Breeding density, habitat selection and reproductive ...

This article was downloaded by: [223.86.102.154] On: 21 March 2014, At: 18:13 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Bird Study Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tbis20

Breeding density, habitat selection and reproductive rates of the Peregrine Falcon Falco peregrinus in Álava (northern Spain) R. Arambarri & A.F. Rodríguez Published online: 29 Mar 2010.

To cite this article: R. Arambarri & A.F. Rodríguez (2000) Breeding density, habitat selection and reproductive rates of the Peregrine Falcon Falco peregrinus in Álava (northern Spain), Bird Study, 47:2, 225-231, DOI: 10.1080/00063650009461177 To link to this article: http://dx.doi.org/10.1080/00063650009461177

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

Bird Study (2000) 47, 225–231

Breeding density, habitat selection and reproductive rates of the Peregrine Falcon Falco peregrinus in Álava (northern Spain) JOSÉ ANTONIO GAINZARAIN1*, RAMÓN ARAMBARRI2 AND ARTURO F. RODRÍGUEZ3 Alavés de la Naturaleza, Apartado 2092, 01080 Vitoria-Gasteiz, Spain, 2Herminio Madinabeitia 14, 5° izda, 01006 Vitoria-Gasteiz, Spain and 3Eras de San Jorge 21, 01200, Agurain, Araba, Spain

Downloaded by [223.86.102.154] at 18:13 21 March 2014

1Instituto

A population of 33–35 pairs of Peregrine Falcon in Álava (north Spain) was studied with the aim of assessing the habitat attributes that influence breeding density, habitat selection and breeding success. A strong relationship was found between density of the species in each UTM (Universal Transverse Mercator) square of 10 × 10 km and cliff availability. Habitat selection was analysed by comparing 15 variables in 33 occupied and 25 unoccupied cliffs located at least 2 km from the nearest Peregrine pair. Significant differences were found in five variables: cliff dominance, distance to the nearest Golden Eagle Aquila chrysaetos nesting cliff, steepness and altitude (all showing larger values in occupied cliffs), and cliff orientation, with occupied cliffs facing preferably south and east. Orientation, dominance and distance to the nearest Golden Eagle pair, and the distance to the nearest Eagle Owl Bubo bubo nesting cliff, were included in a discriminant analysis which classified 82.76% of the cliffs correctly. The productivity of the studied population was 1.44 young/territorial pair (n = 45), and no consistent relationship was found between breeding success and habitat variables. The largest European population of Peregrine Falcon Falco peregrinus occurs in Spain.1 The Iberian Peninsula is one of the strongholds of this almost cosmopolitan species, which has been the subject of a large number of studies in both Europe and North America.2–4 However, published information about the species in Spain is rather scarce, with the exception of the synthesis of Heredia et al.5, which estimated a national population size of 1628–1751 pairs. Quantitative data on habitat selection by Peregrines is hard to find in the literature, and has seldom been treated in depth.4,6 This paper deals with the habitat attributes that influence occupancy patterns of a Peregrine Falcon population in the north of Spain, with respect to density and selection of a particular nesting cliff. To assess selection, occupied and unoccupied cliffs were compared, as in studies of other *Correspondence author. Email: [email protected] © 2000 British Trust for Ornithology

Iberian raptors.7–11 The breeding cycle of a sample of pairs was also monitored to estimate the reproductive rates of the species and to investigate the relationship between breeding success and the features of nesting cliffs and territories. STUDY AREA AND METHODS The study area (42°28’–43°12’ N and 2°13’– 3°15’ W) comprised the province of Álava and the adjacent enclaves of Treviño (Burgos) and Orduña (Bizkaia). Located inland, on the boundary beween the Eurosiberian and Mediterranean regions, it covers a total of 3302 km2, with an average altitude of 765 m (range 120–1480 m) and a mean population density of 85 human inhabitants/km2. Fieldwork was carried out during the years 1996 and 1997, and suitable cliffs were inspected in search of established pairs mainly in February and March, when courtship takes

226

J.A. Gainzarain et al.

place. In this way, we hoped to detect those pairs that did not lay eggs or that suffered early breeding failure and are much more difficult to locate later in the season. The exclusion of such pairs would have led to an underestimation of the breeding population size,12 and an overestimation of the reproductive rates.13

The same 15 variables were employed in a stepwise discriminant analysis,a using the STATISTICA program (Statsoft Inc.). Previously, the distance variables were log transformed, so that their greater influence when referred to the shortest distances was reflected. The robustness of the model was tested by means of Jackknife.b, 15


Density In order to determine the factors that might influence Peregrine density in the study area, a Spearman’s rank correlation coefficient14 was calculated between the number of pairs present in each 10-km UTM (Universal Transverse Mercator) square and the following variables: number of inhabitants, percentage of forested land, cliff availability, and dispersion of human population centres. The last two variables were calculated by dividing the 10-km square into 25 cells of 2 × 2 km, and recording the proportion of these with presence of cliffs and human population centres.

