Journalof AnimalEcology(1988),57, 903-916
FACTORS AFFECTING BREEDING SUCCESS OF
PEREGRINES IN SOUTH SCOTLAND
BY R. MEARNS AND I. NEWTON
Institute of TerrestrialEcology, Monks Wood Experimental Station, Abbots Ripton,
Huntingdon, PE17 2LS
SUMMARY
(1) During 1974-82,peregrinesin south Scotlandproducedan annualaverageof 1 15
young per territorialpair, between0-60 and 1.45 in differentyears.Heavy rain in May
loweredproductivity,by reducinghuntingsuccessand by soakingexposednests. Early
clutches,begunbeforemid-April,werelargerand more successfulthan laterones.
(2)All pairsnestedon cliffs;thefewlargecliffswerepreferred,so that,as thepopulation
grew,morepairsusedsmallcliffs.Cliffheightandaccessibilityto humanclimbersgreatly
influencednestingsuccess,as did exposure.Thosenestsin recessesweremost successful.
(3) Performanceimprovedas femalesaged; laying becameprogressivelyearlierand
clutchesprogressivelylarger,from 1 to 5+ years.
(4) Despitesomeslighteggshellthinning(attributedto DDE), organochlorineresidues
had at most a smallimpacton productivityin the yearsconcerned(in contrastto earlier
years).
(5) The overalldensityof breedingperegrinesmore than doubledduringthe 9-year
study, and also variedbetweendifferentparts of the study area. But over the rangeof
densitiesobserved,no density-dependent
depressionof breedingsuccessoccurred.
INTRODUCTION
The peregrinefalcon FalcoperegrinusTunst, an inhabitantof wild remotecountry,is
unusualamongbirdsin the extentto whichits numbersand breedingdependon man. In
muchof Britainthe specieseatsmainlyferalanddomesticpigeons.Theseeminglyendless
supplyof these prey probablyenablesperegrinesto breedat densitiesfar higherthan
would be possible on naturalprey alone. On the other hand, peregrinenumbersand
breedingsuccesshavein recenttimesbeenmuchreducedby organochlorinepesticides,to
which these falcons are unusuallysensitive(Ratcliffe1980).Also, the speciesis almost
immuneto naturalpredators,buthaslongbeenpersecutedby man.Theadultsareshotby
game-preserversand pigeon-keepers;and their nest contents are removed by eggcollectorsand falconers.Thispredationis independent,not only of the food-needsof the
humanpredators,but also of the density of the falcon population.It may be largely
responsiblefor restrictingperegrinesto nestingon cliffs,and hencefor confiningthemto
particularregions,despitean abundanceof food elsewhere.
In this paper,we examinethe influenceof humanand other factorson the breeding
successof peregrines,basedon a 9-yearstudyin south Scotland.Emphasisis placedon
variousfeaturesof the nestingplace,but the effectsof weather,habitat,diet, femaleage
and organochlorinepesticidesare also discussed.
903
904
Peregrine breedingsuccess
TABLE1. Breeding performance and causes of failure 1974-82
(a) Overallperformance
Numberof pairs Numberwhich Numberwhich
on territory laid eggs (%)* fledgedyoung(%)*
Inland
297
246(83)
131(44)
Coast
100
83(83)
58(58)
Overall
397
329(83)
189(48)
(b) Frequencyof differentclutchand broodsizes
Numberof clutchesof
1
2
3
4
5
Inland 5
0
Coast
Overall 5
11
0
11
68
18
86
119
20
139
6
0
6
Eggsper
clutch
3.53
3.53
3-53
Mean
numberof
Youngper Youngper
pair
broodt
2-40
1-06
2-17
1-26
2-33
1.10
Numberof broodsof
1
2
3
4
26
16
42
48
20
68
34
18
52
22
4
26
(c) Causesof breedingfailureof territorialpairs
Causeof failure
Number
inland
Number
%of all failures
on coast Totalnumber
(N= 208)
17
68
33
5
30
14
3
1
0
2
8
4
11
0
23
2
13
6
1
8
4
2
20
10
2
13
6
22
11
11
%of allbreeding
opportunities
(N= 397)
52
17
Non-laying
24
8
Eggsbroken/addled
3
1
Eggsdeserted
6
2
Eggsdisappeared
23
6
Eggsrobbed$
11
3
Youngdisappearedat hatch
7
2
Youngdied
18
5
Youngdisappeared
11
3
Young robbed:
Unknown
11
6
* % of territorialpairs;t excludesbroodsof 0; t minimumnumberby humanagency(see text).
The figuresin the Tableexcludenineteenrepeatnests.If thesewereincluded,an overallaverageof 50%of
territorialpairsproducedyoung,or 60%of pairswhichlaid, and the overallproductionwas 1-15youngper
territorialpair.
The study population occupied a fairly discrete area in south-west Scotland, in which
the extreme east-west territories were 150 km apart, and the extreme north-south ones
170 km apart. The low ground in the region supports mainly mixed farming, with pasture
and arable, while the hill ground supports open grassy sheepwalk, heather moor and
conifer plantation. Peregrines are year-round residents in the region. In addition to
pigeons, they eat waders, grouse, songbirds and others (Mearns 1982, 1983).
