Population and Breeding of Red Kites in Wales Over a 30-Year Period

Population and Breeding of Red Kites in Wales Over a 30-Year PeriodAuthor(s): P. E. Davis and I. NewtonSource: Journal of Animal Ecology, Vol. 50, No. 3 (Oct., 1981), pp. 759-772Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/4135 .

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Journal of Animal Ecology (19 8w), 51, 111-133

A RADIO-TRACKING STUDY OF THE RANGING BEHAVIOUR AND DISPERSION OF EUROPEAN

SPARROWHAWKS ACCIPITER NISUS

BY M. MARQUISS AND I. NEWTON Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton,

Huntingdon, Cambs PE 7 2LS

SUMMARY

(1) The ranging behaviour of sparrowhawks was studied by radio-tracking in two areas 12 km apart, one rich in prey and the other poor.

(2) In both areas, ranges of adults centred around nesting areas (and in winter around nearby roosts) in woodland.

(3) Range size was smallest when hawks were attending the nest, from pre-lay through to incubation/brooding, and largest outside the breeding season; it was most variable at seasons when range size was rapidly changing, in early spring and (for hens) in the late nestling and post-fledging periods. The ranges of cocks were on average smaller than those of hens, and for the same sex and season, ranges were greater in the area with low prey density than in the area with high prey density.

(4) In their first winter, both cocks and hens were less faithful to any particular roost than were adults; they had more roost sites and used the main site less often. There was a suggestion that adults used their main roost less often when their ranges were large.

(5) When hunting, sparrowhawks used woodland more than expected from its proportion in their ranges, particularly broadleaved woodland, and hens used open country (especially farmland) more than cocks. Adults mostly roosted in thick conifer woodland near nesting areas, and returned repeatedly to the same sites, whereas first-year birds in winter roosted in a variety of small woods in open country, both during the day and overnight.

(6) At some stages of breeding, range parameters could be related to breeding performance. In the pre-lay period, cocks which had small ranges and spent much time near the nesting area became successfully mated, whereas cocks which had large ranges and spent little time near the nesting area remained unmated. In the late nestling period, the range size of hens was negatively correlated with the growth of their young. Provision by us of supplementary food in both periods resulted in hens becoming more sedentary at the nest.

(7) Between members of a pair in summer, and adjacent birds of the opposite sex in winter, a high proportion (74%) of their locations occurred in the overlapping part of their ranges. Much less overlap occurred between adjacent birds of the same sex, and for adults the overlap was relatively greater between larger ranges. For any given altitude, the ranges of adjacent breeding cocks in the prelay period were mutually exclusive at approximately the level predicted from a previously established relationship between nest density and altitude. For any given range size and altitude, first-year cocks overlapped their range with adjacent birds more than did adults.

0021-8790/82/0200-0111 $02.00 ? 1982 British Ecological Society

111



slaughter houses. They have a wide diet, from insects to birds and mammals, and are especially fond of carrion. Dead sheep are eaten mainly in winter and spring, insects mainly in autumn and winter, birds mainly in late spring, and small mammals at all times, but especially in late summer and autumn (Davies & Davis 1973; Davis & Davis 1981). The dispersion of the kite population varies in different parts of the area; most pairs are spaced 3-5 km apart, but some have been as close as 0.4 km to one another, and others as distant as 34 km. Particular pairs (and their successors) breed year after year in the same restricted places, but each may use one wood, or alternate between two or three woods in different years. The regular nesting places are here called 'nesting territories', because they are occupied by no more than one pair at a time, which defends the immediate nest vicinity against other kites and against other predators (such as corvids and buzzards Buteo buteo (L.)). The kites forage in large undefended home ranges, which overlap widely between pairs; some pairs seem to hunt mainly within 3 km of their nests, while others hunt regularly up to 5 km or more away. The birds re-establish their nesting territories by March, and lay mainly during the first half of April, so that the young fledge in late June or July, and disperse from late July onwards. Outside the breeding season, most birds roost communally in woods or hedgerows, and, during the daytime, they are met with singly, or in groups around abundant food sources.

The bulk of the population is resident in central Wales throughout the year, but some juveniles leave in autumn, and return in the following spring or later. They disperse to other parts of Britain, especially to southern England. These facts were established by Davies & Davis (1973) by sightings and ring recoveries, and, in recent years, they have also been substantiated from records of tagged birds. The population has been generally considered as self-contained; the one recovery of a German bird in its first spring in Wales is compatible with juvenile dispersal or migration from that population, and need not imply genetic mixing of Welsh with continental kites, because young birds usually return to breed near their birthplace (Newton 1979).