Reproductive rates The breeding success of a sample of pairs (20 in 1996 and 25 in 1997) was recorded, and the habitual parameters of productivity (number of young fledged per territorial pair), breeding success (percentage of successful pairs) and fledging rate (number of fledged young per successful pair) were calculated. To evaluate the potential influence of habitat features on breeding performance, correlation analyses (using the Spearman’s coefficient) for both 1996 and 1997 were carried out between the number of young fledged per pair and the 15 variables used in the habitat selection analysis.

Habitat selection Fifteen variables were selected to describe both the nesting cliff and the surrounding land, as well as the possible influence of interspecific competition with the Golden Eagle Aquila chrysaetos and the Eagle Owl Bubo bubo (Table 1). In the case of those Peregrine pairs with known nesting attempts on different rock faces, the one most recently occupied was selected. The location of the rock faces occupied by Aquila and Bubo in Álava, where they breed exclusively on cliffs, was known from our own observations and the exhaustive censuses carried out by other local ornithologists. The same 15 variables were also quantified in 25 apparently suitable cliffs chosen randomly from those in which the species was not observed. In order to exclude the effect of intraspecific competition, no crag in close proximity to any pair of Peregrines was included among the unoccupied cliffs. This proximal distance was calculated as half the average separation between pairs in the areas with the highest density of Peregrines and where cliff availability did not seem to be a limiting factor. The values of the variables in cliffs with and without Peregrines were compared using the Mann–Whitney U-test.14 © 2000 British Trust for Ornithology, Bird Study, 47, 225–231

RESULTS Density A total of 33–35 Peregrine pairs were counted in the study area, all of them occupying natural cliffs. Two single individuals holding territory were recorded, but they were not included in subsequent analysis. The mean distance between neighbouring pairs was 5190 m. In the areas with the highest density this value decreased to 3959 m (average for three zones with 5–11 pairs); half of this value was taken as a reference to eliminate the effect of intraspecific competition in the habitat selection analysis, hence no cliff located less than 2 km from any occupied crag was included in the list of unoccupied ones. This distance was approximately the same as the minimum distance observed between pairs (1900 m). The density of Peregrine pairs in each 10-km square was significantly correlated with two of the variables considered in the analysis: cliff availability (rs = 0.880, n = 32, P < 0.0001; Fig. 1) and dispersion of human population centres (rs = –0.387, n = 32, P < 0.05). If the squares devoid of cliffs were eliminated from the analysis, the relationship between density and

Peregrine Falcon in norther Spain

227

Table 1. Variables used in the habitat selection analysis. Variable

Explanation

ALT ORI

Altitude in metres above sea level. Orientation index, with higher scores for the sunniest and most sheltered orientations: 1, NW; 2, N & W; 3, NE & SW; 4, S & E; 5, SE.8 Maximum height of the cliff. Average difference of altitude – measured on the map – between the top of the cliff and the end of three imaginary lines of 1 km that, from the cliff top, progress forward forming a right angle and its bisection. Difference in altitude between the highest and the lowest point within a 500 m circle around the cliff. Distance in metres to the nearest paved road. Distance in metres to the nearest village. Distance in metres to the nearest Peregrine Falcon nesting cliff. Distance in metres to the nearest Golden Eagle nesting cliff. Distance in metres to the nearest Eagle Owl nesting cliff. Percentage of land covered by trees. Percentage of land covered by shrubland and pasture in the 9 km2 square. Percentage of land covered by farmland in the 9 km2 square. Number of kilometres of paved road in the 9 km2 square Number of inhabitants in the 9 km2 square.

HEIGHT DOMINANCE

DISTROAD DISTVIL DISTFALCO DISTAQUILA DISTBUBO %FOREST* %SHRUB* %FARM* KMROAD* INHABIT*

*These variables are referred to a 9 km2 square formed by 3 × 3 UTM cells in the central of which the cliff is located

dispersion of villages fell below the level of significance of 0.05 (rs = –0.374, n = 22, P = 0.086), whereas the association between density and cliff availability remained highly significant (rs = 0.788, n = 22, P < 0.0001). Therefore, the observed relationship between Peregrine density and dispersion of villages seemed to be mainly due to the lack of rock faces in areas where there were many people.

