingent habitat quality (as judged by occupancy and nest-success),
on a good food-supply, and a good food-supply is dependent and its role in the regulation of breeding density. It remains to
partly on the quality of the local habitat and partly on the examine temporal variation, as habitats change through
hunting skills of the bird itself, this is not an implausible idea, time. Over the years the trees in the nesting areas became
as explained below. In the Sparrowhawk the hunting skill of larger and (through thinning) further apart, so that the
the male is probably more important than that of the female habitat gradually gained a more open structure. The question
because the male provides most of the food for the breeding is whether nesting places that were favoured in the short
attempt. term, say over 5-10 years, were similarly favoured in the
In nearby areas (mostly in Annandale) of similar land- longer term! Taking the data for Eskdale over a 15-year
scape. the presence of surplus potential breeders was con- period (1972-1986), I checked whether nesting places that
firmed experimentally in 1975-1977 when seven males and were most used in years 1-5 were also most used in years 6-
seven females were removed from nesting places in late April, 10and in years I 1-1 5. Frequency of occupation was indeed
just before egg laying (Newton & Marquiss 1991).Within 4 correlated between these three 5-year periods (Table 9). but
weeks, at least three males and three females had been all regression slopes were less than unity, which could have
replaced, and in four of the six nests eggs were subsequently implied a decline in mean occupancy over time (see below).
laid. The incomers had not moved from neighbouring nesting This was also evident in the histories of individual nesting
places, whose occupants were marked, and on capture three places. When the study began, many of the woods had been
out of five were found to be first-year birds. In the rest of the used by breeding Sparrowhawks in previous years, as was
population, about 18% of males and 15% of females were evident from the presence of old nests. But other, younger
yearling birds in these years. It seems reasonable to conclude woods were occupied for the first time during the course of
that, without a nesting place being made available to them, the study (Fig. 5). For these latter woods, both occcupancy
these birds would not have bred that year, and that they came
from the non-breeding pool.
The removals were done to test for the presence of surplus Table 9. Linear regression relationships depicting the consistency in
birds, but as it happened, ten were on high-grade nesting occupancy by Sparrowhawks of the same nesting places in successivefive-
places (Grades 4 - 5 ) . and four were on low-grade ones year periods. 1 9 7 2 - 1 9 7 6 ( A ) . 1 9 7 7 - 8 1 (B) and 1 9 8 2 - 8 6 ( C ) . 42
(Grades 1 - 3 ) . All the replacements were on high-grade nestingplaces were available throughout periods A and B, 5 2 throughout
places, and on the low-grade places no further breeding periods B and C. and 3 8 throughout periods A, B and C
activity occurred after the removals. This indicated a signifi-
cant preference for high-grade places ( P = 0 . 0 3 5 , Fisher’s Regression Statistical
Exact Test, two-tailed). Moreover, only one of the six coefficient ( b ) significance ( P )
replacement nests produced young, the rest failing for
reasons usually associated with food-shortage (non-laying, Between periods A and B* 0.81984 I. NEWTON IBIS 133 SIJPPL. 1
ri rz 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 Nesting
n
.1 0 . 0 . 0 opportunities152 114 68 36 11
2 . 0 . 0 . L 0 .
3 0 . 0 0 . . 0 %
4 0 . 0 0 0 . 0 0 . 100 I
5
6
0
0
. .
. 0 0 .
0 C - c 80
7 0 0 0 0 0 0 0 , 0 0 0 ~ . L
8 0 0 . 0 . . 2 60
............
9 a¶
10
40
..........
11 a9
12 0 . 0 20
...........
13
14 . . o r C O 0 . . -
n
..........
15
1-5 6-10 11-15 16-20 21+
........
16 0 0 0 3 0 0 0 0 0 0 0 0
ir
........
18 0 . 0 . 0 3 0 . C
19
20
.......
21 0 . . C 3
22
.....
23 0 3 . 0 0 . C
24 .3..0..,^
25
26 G G O
2r
28 0 . 0 .
29 O C E 1-5 6-10 11-15 16-20 21+
30 0 . .
31 0 . Years from first occupation
32 0 .
Figure 6. Occupancy and nest success at Sparrowhawk nesting
Figure 5. Yearly occupancy and nest success of 32 Sparrowhawk places, according to years from first occupation. Full details in Table
nesting places in young woods occupied for the first time during the 10.
study. Both occupancy and nest success tended to be high in the first
5 - 1 0 years after initial occupation and then declined in later years.