METHODS

Each spring, attempts were made to count all the kites in the area, to find how many were associated with nesting territories, and to record certain details of nest success. Over the years the data were collected by a succession of observers, initially by amateurs only, and from 1960 by full-time staff. In consequence, the earlier records are less complete on some aspects than the later ones; and possibly less reliable. The most complete data were obtained from 1967 onwards, when P.E.D. took over the bulk of the fieldwork. From 1968, most young were ringed, and, from 1975 to 1979, most were also wing-tagged. Different coloured tags were used in different years, and each tag had an individual number that could be read in favourable circumstances. Tagging provided information on the age of first breeding and other aspects.

Attempts were made to relate kite breeding and overwinter survival to supplies of sheep carrion and short-tailed voles Microtus agrestis (L.), which were two of the main foods (Davies & Davis 1973). Sheep carcasses were counted in 5 years in two areas of upland (details in I. Newton, P. E. Davis, J. E. Davis & M. B. Haas, unpublished), and information on vole numbers was obtained from records published over the years in Nature in Wales, together with unpublished observations of P.E.D. and others who worked in the area during the years concerned. It was possible, using past records in this way, only to classify years according to whether vole numbers had been high, intermediate

Population history of kites 760



P. E. DAVIS AND I. NEWTON

or low, and not to distinguish finer variations. As it had been observed that kite nests often failed during periods of heavy rain, we also examined mean nest success each year in relation to May rainfall, as recorded at Tregaron, near the centre of the study area. The figures for May alone were used, because almost all failures at active nests occurred in this month.

RESULTS

Eight more years of data have accumulated since the previous summary by Davies & Davis (1973), and, in addition, some of the earlier figures have been revised in the light of more recent information. Thus the data in this paper supercede all previous published data.

Population Between 1951 and 1960, the known population in spring fluctuated between twenty-four

and thirty-two birds, with no clear trend; after 1960, however, the known population suddenly began to increase, and reached a total of ninety-eight birds in 1980 (Fig. 1, Table 1). The increase was fairly steady, but slightly more marked in some years than in others, and twice on the general upward trend the numbers dropped between one year and the next.

The population in spring consisted of two components. The 'non-territorial' birds showed no obvious attachment to nesting places and were unpaired; in contrast, the 'territorial' birds were clearly attached to nesting places and were almost always paired, though a few such birds were present at nesting places as singles. There was also an age difference between non-territorial and territorial birds, as was found from the individuals that had been tagged (73% of young fledging in 1975-79). With one possible exception, all

first-year tagged birds occurred as non-territorial individuals; nine or ten second-year birds were paired on territories and ten others were not; four or five third-year birds were both paired and territorial and two others were not; and another bird did not settle until it was at least 4 years old. From these data, and from the proportion of tagged birds in each

component of the population, we surmised that the non-territorial component consisted of first-year birds, some second-year birds and a few older birds, whereas the territorial component consisted of some second-year birds, but mainly older birds.

Some tagged individuals were paired and territorial for one season before they bred for the first time, and one occupied a territory for two successive years without breeding. The age of first breeding was recorded for ten individuals; five were 2 years old, one either 2 or 3, three were 3 years old, and one 4 or 5. Another 2-year old, poisoned in early April, contained fully-developed eggs and clearly would have bred, had she survived. The recorded proportion of first-time breeders in the older age-groups will presumably increase, as the remainder of the existing tagged population attains maturity.

Breeding success

An idea of the average breeding performance of the population could be gained from the combined records of different years (Table 2). About 90% of territorial pairs built nests, 82% laid eggs, and 40% produced at least one fledgling. The mean clutch-size was 2.2, the mean brood-size in successful nests was 1.4 and the mean number of young produced per territorial pair per year was 0.5. These figures were appreciably lower than the 1.7 young per successful nest recorded for the same species in Germany (Rockenbauch, in Glutz von Blotzheim, Baver & Bezzel, 1971), than the 0.9 young per territorial pair recorded in

761



762 Population history of kites

Total birds /

ED S o- /?? .?-0'' brds0-0-0

/

80 - ' * -

t 40-

30- /

2 20 - ' .................. ......~~~....~..... 1~.......:......../ .~ .....0...0.0..."