Each year an attempt was made to count the whole territorial population, resident at
cliffs, and record their breeding success. Whenever possible adults were trapped (for
identification) at nesting cliffs; in practice many more females than males were caught
(Mearns & Newton 1984). During the study the population was recovering from a low
level imposed by organochlorine pesticide use; known territorial birds increased from
twenty-eight pairs and six singles in 1974 to sixty-seven pairs and four singles in 1982
(Newton & Mearns 1987). As it was not possible to obtain full details from every nest, the
totals vary between the Tables of data presented. Weather data were from Clatteringshaws, the recording station nearest to the centre of distribution of the birds studied.
ANDI. NEWTON
R. MEARNS
905
RESULTS
success
Overallbreeding
Pooling the data from differentyears gave 397 recordsof pairs seen on territoryin
spring(Table 1), and forty-eightrecordsof singlebirds.Eggs wereknownto have been
laid in 329 cases (at 83%of territorieswith pairs),and young werefledgedin 189cases
(48%of territorieswithpairs,or 57%of clutches).Clutchescontained1-5 eggs,mostly34, with a meanof 3-5(Table1). Most of the femaleswerein blue adultplumage,but two
brownyearlingswhichnestedlaidonly2 eggs.Fourout of sixclutchesof 5 eggswerefrom
the same territoryin successiveyears, and presumablyfrom the same female. Broods
contained1-4 young,witha meanof 2-3.Thisgavean overallproductionat firstattempts
each year of 1 3 young per clutchor 110 young per territorialpair (Table 1).
Nineteenpairswhichfailedsoon afterlayingproduceda secondclutch,of whicheleven
weresuccessful(mean1-7youngperbrood).Thisbroughtthe overallproductivityup to
50%of territorialpairsand60%of pairswhichlaid,givingI 115youngperterritorialpair
peryear.As thesevariousfigureswerebasedon all the pairsfoundeach year,including
those whichfailedat an earlystage, they werenot biasedin any obviousway. The last
figureis the most meaningful,becauseit takes accountof failuresat all stages,and of
repeatlayings.
Besidesnon-laying,themainproximatecausesof breedingfailureincludedbreakageof
eggs,failureof incubatedeggsto hatch,robbingof eggsandyoungby people,anddeaths
of young (Table 1). Most chickmortalityoccurredwithina day or two afterhatch,but
some occurredlater,whenchickswereup to half grown.Suchmortalityseemedoftento
occurduringseveral-dayspellsof rainand mist, whenhuntingwas difficult.Failurewas
attributedto humanactiononly whenthe disappearanceof nestcontentscoincidedwith
signsthatthe cliffhadbeenclimbed;in somecases,menwerereportedat the cliff,or later
caughtwith eggs or young. Otherfailures,involvingthe disappearanceof nest contents
betweenour successivevisits, may also have been due to humanaction or to nestling
starvation,but we had no directevidence.Despitefrequentnest robbing,no indication
was obtainedthat any adultswerekilledat nest cliffsduringthe studyperiod.
Overallproductionvariedgreatlybetweenyears,the mean young producedper pair
rangingbetween0-60and 1-45.Thesefigureswereaffectedmainlyby the proportionsof
clutcheswhichproducedyoung,whichvariedbetween29%and 57%in differentyears.
They wereaffectedmuchless by the mean brood-sizesin successfulnests, whichvaried
between1.60 and 2-68in differentyears.
Eachyear,peregrinesstartedlayingovera 7-weekperiod,fromlateMarchto theendof
April.Pooling the recordsfrom differentyears,clutchesstartedbeforemid-Aprilmore
often producedyoungthandid laterones (Table2). Meanclutchsize also declinedwith
advancein layingdate. Theseseasonaltrendsweresimilarto those foundin some other
raptors(Cave 1968;Newton 1979;Newton & Marquiss1984;Village 1986).
success
Weather
andbreeding
Heavyrainfallis thoughtto depressthe breedingsuccessof Britishperegrines(Ratcliffe
1984),but no formalstudyof the relationshiphas beenmade.During1974-82,rainfallin
southScotlandvariedwidelyfromyearto year,so we examinedthe relationshipbetween
rainfall(totalfall andnumberof raindays)and layingdates,clutchsizesand overallnest
success.Mean layingdates (of firsteggs) variedonly between3 and 8 Aprilin different
years.No significantrelationshipwas found betweenthese mean dates and rainfallin
906
Peregrine breedingsuccess
TABLE2. Seasonaltrendin breedingperformance.Data from firstclutchesonly
(excludingrepeats),wherelayingdateswereknown
Date of
first egg
23-27 March
27-31 March
1-5 April
6-10 April
11-15 April
16-20 April
21-25 April
26-30 April
% clutches which produced young
Number of
Number of Number which known human Including known Excluding known Mean clutch Mean brood
size
size
clutches produced young nest robberies human robberies human robberies
1
2
9
5
5
2
0
0
6
16
37
36
34
8
8
2
8
24
61
52
49
20
12
4
3-71
3-89
3-82
3-77
3-54
2-89
2-70
2-75
86
73
71
77
77
44
67
50
75
67
61
69
69
40
67
50
1-50
2-38
2-46
2-53
2-50
1-88
1-25
2-00
Comparison of proportion of clutches which produced young from those started before and after mid-April: including known
human robberies, x2 = 290, P<0 1; excluding known human robberies, x2 = 571, P< 0-025.