Filled circles-successful nests: open circles-failed nests.
I 5th year from estimated first occupation they showed even
lower occupancy (25%) and nest success (44%) than
younger woods (Fig. 6, Table 10).Some of the oldest woods in
and nest success were high in the first 5-10 years after the the study area have now not been used by nesting Sparrow-
initial occupation, and then declined in later years, In three hawks for more than 10 years. Their histories suggest that
successive yyear periods from first use. mean occupancy of Sparrowhawks did not nest in them later than 20-25 years
the same nesting places dropped from 92% to 75% to 46%. after first occupation, when the trees were 40-45 years old.
and the proportions of nests that were successful dropped At this stage, the mean distance between trees exceeded 6 m,
from 76% to 62%to 45%(Fig. 6, Table 10). Moreover, in any tree heights exceeded 2 5 m, and bramble and other shrubs
one calendar period, the younger, and more recently occu- were prevalent in the ground vegetation.
pied, woods showed greater occupancy and nest success than In conclusion, therefore, for the first 10years or so after the
the older woods (Table 11). In these analyses there was a first nest, when trees were up to I 7 m tall and up to 5 m apart,
slight upward bias in the figures for occupancy in the first 5 the average nesting place showed both high occupancy and
years, in that a territory had to be used at least once in this high nest success. Then as the trees grew and their spacing
period to be included, but this was not the case for later increased through thinning to around 6 m. the average
periods. nesting place showed progressively lower occupancy and
Coincident with the declines in occupancy and nest poorer nest success. Thereafter, at even wider tree spacings.
success, the mean residence periods of females in successive former nesting places were rarely occupied, and the four nests
5-year periods fell from I. 7 to I .4 to I . I years, and the mean that were found in stands at this stage all failed to produce
age of female occupants fell from 3.5 to 3 . 1 to 2 . 7 years young. The relative stability of breeding numbers in the study
(Table 10). First-year females were more frequent as breeders area over the years was thus associated with a system of
on long-established nesting places than on newly occupied rotational forest management, which ensured a continuous
ones (Table I 0). However, none of these differences achieved supply of woods at the preferred stages. Birds were conti-
statistical significance. nually abandoning the older woods in favour of younger
Some woods in the study area were occupied over a period ones. Thus, while the numbers of breeding Sparrowhawks
exceeding I 5 years, having been used both before the study remained fairly constant over the study period, their distribu-
started (as known from old nests) and during it. After the tion over the area changed slowly with time.1991 H A B I T A T V A R I A T I O N A N D POPULATION R E G U L A T I O N I N S P A R R O W H A W K S 85
Table 10.Occupancy and nest success at Sparrowhawk nesting places. according to gears from first occupation
Years from fist known occupation’
Significance of
1-5 6-10 I 1-1 5 16-20 21 variation
Number of nesting opportunities2 114 68 36 I1 -
Number of nests (%)’ 85 (75) 31 (46) 9 (25) o (0) xZ4=115.2,P86 I . NE W T O N IBIS I33 SIIPPL. 1
labie 12. Breeding performance of Sparrowhawks on nesting places between nesting places of different grade was almost cer-
gradrd in two diflerent ways (see text) tainly due partly to habitat but was greater than could be
attributed to habitat alone. In other words, the effects of
Grade of On stable scoring system On sliding scoring system habitat and bird quality were probably additive.
nesting The removal experiments gave further evidence that both
place n S(%) M n S(%) M habitat and bird contributed to performance. The replace-
ment birds, when given access to high-grade nesting places,
1 26 S(31) 1.19 27 8(30) 1.04 at least attempted to breed (and in one case raised young).
2 54 26(48) 1.57 50 21(42) 1.54 Without the experiment they would presumably have
1 161 75 (47) 1.60 82 39 (48) 1.60 remained as non-breeders despite vacancies in low-grade
4 150 90 ( 6 0 ) 2 11 127 63 (50) 1.75 habitat. In other words, the quality of habitat available to
5 123 87(71) 2.63 228 155 (68) 2.44 them influenced their performance. At the same time. as a
group they bred significantly less well than other Sparrow-
n=number of nests, S=number of nests which raised young, hawks in high-grade habitat laying at the same date (Newton
iVl=mean number of young raised per nest (including failed nests). & Marquiss 1991). indicating that they themselves were less
effective than the usual occupants of such habitat. So bird
quality also influenced breeding performance.