0 -

o Te Ir I

0.6- . . . /

60-2 ?

50- .O' ' 0' "

1950 1955 1960 965 1970 1975 1980

FIG. 1. Trends in population, losses and breeding success of kites, 1951-80.

France (Thiollay 1967), and than the 1-2 young per pair recorded for the related black kite M. migrans in Germany (Fiuczynski & Wendland 1968). The red kites in Wales had almost the lowest rate of production recorded among ninety-three studies of twenty-seven temperate-zone raptor species reviewed by Newton (1979).

In the Welsh kites, production varied in different years between 0.22 and 0.87 young per territorial pair (Table 1). This range excluded 1955, when the entire population raised only one young, a mean of 0-08 young per pair. This was a year of extreme food-shortage, caused among other events by the dearth of rabbits Oryctolagus cuniculus (L.) due to myxomatosis. The years 1961-64 were also poor for breeding. They included a particularly hard winter, which reduced most kinds of prey for a year or two, and a peak in human nest



TABLE 1. Numbers and breeding success, 1951-80 (Vole numbers designated as low (L), intermediate (1) or high (H))

Number of Number of non-territorial territorial

birds pairs

1 2 0 1 1 0 0 3 0 1

3 1 1 3 4 3 2 7 6 8

5 9

13 10 6

13 18 10 13 19

15 14 14 15 13 12 13 14 14 12

14 18 18 19 19 21 23 23 26 26

27 28 30 33 33 32 34 38 39 39

Single Number of territorial pairs which

birds built nest

0 1 1 0 1

0 1 0 0

2 0 1 0

0 0 0 0

0 0 1 2 2 2 1 1 0 1

12 11 13 11 11 10 10 12 11 11

13 14 15 17 17 17 23 21 24 26

27 26 27 28 30 32 33 34 36 36

Number of Number of Mean pairs which pairs which clutch

laid eggs raised young size

11 10 12 11 6

10 9

12 11 10

13 13 13 17 17 16 22 19 24 24

24 26 26 26 28 28 28 31 29 28

7 6 7 9 1 6 7 5 6 8

5 6 3 6

10 8 7 9

10 11

12 14 10 10 15 15 12 12 14 21

2-00 2.00 2-00 2.13 2.07 2-00 2-27

2-01 2.30 2.06 1-94 2-26 2.38 2-24 2.36 2.53 2-44

Mean brood size

1.29 1.17 1.57 1.44 1.00 1.16 1.14 1-00 1.33 1.25

1.20 1.16 1.33 1-17 1-10 1-38 1.57 1.33 1.60 1.55

1.33 1.36 1.40 1.30 1.60 1 20 1-42 1-67 1.29 1.29

Mean number Total number of young/ of young

territorial pair produced

0-60 0.50 0-79 0-87 0-08 0-58 0.62 0.36 0.57 0-83 0-43 0-39 0.22 0.37 0-58 0-52 0.48 0.52 0.62 0-65

0.59 0-68 0-47 0-39 0-73 0-56 0.50 0.53 0-46 0-69

9 7

11 13

1 7 8 5 8

10

6 7 4 7

11 11 11 12 16 17

16 19 14 13 24 18 17 20 18 27

19511 19521 1953H 19541 1955L 1956I 1957H 1958L 19591 1960H

19611 1962L 19631 1964H 19651 19661 1967H 19681 19691 1970L

19711 1972H 19731 1974L 1975H 19761 1977L 1978L 19791 1980H

CI

trl tl

z r'n

z 0

z tTi 1--

r:



Population history of kites

TABLE 2. Summary of breeding performance, 1951-80*

A. Overall performance

Number of territorial pairs (years summed) 676 Number (%) which built nests 608 (90) Number (%) which laid eggs 554 (82) Number (%) which fledged young 272 (40) Mean clutch size* 2.20 Mean brood-size 1.35 Mean number young produced per territorial pair 0-54

B. Clutch-sizes and brood-sizes (at fledging) Clutch-size or brood size

1 2 3 4 Mean

Number of clutches 18 194 70 3 2.20 Number of broods 181 87 4 0 1-35

* Mean clutch-size calculated for the period 1964-80, because of lack of data in earlier years.

robbing, which affected at least 13% of pairs which laid in these years. They also immediately followed the introduction of the organochlorine compound, dieldrin, into sheep-dips, discussed later.