4

Mean number of pairs


STEEPNESS

3

2

1

0 0

1–10

11–20

21–30

31–40

Cliff availability (%) Figure 1. Relationship between cliff availability in the UTM square of 10 × 10 km (expressed as percentage of 2 × 2 km squares with presence of cliffs) and the number of Peregrine Falcon pairs (± standard error).

Habitat selection Five of the variables used in the analysis showed significantly different values in occupied and unoccupied cliffs: altitude, dominance, steepness, cliff orientation and distance to the nearest Golden Eagle pair (Table 2). The first three variables were highly intercorrelated (all correlations significant at the level of 0.001; Spearman’s rank correlation test, n = 58), and revealed the tendency of this species to use crags located in mountainous terrain and with a commanding position. The orientation was also significantly different in occupied and unoccupied cliffs. Crags with breeding Peregrines more often faced east or south and 84.8% of them presented an east or south component in their aspect, while for unoccupied crags this percentage decreased to 52%. Finally, the distance to the nearest Golden Eagle nesting cliff also differed significantly between cliffs with and without breeding Peregrines, being larger in the occupied crags. The shortest separation found between neighbouring pairs of these species was 700 m, and only two Peregrine pairs were found nesting closer than 2 km to a pair of Golden Eagle. Four variables were selected in the discriminant analysis: dominance, orientation index, © 2000 British Trust for Ornithology, Bird Study, 47, 225–231

228


Table 2. Means (and standard deviations) of the variables used to describe the territories occupied and unoccupied by the Peregrine Falcon.

ALT** ORI* HEIGHT DOMINANCE** STEEPNESS** DISTROAD DISTVIL DISTFALCO DISTAQUILA* DISTBUBO %FOREST %SHRUB %FARM KMROAD INHABIT

Occupied cliffs (n = 33) 948.2 3.6 63.0 250.3 337.6 1149.7 1589.0 4505.0 6348.5 9337.9 44.4 32.4 20.5 2.5 51.6

Unoccupied cliffs (n = 25)

(206.2) (1.0) (40.5) (149.8) (113.6) (660.0) (701.2) (2307.9) (3781.4) (7441.3) (15.7) (16.4) (16.4) (2.1) (145.4)

856.4 2.8 46.5 142.4 266.8 1047.4 1422.0 3627.6 3896.4 5760.0 51.1 29.8 17.9 2.9 31.7

(182.8) (1.2) (31.2) (131.7) (86.6) (706.0) (807.2) (1199.1) (3524.7) (4866.9) (20.2) (12.7) (14.8) (2.4) (73.0)

Variable abbreviations as in Table 1. Variables with significant differences in the Mann–Whitney U-test are marked with asterisks. Significance levels: *P < 0.05; **P < 0.01.

distance to the nearest Golden Eagle pair and distance to the nearest Eagle Owl pair. The percentage of correctly classified cliffs was 82.76% (90.91% of occupied and 72% of unoccupied ones), and Jack-knife showed the robustness of the model, as it did not reduce the number of correctly classified cases. Classification functions obtained in the discriminant analysis, with variables in the order they were included in the model, are as follows: occupied cliffs: –57.95 + 12.41 (log dist. to Aquila) + 0.024 (dominance) + 5.38 (orientation index) + 11.48 (log dist. to Bubo) unoccupied cliffs: –41.61 + 10.59 (log dist. to Aquila) + 0.016 (dominance) + 4.29 (orientation index) + 9.97 (log dist. to Bubo) The analysis confirmed the importance of the first three variables in the Peregrine habitat selection, and highlighted the influence of cliff dominance with regard to altitude and steepness. The new variable added, distance to the nearest Eagle Owl pair, was larger on average in occupied cliffs. Figure 2 represents graphically the distances of the cliffs included in the © 2000 British Trust for Ornithology, Bird Study, 47, 225–231

analysis to their nearest Golden Eagle and Eagle Owl neighbours, and shows the absence of breeding Peregrines at the closest sites to pairs of these raptor species. Reproductive rates The percentage of successful pairs was practically identical in both years of the study, 65.0% in 1996 (n = 20) and 68.0% in 1997 (n = 25), as 100 000

10 000

Distance to Bubo (m)


Habitat variables

1000

100

10

0 0

10

100

1000 10 000 100 000

Distance to Aquila (m) Figure 2. Graphic representation (in logarithmic scale) of the distances of the cliffs occupied (●) and unoccupied (● ) by Peregrine Falcon to the nearest pairs of Eagle Owl Bubo bubo and Golden Eagle Aquila chrysaetos.