Regression of clutch size (y) on laying date (x) revealed the following relationship: y= 422-0.204x, N=202, r= - 0497,
P<0.001.
100
90-
0)
c
:3
0
>1
0)
U
75
`0
80S
0
70S
a)
.C
73
U
0
0l
60 -
50-
40-
300
S
1
20
40
1
1
1
1
1
1
1I
60
80
100
120
140
160
180
200
220
Rainfall in May (mm)
FIG. 1. Nest successand May rainfallin differentyears.Excludingnests knownto have been
robbedby people,nestsuccess=80-30-0-15(rainfall,mm),r= -0-84, P< 0-01.Thisrelationship
is strongerthanthat givenin Newton& Mearns(1987),whichincludedhumanrobberies.
March (total rainfall, r= -0-42, N.S.; rain days, r= -0-44, N.S.) or in February-March
(total rainfall, r= -0-38, N.S.; rain days, r= -0-37, N.S.). However, in both cases the
slope of the regression was in the expected direction, consistent with later laying in wetter
springs. In addition, mean clutch-sizes varied slightly from year to year, with a tendency
for a smaller mean in years when laying was late (between mean laying date and mean
clutch-size, r=0'55, P< 01).
ANDI. NEWTON
R. MEARNS
907
If the effects of rainfall on production were felt most strongly around the time of hatch,
as observations suggested, then rainfall during May should have been crucial, as this
month covered the hatching period of the population. Overall breeding success, expressed
as the percentage of clutches which produced young, did in fact vary according to rainfall
in May, with poorest success in the wettest years (r = - 084, P < 0-01, Fig. 1). Using the
number of rain days, rather than total rainfall, gave a poorer correlation (r=0 60.
P <01). No significant relationship was found between overall breeding success and
rainfall over a longer period, either April-July, which covered almost the whole breeding
cycle, or May-July, which covered the full nestling period. It seemed, therefore, that yearto-year variation in breeding success was most closely related to variation in rainfall
during the time of hatch in May.
All the above analyses were repeated, using mean temperatureinstead of total rainfall,
and number of days below 0 "C instead of number of rain days. No significant
relationship emerged, suggesting that temperature was unimportant, compared to
rainfall, in the south Scottish context. Nests spanned a wide range of altitude (from sea
coast to 570 m). Laying dates became progressively later on higher ground, by about one
day for every 100 m rise (N=230, r=0-22, P<0-01)-similar to the one day per 122 m
earlier found by Ratcliffe (1980)-but no other aspect of performance was related to
elevation.
Territories,nest sites and breedingsuccess
Because the same territorieswere monitored in successive years, we could examine the
variation in performance in the same territoriesfrom year to year, as well as the variation
between territories. In terms of laying dates, clutch-sizes and the number of young
produced, the variation within territories was significantly less than that between
for laying dates, F54174= 331, P< 0'001; for clutch sizes, F58188= 1 84,
territories (ANOVA:
P< 0-01; for number of young, F60266=1 38, P< 0-05). This implied that the territories
themselves, or their occupants, had a significant effect on performance.
Overall, the coastal sites were more successful than the inland ones, with 70% and 53%,
respectively, of clutches producing young (Table 1). This was due mainly to more frequent
nest robbery by people inland, and in other respects no clear differences in performance
were found. Nor did performance vary according to the surrounding land use, when this
was classed according to the proportions of farmland, grass moor, heather moor or forest
within 3 km of the nest.
Some territories were occupied every year during 1974-82, while others were taken up
during this period, as the population grew. (At some of the latter sites, peregrines were
known to have bred earlier this century, while at others they were not.) Once a territory
became occupied, it usually remained so, at least to the end of the study. In consequence,
therefore, territoriesvaried in the number of years they were used, from 1 to 9 during the
study period.
A relationship emerged between duration of use and breeding success. In general, the
longer ( = earlier)the territorywas occupied, the better the nest success (Table 3). This was
not because success deteriorated during the course of the study (Newton & Mearns 1987),
so that the territories with shortest occupation were in the worst years. But it could have
been because the longer-used sites were better, and therefore occupied first, or because in
general they were occupied by older, more experienced birds. In fact, evidence was
obtained for both these possibilities. As the population grew, new settlers took to smaller
cliffs, where they were more vulnerable to human and other predation (see later), and such
Peregrine breedingsuccess
908
TABLE3. Breeding success in relation to number of years territory occupied during
1974-82
Numberof yearsterritoryoccupied
7-8
9
1-3
4-6
32
59
58
248
Numberof pairs
25
40
48
216
Numberwhichlaid
10 (31) 23 (39) 26 (45) 130 (52)
Number(%)whichproducedyoung
21
3
10
Numberof failuresdueto knownhumanrobbery 2
Significanceof variation in pair success between categories:includingknown human
robberies,x2= 766, P < 001; excludingknownhumanrobberies,x2= 941, P<0-005.