During the study period, local vole numbers were classed as high in 8 years, as intermediate in 15 years and as low in 7 (Table 1). On average, kites produced slightly more young per territory in peak years for voles than in intermediate ones, and slightly more in intermediate years than in low ones (Fig. 2). The differences in production between years of different categories were significantly greater than between years of the same category (F = 4.5, P < 0.025). This implied that kite breeding was influenced by vole numbers. Excluding nests known to have been robbed by egg-collectors did not greatly improve this relationship (F 4.6, P < 0.025). On the other hand, during the 5 years in which systematic counts of sheep carcasses were made, no relationship was found between carcass numbers in winter/spring and kite breeding (Table 3).

There seemed to be some relationship between nest success and May rainfall, with more failures in the wetter years, but the relationship was not quite significant at the 5% level (Fig. 3). Adjusting the figures to allow for human nest robbing made little difference to the correlation coefficient or to the significance level. Hence the annual variations in kite breeding success could be partly explained by annual variations in certain key foods, and to a much less extent by annual variations in May rainfall. If rain did cause nest failures, it probably did so through reducing the hunting efficiency of the male, depriving the small young of adequate food or causing the female to abandon the nest and seek her own food. Our data were too imprecise to allow us to examine the relationship between nest failures and daily variations in the rainfall figures.

Of the nest-failures that occurred after laying, it was possible to diagnose the proximate cause in about 74% of cases (Table 4). Some of the most prevalent causes were the failures of fully incubated clutches to hatch and of broods to survive. Most unhatched eggs showed no sign of development, while others contained embryos that had died at various stages. Most nestling mortality occurred within a few days after hatch, but some occurred among large young, of up to 5-6 weeks old. In several broods of two young, one died soon after hatch and the other some weeks later. Much of this mortality occurred on days of heavy rain. A few clutches were deserted, and others were lost through nest collapse. A second

764




019 -

06

0 a) w

a, a) U)

0

0 0 ?

S

S7

I

6S

I

Low Intermediate High Low Intermediate High

Vole numbers FIG. 2. Relationship between vole population (H-high, I-intermediate, L-low) and kite overwinter losses and breeding success. Each point refers to a different year in the period 1951-80. On analysis of variance for overwinter losses, F = 1.15, P > 0.025; for breeding success, F = 4.50, P < 0-025. Correcting the breeding figures for known human nest robbing did not appreciably

improve the relationship: F= 4-59, P < 0-025.

major category of failures was attributed to interference of some kind. Kites nesting close to ravens Corvus corax Linn. almost invariably failed (unless preventative measures were taken), and at one time or another, magpies Pica pica Linn., jackdaws Corvus monedula Linn. and crows C. corone Linn. were seen on kite nests, eating the eggs. Other failures occurred at times of agricultural or other human activity near the nest, when the bird was

TABLE 3. Lack of relationship between winter/spring sheep carrion, kite breeding success and overwinter loss. NS-not significant

Number of winter/ spring sheep carcasses*

(a) 58 60 39 40

145

% decline in kite numbers overwinter

(b) 19 19 10 16 15

Mean number in spring/summer of: Eggs per Young per

territorial pair territorial pair (c) (d)

1.92 2.08 1.84 1.93 1.88

0.73 0.56 0.50 0.53 0.46

Relationships between (a) and (b): r, = 0.4, NS; (a) and (c): r, = 0.3, NS; (a) and (d) r, = -0.1, NS. * Carcasses counted in 40 x 0.25 km2 sample areas selected at random in the sheepwalk of the study area

(for further details, see I. Newton, P. E. Davis, J. E. Davis & M. B. Haas, unpublished).

0

765

o2 24

a)

C

8, I 8 -0

12

1

S 0

?,

1974-5 1975-6 1976-7 1977-8 1978-9

0




100 -

80-

o0

0 60-

20 -

U)~o^~~~0 40- P > 5).

kept off for long periods, exposing the nest-contents to chilling or predation, or causing desertion. Most such failures occurred before the time of hatch, and a few soon after. Other failures were attributed to human robbing of eggs (thirty-two cases) and young (three cases); they were recognized from the marks of climbing irons on the tree. Some of the shells taken were subsequently recovered when the collector was caught and prosecuted. Egg collecting may have declined in recent years (Table 4). At four nests, dead adults were found. Over the years, seven repeat nestings were recorded after an initial failure, and three of these failed, from desertion, female death and nestling death respectively.