Peregrine Falcon in norther Spain was the number of young fledged/territorial pair (1.45 in 1996 and 1.44 in 1997) and the number of young fledged/successful nest (2.23 in 1996 and 2.12 in 1997). In 1996 a positive relationship was detected between the number of young fledged per pair and two of the variables of the analysis: the distance to the nearest Peregrine pair (rs = 0.588, n = 20, P < 0.01) and the distance to the nearest Golden Eagle pair (rs = 0.454, n = 20, P < 0.05). In contrast, in 1997 no significant association was obtained, and, moreover, the relationships between reproductive output and the distances to pairs of Peregrine Falcon and Golden Eagle showed the opposite tendency to those for 1996. For the remaining variables, even when the level of significance was reduced to 0.1, no significant relationship was found in both years. DISCUSSION Peregrine density in the study area, measured by means of the average distance between pairs, was comparable to that of some of the healthiest populations reported in the literature,4,6,16–18 and similar to that of the best areas in the Iberian Peninsula.5 As in the adjacent province of Navarra,19 the only factor with a noticeable influence on the species density was cliff availability, of which the direct relationship with the abundance of several raptor species has been confirmed in many instances.20,21 This relationship appeared with particular strength in our data because of the absence of Peregrines occupying artificial habitats such as buildings or quarries, which are frequently used in other areas of the species’ range.3 In the study area, the position of the nesting cliff seemed to be one of the main factors influencing the habitat selection by the Peregrine Falcon, which used mostly those located at high altitude, in steep terrain and with a commanding position over the surrounding land. Preference for the largest cliffs is often cited in the literature, a phenomenon attributed mainly to the greater security from interference that they offer.4,6 Although height is usually considered the main characteristic of a preferred nesting cliff, we did not find significant differences in this aspect between occupied and unoccupied sites. Probably, the total descent and steepness of the entire cliff range has a

229

stronger influence on its accessibility than the height of the specific rock face where the Peregrine nests and therefore plays a more important role in habitat selection.16 Dominance is another feature that the Peregrine seems to look for when selecting a nesting cliff, although it has been considered mainly a consequence of the species’ preference for the highest cliffs.4 Commanding cliffs provide a wider outlook and presumably an advantage in capturing prey flying high over the ground.16,22 This selection of dominant rock faces contrasts with the preferences of other raptor species in the north of Spain, such as the Egyptian Vulture Neophron percnopterus8 and the Eagle Owl,7 which generally capture their prey on the ground and select those cliffs located near the bottom of valleys for breeding. As far as the orientation of the nesting cliff is concerned, the preference of this species for those with an aspect that provides adequate shelter from severe weather has been pointed out in other studies.6,23–27 This phenomenon is attributed to the negative effect of rainy and misty weather on breeding for this species, as it reduces hunting opportunities and is also a direct cause of mortality of chicks.4 Our results agree with many other studies on the importance of spatial segregation between breeding Golden Eagles and Peregrines.4,16,19,24,26,28 Peregrines avoid the vicinity of the dominant Golden Eagle, and in Álava they have never been recorded sharing the same cliff. The underlying reason for this intolerance is uncertain: it may be competition for food, as the diets of the two species partially overlap, but it is probably the avoidance by the Peregrine Falcon of a potential predator.4,29 Although less intense, segregation with the Eagle Owl was also apparent, probably because of avoidance by the Falcon of this nocturnal raptor, which is capable of killing Peregrines and shows a strong aggressiveness in its territory towards other raptors.30 The results of this study suggest therefore that interspecific competition and the features of the nesting cliff have a decisive influence on Peregrine habitat selection, which, according to our data, does not seem to depend appreciably on the characteristics of the surrounding habitat. However, it must be said that, in this sort of analysis, conclusions are exclusively limited to the parameters included in the © 2000 British Trust for Ornithology, Bird Study, 47, 225–231