On regressionanalysisof numberof youngraisedper attempt(y) againstyearsof territory
occupation(x):y=0.47 +0.08x, r=0 135,n = 397, P < 0001.
r= 0 168,N=362, P < 001.
Excludingknownhumannest robberies:y = 045+0 10Ox,
TABLE
4. Comparison of cliff sizes occupied at the start and end of the study, during
which time the population rose from 28 to 67 pairs
Numberoccupied Numberoccupied
in last yearof
in firstyearof
Heightof
Proportionate
cliff(m)
increase
study(1974)
study(1982)
Inland
5
17
3-4
-10
17
11-20
10
1-7
9
12
211-3
Coastal
-10
0
1
+
11-20
21-
1
3
6
14
6-0
4-7
Inlandand coastalrecordsare separatedbecausein generalthe coastalcliffs
werelarger,and during1974-82the populationincreasewasmoremarkedon the
coast thaninland.
settlers were probably in their first few years of life. However, in a statistical analysis of
known age birds, the relationship between duration of use and female age was not
significant (see later).
Breeding success was clearly related to features of the nest-cliff and nest-ledge itself.
Within the study area, peregrinesused the full range of cliffs available, from tiny crags less
than 2 m high to large rockfaces more than 60 m high. As the breeding population
expanded, more cliffs in all size categories were occupied, but with an increasing
proportion of pairs on smaller cliffs (Table 4). This trend probably resulted from
preferential occupation of a restricted number of large cliffs, which became progressively
less available as the population rose, forcing a bigger proportion of new settlers to take
small cliffs.
Nest success was related both to the height of the cliff, and to the accessibility of the
nest-ledge to human climbers (Tables 5). Only 42% of clutches on the lowest cliffs
produced young, rising to 71% on the highest. Non-breeding was also more frequent on
the lowest cliffs (30%) than on the tallest (12%), and clutch and brood-sizes tended to be
smaller on the lowest cliffs, but not significantly so. Turning to accessibility, the
proportion of clutches which produced young varied from about 51% at 'walk in' sites to
R. MEARNSAND I. NEWTON
909
TABLE
5. Breedingsuccessin relationto heightof nestcliff(a) andaccessibility(b)of
nest ledgeto humanclimbers
(a)
-10
Heightof nestcliff(m)
11-20
4121-40
Numberof pairs
74
145
89
89
Numberwhichlaid
52
126
72
78
Number(%)whichproducedyoung* 22 (42)
70 (56) 42 (58) 55 (71)
Numberof failuresdue to known
humannest robbery
7
17
4
2
* %of clutches
Significanceof variationin clutchsuccessbetweencategories:includingknown
human robberies, x2=13-15, P< 0005; excluding known human robberies,
x2= 17-04,P < 0001.
On regressionanalysisof numberof young raisedper attempt(y) againstcliff
height(x) measuredin 10-munits:y=0-74+0-12x, r=0-167, N=397, P<0-001.
Excluding known human nest robberies:y=0.86+0-12x, r=0 159, N=362,
P<0-01.
(b)
Accessibilityof nest ledge
Accessiblewithout Accessiblewith
Accessextremely
a rope
a rope
difficultor 'impossible'
Numberof pairs
whichlaid
148
94
65
Number(%)which
76 (51)
56 (60)
48 (73)
producedyoung*
Numberof failures
dueto knownhuman
nest robbery
22
10
1
* %of clutches
Significanceof variationin clutchsuccessbetweencategories:includingknownhuman
robberies,x2= 22-46,P < 0-001;excludingknownhumanrobberies,x2= 17-32,P< 0-001.
On regressionanalysis of number of young raised per attempt (y) against ledge
accessibility(x) in five categories:y= 1-03+0llx, r=0 135,N=309, P<0 02. Excluding
knownhumannest robberies:y= 1 29+0 08x, r=0-093, N.S.
73% at practically inaccessible sites. At a subset of the latter, on twelve unassailable cliffs,
83% of clutches were successful. The trend did in fact correlate with the extent of known
human robberies, and the last figure reflected the success of peregrines in the absence of
human intervention. Some of the 'walk in' sites would also have been accessible to foxes
Vulpes vulpes (Linn.), which were the only other likely mammalian predators in the
region. On two occasions, peregrine pairs were seen to attack single foxes and drive them
away. No other aspect of performance was related to the accessibility of the site. The
relationship between nest success and accessibility could have accounted for the better
success of coastal pairs, many of which were on tall, difficult cliffs.