as a major factor influencing habitat quality for Sparrow-
hawks. But it was not the only factor involved. Previous
Habitat destruction and breeding numbers
analyses revealed that size (i.e. extent) of wood influenced
occupancy and success (Newton 1986).as did the location of Interaction between habitat and bird quality complicates any
the wood within the landscape, and the nature of surround- prediction of the effects of habitat destruction on breeding
ing habitat (Newton et nl. 1979).This was as expected numbers. Clearly, in Eskdale the removal of a proportion of
because, as explained above, Sparrowhawks hunted not only low-grade (‘sink’)habitat need have caused no reduction in
in the nesting wood itself, but also further afield in the overall breeding numbers because surplus, unoccupied, low-
surrounding terrain. grade habitat was always available. But the removal of a
proportion of high grade (‘source’) habitat could have
affected pair numbers not only in that habitat but also in low-
DISCUSSION grade habitat, because occupancy here was dependent on
surplus production in high-grade habitat. In other words, the
Assessment of habitat quality
impact on overall breeding numbers of destroying ‘source’
Habitat quality can only be assessed from the performance of habitat would be greater than predicted from the area
the birds which occupy it (or similar habitat). However, involved. In theory, if only low-grade (‘sink’) habitat
breeding success does not depend solely on the habitat, but remained, and there was no further immigration, the local
also on the particular individuals which live there. If the best population could have died out.
birds (the most competent breeders) occupied the best Events would have been altered, however, if the best birds,
habitat, the effects of habitat and bird quality on observed which would normally occupy good habitat, had no choice
performance would be confounded. but to occupy poor habitat, because productivity in the poor
In Eskdale, both habitat and occupants probably contri- habitat could then be raised. The impact on overalt breeding
buted to recorded performance, because young birds (with numbers would depend on whether the rise in mean
low nest success) more frequently nested in habitat classed as occupant quality in poor habitat could change the balance
low-grade. Strong (but not wholly conclusive) evidence that between reproduction and mortality from negative to posi-
habitat quality influenced occupancy and success, indepen- tive: in other words, whether the improvement in occupant
dently of occupant, was of three kinds: ( I ) among birds of a quality could change ‘sink’habitat into ‘source’habitat. This
single age group, breeding was more successful in the habitat is a critical point which only further experimentation could
classed as high-grade (see above): ( 2 ) high occupancy and illuminate.
nest success was maintained on some nesting places over
periods of 10-1 5 years, despite frequent changes in occupant
Problems in the estimation of adult survival
(mean residence period < 1.5 years) (performance in one 5-
year period could be used to predict performance of a largely Insofar as one could judge from subsequent ring recoveries,
different group of birds in the same habitat in the next 5-year post-fledging survival was similar for young produced on all
period, and the one after): and ( 3 ) a ‘repeatability’ analysis. grades of nesting place. This was perhaps not surprising
comparing productivity of the same nesting places with because, within four weeks after leaving the nest, the young
different females and of the same females on the same or dispersed and were free to compete on their own merits.
different nesting places (Newton I 988). revealed that habitat Unfortunately, it was not possible to compare the survival of
contributed to nest success over and above any effect of breeding adults from different grades of nesting place. As
occupant. Thus the variation in recorded performance birds more often moved away from low-grade nesting places,1991 H A B I T A T V A R I A T I O N A N D POPULATION R E G U L A T I O N I N SPARROWHAWKS 87
it was usually unknown whether their disappearance was evaluate. In the study area, Sparrowhawks preferred thicker
due to movement from the study area or to mortality. The woods for roosting in winter (Marquiss & Newton 1982). as
analysis of ‘source’and ‘sink’habitat was therefore done on well as for nesting, and some of the more mature stands were
the assumption that breeder survival was similar across all extremely open and windswept in early spring, when nesting
grades of nesting place. If this was not the case, and breeders started.
survived less well in low-grade habitat, the effect would be to Whatever the reasons for Sparrowhawks favouring and
widen the difference between ‘source’and ‘sink’habitat: i.e., breeding most successfully in young woods, commercial
the ‘source’ habitat would contribute more to the mainte- forest in much of Britain (as in the study area) is now
nance of the population, and the ‘sink’ habitat would managed in a way that promotes consistently high Sparrow-
contribute less, than was evident from reproduction alone. hawk populations, namely on a fairly short rotation that
ensures at any one time a large proportion of forest in the zo-
35 year class. Of course, there are local variations: where
Habitat deterioration
extensive areas of forest suddenly reach a suitable stage,
In Eskdale, dense young woods (of 20-35 years old) were Sparrowhawk breeding numbers can increase rapidly, while
preferred for nesting and facilitated good reproduction. This in other areas where much of the available forest suddenly
may have been because woods of this age provided a better exceeds this age, then declines occur. Examples of both
food-supply, less risk of predation or more shelter from occurred in other areas during the study. In monitoring
inclement weather. Because most nest failures could be hawk populations, it is therefore important to record the state
attributed, directly or indirectly, to food-shortage, food of the woodlands, and at the same time to search the younger
availability might have declined with increasing age of forest. stands continually as they become suitable.