230


model. There are many other variables with a potential influence (e.g. availability of prey species, microclimatic conditions), whose effects, if any, remain unknown. Nonetheless, the high percentage of pairs correctly classified by means of discriminant analysis and the agreement with similar studies4,19,24,28 strongly suggest that the selected variables play a major role in the Peregrine’s choice of nesting site. Also, the species’ requirements probably relate to an adequate prey supply,4 which, on account of the large number of bird species it can prey on, seems to be a widely fulfilled condition in this part of the species’ range. The breeding performance of Peregrines in the study area was similar to or slightly larger than that of other populations in the absence of significant organochlorine contamination or human disturbance.20–22 Many authors have pointed out that breeding success is strongly influenced by weather during the breeding season, especially by heavy rain in the first weeks after hatching,23,25,31,32 which in Álava takes place in April. Rainfall in April during the two study years was below average (43.7 mm in 1996 and 17.7 mm in 1997, compared to an average of 73.1 mm in the meteorological station of Arkaute), and this may have led to the good productivity levels observed. In the south of Scotland, Mearns & Newton23 have shown the negative effect of human disturbance on the breeding success of species in territories with high human pressure and in the most accessible nest sites. In Spain, Peregrine Falcons in heavily populated areas also showed considerably lower productivity than their congeners in more remote areas, which was attributed mainly to nest robbing.5 In the present study, no negative relationship was observed between breeding performance and any of the variables related to human presence in the surroundings of the nesting cliff, nor with its accessibility. Thus, and with the caution that the brief period this study covered demands, human pressure did not appear to be a factor with an important influence on Peregrine breeding success in this area.

Salvada. We are also grateful to the many ornithologists who notified us about their observations of Peregrine Falcon: Santi Abascal, Gorka Belamendia, Aurelio Canabal, Pedro Cruzado, José María Fernández, Luis Lobo, Iñaki Martínez, José Angel Nuevo, Alejandro Onrubia, Txema Pérez Ugarriza and Pablo Ruiz de Arkaute. José María Fernández, Carmelo Fernández, Iñigo Zuberogoitia, M. Marquiss, D. Ratcliffe and J. O'Halloran made valuable suggestions on earlier drafts of this manuscript. Brian Webster and Roberto Ruiz helped with the English translation. This study was partially funded by a grant from the Department of Industry, Agriculture and Fisheries of the Basque Government.

ACKNOWLEDGEMENTS

REFERENCES

We especially thank Alfredo Conde for his invaluable help with the Peregrines of Sierra

1. Hagemeijer, W.J.M. & Blair, M.J., eds (1997) The EBCC Atlas of European Breeding Birds: Their Dis-

© 2000 British Trust for Ornithology, Bird Study, 47, 225–231

ENDNOTES a. Discriminant analysis is a multivariate statistical technique employed to investigate differences between groups (in our case two groups: occupied and unoccupied cliffs), using data from a set of variables which is thought to contribute to group differences. It produces a set of discriminant equations, one associated with each group, resulting from weighted, linear combinations of the independent variables. Each case is assigned to the group with the larger linear combination score. The individuals used to produce the discriminant function can subsequently be classified by it. A forward stepwise procedure was adopted to filter out those variables that did contribute to the discrimination of territories with and without Peregrine pairs. The minimum value of F to include a variable in the model was 7. b. In the Jack-knife method, the proportion of correctly classified cases obtained by the model is compared with that obtained when each observation is classified using the discriminant equations derived from all samples except the one currently classified. In order to show the robustness of the model, the number of correctly classified cases with Jack-knife would be the same or slightly less than that obtained with the initial model.