Regardless of height, some cliffs were situated in commanding positions, high on
mountainsides, while others were at low points in the local landscape, and often afforded a
more restricted view. To take some account of topographic position, for inland cliffs we
measured 'outlook', which was the height of the nest ledge above the valley floor or
adjoining low ground. The proportion of pairs which produced young rose slightly with
increase in outlook, but this trend was not significant whether nest robberies by humans
were taken into account or not. Height of cliff and outlook were in no way correlated
(r=0'079, N.S.), so evidently the height of the cliff was important, regardless of its
910
Peregrine breedingsuccess
TABLE6. Breedingsuccessin relationto nest shelter
Nest site
(3)
(4)
(1) (2)
(5)
(6)
In hole or
Open Sheltered Sheltered Overhung Well
one side two sides
overhung otherrecess
Numberof clutches
47
55
42
86
55
19
Number(%)which
24 (51) 27 (49) 22 (52) 52 (60)
38 (69) 15 (79)
producedyoung
Numberof failures
dueto knownhuman
nest robbery
4
10
5
7
5
2
Significanceof variationin clutchsuccessbetweencategories1-2, 3-4 and 5-6: including
knownhumanrobberies,x22=838, P< 0025; excludingknownhumanrobberies,x2= 800,
P< 0-025.
On regressionanalysisof numbersof youngraisedperattempt(y) againstledgeshelter(x)
in six categories:y= 1 03+0.14x, r= 0153, N=305, P<0.01. Excludingknownhumannest
robberies:y= 1 19+0-14x, r=0-155, N=272, P<0-02.
TABLE7. Breedingsuccess in relation to nest type. Known human robberies
excluded
Disusedsticknestof otherspecies Bareledge
Numberof clutches
88
185
Number(%)which
65 (74)
113 (61)
producedyoung
Significanceof differencebetweentypesx:2= 454, P < 0.05.
topographic position. No facet of performance was related to the compass aspect of the
nest cliff.
Another trend in nest success occurred irrespective of human predation, namely
towards better success in the best protected nest sites (Table 6). The ledges used by
peregrinesvaried from those well protected from the elements (e.g. a recess in a rock face)
to those almost completely exposed (e.g. a poorly drained ledge, with no overhang above
and no shelter at the sides). When nest-sites were classed according to the degree of shelter
they conferred, the proportion of clutches which produced young increased from 51% in
the most exposed sites to 79% in the most protected (Table 6). This finding was not
unexpected, considering the effect of rainfall on overall success. In general, exposed ledges
were used on cliffs where nothing better was available, and some apparently dry sites
recieved a constant drip from above during rain. As a related trend, peregrineclutches laid
in old stick nests of other species (mainly raven Corvus corax L.), more often produced
young than did clutches laid on bare substrates (Table 7). This was probably because, in
general, stick nests were in sheltered positions, because their builders were not
constrained to using flat substrates, and so had more choice.
Diet and breedingsuccess
Performance was examined in relation to diet for those nests where enough prey
remains were found (N= 88). Diet changed during the season (Mearns 1983), so if all prey
R. MEARNS
ANDI. NEWTON
911
TABLE8. Breedingperformancein relationto femaleage
1
Numberof clutches
Number(%)whichproducedyoung
%whichproducedyoungafter
excludingknownhumanrobberies
Meanlayingdate in April
Meanclutch-size
Meanbrood-size
2
Age in years
3
4
2
0
17
10 (59)
11
7 (64)
0
63
12
3-0
24
70
9
3.7
2-0
8
4 (50)
57
6
3-8
2-8
5+
24
13 (54)
72
4
3.9
28
Regressionanalysisof performance(y) againstfemaleage (x) gave the followingequations:
Layingdate = 5 451-0 590x,N= 54,r= -0-593, P< 0-001
Clutch-size=2-488+0-273x,N=60, r= 0-570,P<0-001
Brood-size = 1905 + 0166x, N=34, r= 0208, N.S.
TABLE9. Organochlorine levels, shell indices and breeding success. N= number of
cases. N.S. = not significant
Relationshipbetween(x &y)
Constant Regression Correlation
N
coefficient(b) coefficient(r) Significance
(a)
90
1-806
P< 0001
Shellindex& log DDE
-0-186
0-394
Shellindex& log HEOD
90
1-703
-0-034
0-142
N.S.
1-782
90
-0-128
Shellindex& log PCB
0-339
P<0.001
Brood-size& log DDE
87 1-161
0-103
0-034
N.S.
Brood-size& log HEOD
87 1-197
0-024
-0-015
N.S.
87 1-193
Brood-size& log PCB
0-029
0.011
N.S.
Brood-size& shellindex
79 1-278
-0-066
-0-009
N.S.
99
0500
P<0-001
log DDE & log HEOD
99
0614
P<0-001
log DDE & log PCB
99
0370
P<0-001
log HEOD& log PCB
remains had been included, the recorded diet would have varied between pairs, according
to how long the nest survived. To achieve comparability, therefore, only items found
before the end of laying were included. For analysis, they were grouped in five categories,
namely pigeons, song-birds, grouse (mainly Lagopus 1.scoticus (Lath.)) waders and others
(mainly Larus ridibundusL.). On this basis, no significant relationships between diet and
performance emerged.