In some other birds-of-prey, breeding failures in recent years have often been associated with substantial residues of organochlorine compounds in the eggs (Ratcliffe 1970; Newton & Bogan 1978). In the period 1964-78, unhatched eggs from thirty-five kite clutches were analysed. The main residues found were of DDE (the main metabolite of the

TABLE 4. Recorded causes of clutch-failure, 1951-80, including a few repeat clutches

All years (% of

20 -

1951-20 40 60 80 10 10 1971- 80 fail 200es)

Rainfall in May (mm)

TotalFIG. 3ailures 41 104 141 286 (100)39,

0.1 > P> 0.05).

k ept off for long periods, exposing the nest-contents to chilling or predation, or causing desertion. Most such failure to hatch, and a few soon after.(20)

Other failures were attributed to human robbing of eggs (thirty-two cases) and young

(threeood deathe marks of climbing irons on the tree. Some of4 19 25 48 (17) th e shells takedation & interferecovered when the collector was caught and

prosecu ted. Egg collectmay have declindce 4). At four nests, dead adults were found. Over the years, seven repeat nestings were recorded after an initial(2) failure, and three of these failed, from desertion, female death and nestling death rest collapse 3 3 1 7 (2)tively.

In some other birds-of-prey, breeding failures in recent years have often been associated14 27 34 75 (26)

with substantial residues of organochlorine compounds in the eggs (Ratcliffe 1970; Newton & Bogan 1978). In the period 1964-78, unhatched eggs from thirty-five kite clutches were analysed. The main residues found were of DDE (the main metabolite of the

TABLE 4. Recorded causes of clutch-failure, 1951-80, including a few repeat clutches

All years (% of 1951-60 1961-70 1971-80 all failures)

Total failures 41 104 141 286 (100) Clutch desertion 3 2 8 13 (5) Clutch failure to hatch* 4 19 33 56 (20) Brood death 4 19 25 48(17) Natural predation & interference 1 6 15 22 (8) Human disturbance 2 11 13 26 (9) Human nest-robbing 9 14 12 35 (12) Adult death 1 3 0 4 (1) Nest collapse 3 3 1 7 (2) Unknown 14 27 34 75 (26)

* After being incubated full-term.

766




insecticide DDT), PCB (industrial polychlorinated biphenyl) and HEOD (from the insecticides aldrin and dieldrin), but these residues were at much lower levels than were found to cause breeding failure in other birds (Newton 1979). In most clutches, these compounds were recorded in only trace amounts. Moreover, the degree of shell-thinning (in other birds caused by DDE) in eggs from thirteen recent clutches averaged only 4.5%, and was not statistically significant compared with seven clutches collected in 1838-1917. Other raptors subject to marked breeding failure from organochlorines have shown more than 20% shell-thinning. We concluded therefore, that organochlorine residues probably had no important influence on the breeding of kites in the period when eggs were obtained. Organochlorines may have contributed to the poor breeding in earlier years (1961-64) when dieldrin was used in sheep dips.

Annual loss

From knowledge of the number of birds present each spring, and of the number produced each year by breeding, the annual losses of full-grown birds from the population could be calculated from year to year, as shown in Table 5. These net losses reflected the combined effects on the population of mortality and of permanent emigration/immigration (if they occurred). Net losses reached 36.4% per year, but over the whole 30-year period,

TABLE 5. Annual losses of full-grown birds, 1951-80 (vole numbers designated as low (L), intermediate (I) or high (H))

Full-grown birds (breeders & non-breeders)

31 31 29 31 28 25 26 32 28 25

33 37 38 41 44 45 48 53 58 60

59 65 74 78 74 79 87 87 91 98

Chicks fledged

9 7

11 13

1 7 8 5 8

10

6 7 4 7

11 11 11 12 16 17

16 19 14 13 24 18 17 20 18 27

Total birds if no mortality

40 38 40 44 29 32 34 37 36 35

39 44 42 48 55 56 59 65 74 77 75 84 88 91 98 97

104 107 109 125

Losses since previous year

9 9 9

16 4 6 2 9

11

2 2 6 1 4

10 8 6 7

14

18 10 10 10 17 19 10 17 16 11

% birds lost since previous year

22.5 23.7 22.5 36.4 13-8 18.8 5.9

24.3 30.6

5.7 5.1

13.6 2.4 8.3

18.2 14-3 10.2 10.8 18.9

23.4 13.3 11.9 11.4 18.7 19.4 10.3 16.3 15-0 10.1

19511 19521 1953H 19541 1955L 19561 1957H 1958L 19591 1960H

19611 1962L 19631 1964H 19651 19661 1967H 19681 19691 1970L 19711 1972H 19731 1974L 1975H 1976I 1977L 1978L 19791 1980H

767




they averaged 16.0%. They referred to all full-grown kites, and did not distinguish the age-groups.