Peregrine Falcon in norther Spain tribution and Abundance. Poyser, London. 2. Hickey, J.J., ed. (1969) Peregrine Falcon Populations: Their Biology and Decline. The University of Wisconsin Press, Madison. 3. Cade, T.J., Enderson, J.H., Thelander, C.G. & White, C.M., eds (1988) Peregrine Falcon Populations: Their Management and Recovery. The Peregrine Fund Inc., Boise, Idaho. 4. Ratcliffe, D.A. (1993) The Peregrine Falcon. Poyser, London. 5. Heredia, B., Hiraldo, F., González, L.M & González, J.L. (1988) Status, ecology and conservation of the Peregrine Falcon in Spain. In Peregrine Falcon Populations: Their Management and Recovery (eds T.J. Cade, J.H. Enderson, C.G. Thelander & C.M. White), pp. 219–226. The Peregrine Fund Inc., Boise, Idaho. 6. Mooney, N.J. & Brothers, N. P. (1987) The Peregrine Falcon Falco peregrinus macropus in Tasmania. I. Distribution, abundance and physical characteristics of nests. Austral. Wildl. Res., 14, 81–93. 7. Donázar, J.A. (1988) Selección de hábitat de nidificación por el Búto Real Bubo bubo en Navarra. Ardeola, 35, 233–246. 8. Ceballos, O. & Donázar, J.A. (1989) Factors influencing the breeding density and nest-site selection of the Egyptian Vulture (Neophron percnopterus). J. Ornithol., 130, 353–359. 9. Donázar, J.A., Hiraldo, F. & Bustamante, J. (1993) Factors influencing nest site selection, breeding density and breeding success in the Bearded Vulture (Gypaetus barbatus). J. Appl. Ecol., 30, 504–514. 10. Fernández, C. (1993) Sélection de falaises pour la nidification chez l’aigle royal Aquila chrysaetos. Influence de l’accesibilité et des dérangements humains. Alauda, 61, 105–110. 11. Gil, J.M., Molino, F. & Valenzuela, G. (1996) Selección de hábitat de nidificación por el Águila perdicera (Hierraaetus fasciatus) en Granada (SE de España). Ardeola, 43, 189–197. 12. Village, A. (1984) Problems in estimating kestrel breeding densities. Bird Study, 31, 121–125. 13. Steenhof, K. & Kochert, M.N. (1982) An evaluation of methods used to estimate raptor nesting success. J. Wildl. Manage., 46, 885–893. 14. Sokal, R.R. & Rohlf, F.J. (1969) Biometry. Freeman, San Francisco. 15. Lachenbruch, P.A. (1975) Discriminant Analysis. Hafner, New York. 16. Ratcliffe, D.A. (1962) Breeding density in the Peregrine Falco peregrinus and Raven Corvus corax. Ibis, 104, 13–39. 17. Cramp, S. & Simmmons, K.E.L., eds (1980) Hand-

231

book of the Birds of Europe, the Middle East and North Africa. Vol. II. Oxford University Press, Oxford. 18. Olsen, P.D. & Olsen, J. (1988) Breeding of the Peregrine Falcon. I. Weather, nest spacing and territory occupancy. Emu, 88, 195–201. 19. Dónazar, J.A., Ceballos, O. & Fernández, C. (1989) Factors influencing the distribution and abundance of seven cliff-nesting raptors: a multivariate study. In Raptors in the Modern World (eds B.U. Meyburg & R. Chancellor), pp. 545–552. WWGBP, Berlin. 20. Newton, I. (1979) Population Ecology of Raptors. Poyser, Berkhamsted. 21. Newton, I. (1998) Population Limitation in Birds. Academic Press, London. 22. Newton, I. (1988) Population regulation in peregrines: an overview. In Peregrine Falcon Populations: Their Management and Recovery (eds T.J. Cade, J.H. Enderson, C.G. Thelander & C.M. White), pp. 761–770. The Peregrine Fund Inc., Boise, Idaho. 23. Mearns, R. & Newton, I. (1988) Factors affecting breeding success of peregrines in South Scotland. J. Anim. Ecol., 57, 903–916. 24. Poole, K.G. & Bromley, R.G. (1988) Interrelationships within a raptor guild in the Central Canadian Arctic. Can. J. Zool., 66, 2275–2282. 25. Olsen, P.D. & Olsen, J. (1989) Breeding of the Peregrine Falcon. III. Weather, nest quality and breeding success. Emu, 89, 6–14. 26. Carlier, P. (1993) Choix des sites de nidification du Faucon Pèlerin Falco peregrinus brookei dans le parc naturel des Sierras Subbéticas Cordobesas. Alauda, 61, 111–117. 27. Norris, D.W. (1995) The 1991 survey and weather impacts on the Peregrine Falco peregrinus breeding population in the Republic of Ireland. Bird Study, 42, 20–30. 28. Manzi, A. & Perna, P. (1994) Relationships between Peregrine Falco peregrinus and Lanner F. biarmicus in the Marches (Central Italy). In Raptor Conservation Today (eds B.U. Meyburg & R. Chancellor), pp. 157–162. WWGBP, Berlin. 29. Watson, J. (1997) The Golden Eagle. Poyser, London. 30. Mikkola, H. (1976) Owls killing and killed by other owls and raptors in Europe. Br. Birds, 69, 144–154. 31. Ratcliffe, D.A. (1984) The Peregrine breeding population of the United Kingdom in 1981. Bird Study, 31, 1–18. 32. Olsen, P.D. & Olsen, J. (1989) Breeding of the Peregrine Falcon. II. Weather, nest quality and the timing of egg laying. Emu, 89, 1–5.

(MS received 23 March 1999; revised MS accepted 3 September 1999)

© 2000 British Trust for Ornithology, Bird Study, 47, 225–231