Density and breedingsuccess
Peregrine pairs were not uniformly distributed over the whole study area, as they
depended partly on the locations of suitable cliffs. Some pairs were as close as 3 km to one
another, but others were up to 30 km from their nearest neighbours. To explore whether
there was any effect of local density on breeding success, we examined the number of
young produced at each attempt in relation to the number of other pairs resident at the
time within a 10 km radius. This number varied between 0 and 8 for pairs in differentparts
of the study area. No significant relationship emerged, whether failures from human
interferencewere included (r = 0-05, N.S.) or excluded (r = 0-09, N.S.). In other words, at
the range of densities observed, no evidence was found for density-dependent suppression
of breeding success.
912
Peregrine breedingsuccess
Female age and breedingsuccess
Some females trapped at the nest were of known age, determined either from their
plumage (first year only) or from their ring numbers, while others could be ascribed a
minimum age. This enabled the performance of females aged 1, 2, 3, 4 or 5 + years to be
compared (Table 8). The incidence of non-laying could not be reliably assessed, however,
because normally only females which laid could be trapped and identified.
Neither of the two yearling females which laid was successful, but among older females
no obvious trend emerged in the proportion which produced young. However, laying
dates tended to become earlier, and clutches and broods larger, with increasing age. On
regression analysis, the trends in laying date and clutch size emerged as statistically
significant (Table 8), but the data were not wholly independent, as some females were
represented in more than one year. These age-related differences could partly, but not
wholly, account for the seasonal decline in clutch-size mentioned earlier, as young birds
laid later and smaller clutches than older ones. Improvements in some aspects of breeding
performance during the first few years of life have been observed in some other raptors
(Newton, Marquiss & Moss 1981; Village 1986), and in some other birds (Kluijver 1951;
Coulson 1966; Hamann & Cooke 1987).
Another tendency was for females which had been present on a territoryin the previous
year to breed more successfully (excluding known human robberies, 69% of 54 raised
young) than females which were new to that territory, 39% of 23 raised young). This, too,
may have been age-related, for new settlers would generally have been younger birds with
less experience of the area (Mearns & Newton 1984). Finally, individuals which changed
nest ledge from the previous year bred no more, or less successfully, on average, than did
birds which used the same ledge.
Pollutants and breedingsuccess
Addled and deserted eggs were collected for assessment of pollutant residues and shell
thickness indices (calculated as shell-weight (mg)/length x breadth (mm), Ratcliffe 1980).
The main pollutants examined were DDE (from the insecticide DDT), HEOD (from the
insecticides aldrin and dieldrin), and PCBs (industrialpolychlorinated biphenyls). During
the study period the levels of DDE and HEOD in eggs declined (Newton, Bogan & Haas
1989), as would be expected from restrictions in the use of the parent insecticides. The
levels of PCBs rose slightly.
Shell indices showed a strong linear relationship with log DDE level, a less strong
relationship with log PCB and a weak (non-significant) relationship with HEOD (Table
9). However, the levels of these various chemicals were intercorrelated,and when the role
of DDE was allowed for in a multiple regression analysis, the other chemicals made no
significant contribution to shell index. This was consistent with previous findings (Peakall
1976; Newton et al. 1982; Newton, Bogan & Rothery 1986), including those resulting
from feeding trials on captive birds of other species (Cooke 1973; Newton 1979). The
implication was that only DDE had any effect on shell index.
During the study period, the levels of DDE were too low to cause marked shell
thinning. Ratcliffe (1980) gave the mean shell index of peregrinesin south Scotland in preDDT times as 1 81. The mean index of eggs obtained in 1974-82 varied between 1 65 (7%
lower than the pre-DDT value) and 1-84 (2% higher) in different years. The few egg
breakages recorded were often associated with egg addling, and the two together
accounted for 7% of clutches laid and 14% of all failures. Breakage and addling also
R. MEARNSAND I. NEWTON
913
caused the loss of an average of 0-86 eggs per clutch in otherwise successful nests.
Organochlorine residues probably accounted for at least part of this loss.
Overall, however, no relationship was found between final brood-size on the one hand
and chemical residues and shell indices on the other (Table 9). Hence, during the study
period, organochlorine residues and shell indices appeared unimportant, compared to
other factors, in the productivity of south Scottish peregrines.
Assessment offindings
Summing up, the breeding success of peregrinesin south Scotland varied between years
according to rainfall in May, with lower production in the wetter years. The success of
individual pairs was related to several features of the nesting cliff, especially its height, the
accessibility of the nest ledge to human climbers, and the degree of shelter the ledge
conferred. Success was also related to the period of years over which the cliff had been
used and to the age of the female. It was not related to surrounding land use, probably
because passing pigeons, together with migrating waders, made up the bulk of the diet,
and their numbers were unrelated to local habitat. Nor was breeding success related to
diet composition early in the season or to the local density of resident pairs. Some of the relevant features were inter-correlated, and the problem was to determine
which were the primary factors affecting success, and which merely appeared important
through association. As expected, the feature 'ledge accessibility' was related to cliff
height (r = 0 409, P < 0 001), but so were several other features. A correlation between cliff
height and ledge exposure (r = 0 243, P < 0 001) probably arose because large cliffs offered
more choice of ledges, so the birds could usually select a sheltered one, whereas some small
cliffs only had a single ledge, often quite exposed. A correlation between cliff height and
duration of occupation (r=0.216, P<0001) could be explained as above by the
preferred, larger cliffs being occupied first, as the population grew. A similar preference
could account for the correlation between outlook and duration of occupation (r = 0-152,
P<0-01). Interestingly, no important features appeared related to female age, except
ledge accessibility. But as this was significant only at the 5% level in a matrix involving
twenty-eight separate correlations, its real significance was doubtful.