The greatest error in these estimates of loss was likely to result from birds being missed at the spring count, including birds that were temporarily absent from the area. Some tagged birds were apparently absent from central Wales (= not seen) during their first spring, but had returned to the area (= were seen) in a later spring. This apparent absence was true of only some of the first-year birds (more in one year than in the others), but in any one spring they formed at most a small proportion of the total population. These birds would have slightly inflated the estimate of loss in their year of absence, and slightly depressed the estimate in their year of return. Over the period as a whole, the absence of one group of birds in each spring would be partly compensated by the return of another, thus reducing the overall error.

Comparing years, overwinter losses seemed to be related partly to vole numbers, with the greatest losses in years when vole numbers were low, but the tendency was not statistically significant (Fig. 2). Nor was there any relationship between the losses of kites and supplies of sheep over the five winters when carcasses were counted (Table 3). The winters in mid-Wales were especially hard in 1955-6, 1962-3, and 1978-9, but losses of kites were not especially great in any of these years either. The most striking trend was for much higher losses before 1960 than after.

During the study period, thirty-two kites were reported dead (including some ringed birds), and nineteen of these were in good enough condition for autopsy. Eight adults found dead or dying under nests included one with a broken wing, one that was arthritic and diagnosed as senile, one that had been electrocuted by overhead wires, two poisoned, and three that were too far decomposed for examination. Seventeen birds found away from nests in mid-Wales included three that died in the hard weather of early 1963 (one obviously emaciated, the others decomposed), one that succumbed to a Pasteurella infection, six poisoned, and seven not examined. Seven Welsh birds found outside mid-Wales included two dead from collisions with train and wires respectively, one poisoned, one shot, and three not examined. Thus, half of the nineteen birds examined had died from human persecution, mainly poisoning. Strychnine was the usual poison identified (available legally for the control of moles Talpa europaea L.), but one bird had died from endrin (which is sold for dilution and use as an insecticide) and another from alpha-chlorolose (which is sold for us as a narcotic in baits for pigeons and other pests). Poisoned meat baits are used illegally in many parts of Britain against crows and foxes (Cadbury 1980), and it seems that kites are frequent victims of this practice. We have no way of knowing whether the deaths that came to our notice were representative of all kite deaths.

Production, loss and population trend The increase in the kite population began around 1960, and affected both territorial and

non-territorial components. There was no increase in breeding rate over the period considered (Fig. 1); the mean number of young produced per territorial pair was 0-58 in the years 1951-60, and 0.51 in the years 1961-78. Hence, the general increase after 1960 could not have resulted from improved breeding success, but must have been due to the reduced net losses of full-grown birds. From the figures on population, these annual losses were estimated as being about twice as high in 1951-60 (22%) as in 1961-80 (13%). They were associated with an increase in the proportion of non-territorial birds in the population, from an average of 3% in 1951-60 to 13% in 1961-80. However, observation of non-territorial birds is the measure which is most dependent on observer effort, and as

768



(a) (b) - (c)

o~~~~~0U) - -- o

- -

a, 40- 40- a 40-

a - a - rU - -

30- 30- 30-

5 ?0 >

20- o20- ? 20- *

t - . . - . I 20 - 0

0- 0- 0 - -

- . 0 -0 I4- 0 0 70 - e0 -e

4)-10? o I- . - -10- 0 -1-

- 0 ?O ? O1 ? t

20 - 0 -20 -

0 0-2 0'4 0-6 0.8 I0 0 0-2 0-4 0-6 0-8 '0 0 0-2 0-4 0-6 0-8 1.0

Young per territorial pair

FIG. 4. Breeding success and (a) % change in total population 1 year later; (b) % change in total population 2 years later; and (c) % change in territorial population 2 years later.



this also improved after 1960, evidence for the increase in non-territorial birds is less firmly based than that for territorial ones.