Overall densities of song-bird prey did not decline in conifer Although this study was in managed plantations, the same
stands between 2 0 and 5 0 years of age (Moss 1978, Moss et picture would almost certainly hold in natural forest where
al. 1979). But at any given density, prey may have been more Sparrowhawks prefer the younger areas, for example where
readily caught in young woods, if only because the denser regeneration follows a fire or landslide, where forest invades
cover allowed the hawks a closer undetected approach. open land, or where short, dense tree-cover occurs for some
Radio-tagged male Sparrowhawks (which provided most of other reason. In the same way that woodland habitat is
the food during the breeding season) showed a marked divided between two Accipiter species in Europe, it is shared by
preference for hunting in woodland as opposed to open three in North America, where the small Sharp-shinned
country and, within woodland, preferred the younger, denser Hawk A . striatus breeds mainly in the youngest, densest
areas. It would be hard to conceive of any reason for this areas: the medium-sized Cooper’s Hawk A . cooperi mainly in
preference, other than increased hunting success, but this the intermediate areas, and the large Goshawk in the most
was not measured. mature, most open areas (Reynolds et al. 1982, Moore &
Another possibility was that young woods were favoured Henny 1983). Thus the same woodland during its growth
for reasons other than food-supply but, because they can become suitable for each species in turn.
attracted the ‘best’birds (say, the most competent hunters),
food-related failures were less frequent there. Irrespective of Parallels with other birds of prey
food-supply, predation risk was greater in older woods,
because they provided less cover than younger woods and Some other species of raptors which have been studied -
were more attractive to relevant predators. Over most of the including both forest and open country species - show
range the most important predator of adult and nestling similar features to the Eskdale Sparrowhawks: (a) only a
Sparrowhawks is the Goshawk Accipiter gentifis. a larger proportion of available nesting places occupied in any one
species which occupies older, more open, stands (Hald- year: (b) some places favoured over others during a period of
Mortensen 1974, Newton 1986). This species was absent years: (c)better nest success on the favoured (most frequently
from Eskdale (owing to past persecution), but the preference occupied) places: and (d) the presence of ’surplus’ birds,
of Sparrowhawks for young woods could have represented an which can replace established birds lost from prime nesting
inherent response, evolved in the past to avoid predation. places. All four features have been shown for the Peregrine
Another predator of nestling Sparrowhawks, the Tawny Owl Falcon Falco peregrinus (Hickey 1942. Hagar 1969, Ratcliffe
Strix aluco, was common in Eskdale. This species also hunts 1980). while features (a) and (b) have been shown for Gyr
more in open stands than in dense ones (S. I. Petty, pers. Falcon F . rusticolus (Nielsen & Cade 1990). Merlin F.
comm.), probably because, with better ground vegetation, columbarius (Newton et al. I 986),Buzzard Buteo buteo (Tubbs
open stands provide more abundant rodent prey. Crows and 1974) and Rough-legged Buzzard B. lagopus (White & Cade
other corvids are perhaps also more likely to find nests in open I 971). among others. In the Peregrine, there were even more
than in dense stands, but they were scarce in the study areas. parallels, because first-year birds were more frequent as
because they were killed by gamekeepers. breeders on low-grade nesting places. and continual replace-
The third benefit that young stands might provide for ment of breeders occurred on favoured nesting places, despite
nesting Sparrowhawks, namely greater protection from ever-present vacancies among less used places (Hagar I 969).
adverse weather than more open stands, is more difficult to If, as in the Sparrowhawk, variation in both habitat quality88 I. NEWTON IBIS 1.33 SIJPPL. I
and bird quality, and some interaction between the two, is REFERENCES
postulated, then on good places most individuals could breed
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I am grateful to the Buccleugh Estates and other landowners for Tubbs, C.R. 1974. The Buzzard. London: David & Charles.
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Redpath. Dr T. M. Roberts and two referees for comments on the
manuscript.You can also read