In simple linear regressions of number of young raised against site features, some
relationships were weakened when known human robberies were excluded, while others
were improved (Tables 3, 5 and 6). These changes were as expected, considering the
factors which facilitate human access (cliff height and ledge accessibility) and those that
do not (ledge shelter and territory occupancy).
In a further attempt to assess the role of different features, we carried out a multiple
regression analysis with the number of young raised at each attempt as the dependent
variable, and cliff height, ledge accessibility, ledge shelter and duration of occupation as
dependent variables. Ledge accessibility emerged as the most important variable
(r=0.149, P<0.03), but cliff height also contributed substantially to variation in
productivity.
In general, therefore, all these various physical features of the nest cliff may have
affected the success of peregrines, but were inter-correlated to varying degrees. The
duration of occupation emerged as important because it depended on cliff height.
DISCUSSION
Peregrineshave long been known to prefer the biggest and steepest of the cliffs available in
a region (e.g. Hickey 1942; Hagar 1969), and it is possible that this preference, originally
914
Peregrine breedingsuccess
evolved in the presence of a range of mammalian predators, has been maintained in
Britain largely by human predation. Since the mid-nineteenth century, peregrines have
been heavily persecuted by game keepers and egg collectors, and for a much longer period
were regularly robbed of their young by falconers. They may even have been exploited by
early man for food. In other words, for at least 2000 years in Britian man has probably
been the major predator of peregrine eggs and young. In recent centuries, other potential
climbing predators, such as pine martens Martes martes (Linn.), were eliminated from
much of the country, again by human action.
Despite the importance of human predation during the study period, it was much less
than in the first half of this century, when most of the nests in south Scotland were robbed
of their eggs each year, and many adults were shot. This decline in persecution may have
enabled the birds to occupy and breed successfully on less daunting crags, a trait which
has occurred not only in south Scotland as numbers have grown, but in other regions too
(Ratcliffe 1980, 1984). Moreover, soon after our study ended, several peregrines in south
Scotland and elsewhere took up new sites among rank vegetation on slopes. This was
another step in the direction of reduced security, as numbers grew.
Even discounting human predation, pairs on large cliffs were still the most successful,
especially when the cliff was situated at a high point in the local landscape. In any one
year, a greater proportion of such pairs produced eggs and their average clutch and brood
sizes were slightly (but not significantly) largerthan those of pairs on small cliffs. This may
have been because large cliffs facilitated hunting in a way that small cliffs did not, and thus
made prey more available (Newton 1987). A falcon on a large cliff could do much of its
hunting from an energy-saving perched position. It could keep watch over a wide area,
and had better height and updraught when it started a chase. A bird on a small cliff, at a
low point in the local topography, had none of these advantages, and many of the
potential prey it saw would be at a higher level than itself.
Unfortunately, on the data available we could not exclude an alternative explanation of
the better success of pairs on large cliffs. Even if a preference for large cliffs evolved solely
as an anti-predator tactic, then the 'best' birds may have got the best cliffs. And because
they were the best birds, they may also have hunted more effectively and thereby produced
more young. This would also have given the observed correlation between cliff height and
performance, which held even when human robberies were excluded. Observations of
birds hunting from different types of cliff may help to separate these possibilities, but no
relationship was found between cliff height and female age.
In this wet region, with a mean annual rainfall in the hills exceeding 100 cm, rain
emerged as the major natural factor depressing productivity. A non-significant trend was
apparent for later and smaller clutches in years with heavy rain in March, but in addition
the proportion of clutches which produced young declined significantly with increasing
rainfall in May. Observations showed that the birds were especially vulnerable when
heavy rain occurred during the hatch. Rain probably acted partly through reducing the
hunting opportunities of the males, and hence the food supply to the chicks. Many prey
species, such as pigeons, do not readily fly during rain, and would thus become
unavailable to peregrineswhich catch most of their prey in the air. The falcons themselves
may also have been encumbered by rain. Moreover, in south Scotland, rain periods were
often associated with low cloud and mist, which could envelop the nesting cliffs for days at
a time, reducing visibility to a few metres. In addition to chick starvation, exposure and
chilling occurred, especially among older young too large for the female to cover
effectively. We several times visited nests after a downpour to find young cold and soaked,
R. MEARNS
ANDI. NEWTON
915
sometimes huddled in a pool of water on the flooded ledge, but still alive. The importance
of shelter was suggested by the correlation between nest type and nest success, with the
more protected sites, such as recesses, most often producing young.