As a further check on the role of breeding success in the population trend, the annual data were examined, with the help of three scatter diagrams (Fig. 4). In the first of these diagrams, the number of young produced per pair in any one year was examined in relation to (a) the percentage change in the population by the following year. However, as some of the young were absent from the area in their first spring (thus lowering the population count), the relationship was also examined between the production of young per pair in any one year and (b) the percentage change in total population 2 years later, and (c) the percentage change in the territorial population 2 years later. Statistical tests on these data would not have been valid, because of the built-in dependence between the percentage changes from year to year (i.e. the change from year A to year B, and from year B to year C both involve the count from year B). The scatter diagrams indicated a variable relationship between breeding success and subsequent population change. However, they suggested that good breeding was necessary for big increases in population the following year. Not all good seasons lead to a population increase, because their effects were often countered by subsequent losses; however, no poor breeding season was followed by a big increase (Fig. 3(a)). Two years after a good breeding season, by the time the birds were reaching breeding age, the effects on population were somewhat less apparent (Fig. 3(b), (c)).

In conclusion, the increase in kite numbers over the last 20 years could be attributed primarily to reductions in the net loss of full-grown birds, compared with previous years, rather than to a general improvement in breeding success. Only good breeding seasons permitted relatively big increases in population, but the effects of most good seasons were offset by losses before the birds reached breeding age.

This conclusion is not wholly consistent with that of Davies & Davis (1973) who, by comparing years, found significant correlations between the numbers of young produced in year A and the number of non-breeders present in year B, and between the numbers of non-breeders in year B and the numbers of breeders in year C. However, sinced they used actual numbers, rather than percentage change, they were almost certain to find correlations between these parameters over a period of population increase, whatever the real relationship.

DISCUSSION

Kites apparently began to increase in numbers from around 1960, following the appointment of full-time field-staff. But the increase in known pairs could not have resulted solely from improved efforts, because it continued more or less steadily for the next 20 years. Possibly the population was slightly higher in the 1950s than was realized, but there can be little doubt that most of the recorded increase was genuine. Throughout, the most difficult birds to find were the non-territorial ones, so it is chiefly this group that may have been under-represented more before 1960 than afterwards.

As the birds increased, they occupied a wider area in central Wales. Evidently, at no stage were they restricted by lack of habitat, and similar landscapes and food-sources are widely available today elsewhere in Wales and in other parts of Britain. On the continent, kites occupy an even wider range of habitat than in Wales, and by analogy, they could be accommodated again in almost every British county.

The extremely poor reproduction was puzzling. The main causes of breeding failure (non-laying, non-hatching of incubated eggs and brood mortality) occur commonly in

Population history of kites 770




other raptor-species, in which they have been clearly linked with measured food-shortages (Newton 1979). The kites may also have been short of food, despite their varied diets. The poorest year for breeding (1955) occurred when rabbits disappeared, and other poor years (1961-64) included a very hard winter, when several kinds of prey remained scarce for a year or two. In addition, breeding success throughout the 30 years correlated to some extent with vole numbers, with slightly better production in the peak vole years.

We could not tell whether the poor breeding influenced the general population status of Welsh kites. If breeding had been much better than observed, the population may have increased faster, and if breeding had been worse, the population may have increased more

slowly, or not at all. However, we could find little evidence, within the variation observed in 1951-80, that the year-to-year trends in breeding success had any effect on the subsequent year-to-year trends in the territorial population. The population increase after 1980 could be explained primarily in terms of a reduction since the 1950s in the net losses of full-grown birds, the losses themselves the result of mortality and immigration/emigration.

With one recovery of a German-ringed kite in Wales, the idea of immigration is at first appealing, but this single recovery is consistent with juvenile dispersal or with drift off-course on the known spring migration of German birds from Spain (Glutz von Blotzheim, Bauer & Bezzel 1971). Such birds normally return to breed nearer their natal area. Against the idea of permanent immigration on a scale sufficient to cause the observed increase of the Welsh population is (a) the considerable distance to the next (very sparse) populations in southern Sweden, north-west Germany and eastern France, (b) the lack of ring recoveries of adults, and (c) the improbability, if immigration occurred, of birds settling in mid-Wales rather than in other suitable habitats widely available elsewhere in Britain. Furthermore, the occasional sightings of kites is most of the rest of Britain are consistent with the known temporary dispersal of juveniles from Wales. This is not true of the more frequent east coast records, however, most of which occur in March-May, and are presumed to be of continental birds on migration. The one relevant ring recovery (in Essex on 6 April 1976) was of an East German bird in its second year. Some autumn and winter birds in the east and south are evidently also of continental stock; several unringed birds have been found dead in recent years, and since most Welsh birds carry rings, they are very unlikely to be all of Welsh origin. On present evidence, therefore, we are inclined not to attribute the increase in the Welsh population since 1960 to substantial immigration. A more likely possibility is that the increase in the Welsh population was due to reduced losses of Welsh birds; and as no kites are known to have bred in Britain outside mid-Wales since 1947, the reduced losses were presumably due to reduced mortality. Most of this mortality took place in Wales, but some outside, in the areas reached by emigrant juveniles in autumn.

The main period of reduced mortality in kites, and the associated rise in numbers, coincided with a period of major land-use changes in mid-Wales, namely the conversion of

large parts of the sheepwalk to plantation forest. Although this steadily reduced the area over which sheep carrion was available, it had two benefits for kites. For a few years after

tree-planting, vole populations reached much higher levels in young forest than on nearby sheepwalk (though still cyclic). As the forests were planted piecemeal, mainly between the mid 1950s and the early 1970s, throughout their period of increase the kites had vole-rich areas to feed in. The year-to-year relationship between kite losses and vole numbers was

poor (and not statistically significant), but we could not rule out the possibility that kites benefitted from this extra food. (As most of the young forest has now reached thicket

stage, and is unsuitable for kite foraging, it will be interesting to see whether the population

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rise continues.) Perhaps more important, however, was that forestry may have steadily reduced the area over which poison baits were used (shepherds being the main users). One-half of all kite deaths that came to our notice were attributed to human persecution, especially poisoning. These may not have been representative of all deaths, but they accounted for territorial adults, as well as juveniles. A reduction in this type of mortality over the years, consequent upon change in land-use and human attitudes, would almost certainly have helped the population to expand.

ACKNOWLEDGMENTS

We are grateful to all observers who contributed information over the years, to J. E. Davis who helped with fieldwork for the last five years, to the Government Chemist and to Dr J. Bogan for analyses of organochlorine levels in eggs, to Peter Rothery for advice on statistics, and to Drs J. P. Dempster and M. Marquiss for criticism of the manuscript.

REFERENCES

Cadbury, J. (1980). Silent Death. The Destruction of Birds and Mammals through the Deliberate Misuse of Poisons in Britain. Royal Society for the Protection of Birds, Sandy, Beds.

Davies, P. W. & Davis, P. E. (1973). The ecology and conservation of the red kite in Wales. British Birds, 66, 183-224, 241-270.

Davis, P. E. & Davis, J. E. (1981). The food of the Red Kite in Wales. Bird Study, 28, 33-40. Fiuczynski, D. & Wendland, V. (1968). Populationsdynamik des Schwarzen Milans (Milvus migrans) in

Berlin. Beobachtungen, 1952-1967. Journalfiir Ornitologie, 109,462-471. Glutz von Blotzheim, U. N., Bauer, K. & Bezzel, E. (1971). Handbuch der Vogel Mitteleuropas. Vol. 4.

Akademische Verlagsgesellschaft: Frankfurt am Main. Newton, I. (1979). Population Ecology ofRaptors. Poyser, Berkhamsted. Newton, I., Davis, P. E. & Moss, D. (1981). Distribution and breeding of red kites in relation to land-use in

Wales. Journal of Applied Ecology, 18, 173-186. Newton, I., Davis, P. E., Davis, J. E. & Haas, M. B. (1980). Carrion-feeding birds in Wales, Part III, Ravens

and buzzards. Unpublished report to Nature Conservancy Council. Newton, I. & Bogan, J. (1978). The role of different organo-chlorine compounds in the breeding of British

Sparrowhawks. Journal of Applied Ecology, 15, 105-116. Ratcliffe, D. (1970). Changes attributable to pesticides in egg breakage frequency and eggshall thickness in

some British birds. Journal of Applied Ecology, 7, 67-107. Salmon, H. M. (1957). The rarer birds of prey, Kite (Milvus milvus). British Birds, 50, 137-141. Salmon, H. M. (1970). The Red Kites of Wales: the story of their preservation. Welsh Wildlife in Danger (Ed.

by W. S. Lacey), pp. 67-79. North Wales Naturalists' Trust, Bangor. Thiollay, J. M. (1967). Ecologie d'une population de rapaces diurnes en Lorraine. La Terre et la Vie, 114,

116-183.

(Received 19 January 1981)

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Documents

Population and Breeding of Red Kites in Wales Over a 30-Year Period