In contrast, organochlorine residues and shell-thinning, which were so important in
earlier years (Ratcliffe 1980), had at most a small effect during the study period. In the
1960s, shell indices in south Scotland were 20% lower than in pre-DDT days, but during
our study was reduced to an average of 2% lower. This, together with the population
recovery, followed reduction in the use of organochlorine pesticides, and in the amounts
of residue present in eggs.
In conclusion, compared to other studies on birds of prey, two findings were especially
striking. The first was the lack of indication that peregrines were in any way constrained
by food-shortage while breeding, except during rain when hunting was impaired. They
clearly benefited from the ready supply of domestic pigeons, whose numbers in the area
were continually augmented through the season regardless of the numbers eaten. The
second was the heavy dependence of breeding success on the safety of the nest-site from
human and other mammalian predators. As peregrine numbers expanded, and the more
secure sites became occupied, the birds spread increasingly to nest at less safe sites and
even on the ground. At less safe sites they bred less successfully, which provided a
mechanism for density-dependent reduction in breeding success. That this was not
evident during our study was perhaps because numbers were not high enough and because
factors other than predation also affected nest success.
ACKNOWLEDGMENTS
We are grateful to G. Carse and R. Roxburgh for valued assistance in the field, to Dr
D.A. Ratcliffe for help in getting us started, and to Drs J.P. Dempster, M. Marquiss and
D.A. Ratcliffe for constructive comments on the manuscript.
REFERENCES
Cave , A. J. (1968). The breeding of the kestrel, Falco tinnunculus,in the reclaimed area Oostelijk Flevoland.
Netherlands Journal of Zoology, 18, 313-407.
Cooke, A. S. (1973). Shell thinning in avian eggs by environmental pollutants. EnvironmentalPollution, 4, 85152.
Coulson, J. C. (1966). The influence of pair bond and age on the breeding biology of the kittiwake gull, Rissa
tridactyla. Journal of Animal Ecology, 35, 269-279.
Hagar, J. A. (1969). History of the Massachusetts peregrine falcon population, 1935-57. Peregrine Falcon
Populations: TheirBiology and Decline (Ed. by J.J. Hickey), pp. 123-132. Madison, Milwaukee & London;
University of Wisconsin Press.
Hamann, J. & Cooke, F. (1987). Age effects on clutch size and laying dates of individual female lesser snow geese
Ansercaerulescens.
Ibis, 129, 527-532.
Hickey, J. J. (1942). Eastern populations of the duck hawk. Auk, 59, 176-204.
Kluijver,H. N. (1951). The population ecology of the great tit, Parus major (L.). Ardea, 39, 1-35.
Mearns, R. (1982). Winter occupation of breeding territories and winter diet of peregrines in south Scotland.
OrnisScandinavica,
13, 79-83.
Mearns, R. (1983). The diet of the peregrine Falco peregrinusin south Scotland during the breeding season. Bird
Study, 30, 81-90.
Mearns, R. & Newton, I. (1984). Turnover and dispersal in a peregrine population. Ibis, 126, 347-355.
Newton, I. (1979). Population Ecology of Raptors. Poyser, Berkhamsted.
Newton, I. (1987). Regulation of peregrine populations. Peregrine Falcon Populations: Their Management and
Recovery (Ed by T.J. Cade, J.H. Enderson, C.G. Thelander & C.M. White). The Peregrine Fund, Boise,
Idaho (in press).
Newton, I., Bogan, J. & Haas (1989). Organochlorines and mercury in British peregrine Falco peregrinuseggs.
Ibis (in press).
916
Peregrine breedingsuccess
Newton,I., Bogan,J., Meek,E. & Little,B. (1982).Organochlorine
compoundsand shell-thinningin British
merlinsFalcocolumbarius.
Ibis, 124, 328-335.
Newton,I., Bogan,J. A. &Rothery,P. (1986).Trendsandeffectsof organochlorine
compoundsin sparrowhawk
eggs.Journalof AppliedEcology,23, 461-478.
Journalof
Newton,I. & Marquiss,M. (1984).Seasonaltrendin the breedingperformanceof sparrowhawks.
AnimalEcology,53, 809-829.
Journalof AnimalEcology,16,
Newton,I., Marquiss,M. &Moss,D. (1981).Ageandbreedingin sparrowhawks.
777-793.
Newton,I. & Mearns,R. (1987). Populationdynamicsof peregrinesin south Scotland.PeregrineFalcon
andRecovery(Ed.by T.J.Cade,J.H. Enderson,C.G. Thelander& C.M.
Populations:TheirManagement
White).The PeregrineFund, Boise,Idaho(in press).
Peakall,D. B. (1976).The peregrinefalconand pesticides.CanadianFieldNaturalist,90, 301-307.
Ratcliffe,D. A. (1980).ThePeregrineFalcon.T. & A. D. Poyser,Calton.
Ratcliffe,D. A. (1984).Theperegrinebreedingpopulationof theUnitedKingdomin 1981.BirdStudy,31, 1-18.
Village,A. (1986). Breedingperformanceof kestrelsat Eskdalemuir,south Scotland.Journalof Zoology,
London(A), 208, 367-378.
(Received 24 November 1987)