Long-term monitoring of an insular population of Barbary Falcon
Falco peregrinus pelegrinoides
Manuel Siverio1*, Felipe Siverio2, Beneharo Rodríguez3 and Airam Rodríguez4
1
Constitución 17-3, E-38410 Los Realejos, Tenerife, Canary Islands, Spain
2
Los Barros 21, E-38420 Los Realejos, Tenerife, Canary Islands, Spain
3
La Malecita s/n, E-38480 Buenavista del Norte, Tenerife, Canary Islands, Spain
4
Department of Evolutionary Ecology, Estación Biológica de Doñana, CSIC, Avda. Américo Vespucio s/n, E-41092 Seville, Spain
* Corresponding author, e-mail: mansiverio@telefonica.net
Territory spacing and breeding rates of an insular population (north-western Tenerife, Canary Islands) of Barbary Falcon
Falco peregrinus pelegrinoides was studied from 1993 to 2008. The population increased constantly since the outset, from
two pairs in 1993 to 12 in 2008. Mean density was 5.48 pairs per 100 km² and mean nearest neighbour distance was
3 119 m. The regularity of the spatial distribution pattern of the nests, observed in most years, may be maintained in the future
despite the expectation that new pairs may occupy still-vacant territories. Considering the 79 breeding attempts analysed, the
mean number of fledged young per territorial pair was 1.92, per laying pair was 2.0 (n = 76), and per successful pair was 2.24
(n = 68). No significant variations were observed between the annual mean number of fledged young per laying pair, nor
between the number of fledged young of pairs according to density in a 5 km radius. All fledglings (brood size one to four) left
the nest in the month of May. In order to avoid affecting breeding success, sporting activities practised in the breeding areas
must be correctly managed by the appropriate authorities.
Introduction
The taxonomic status of the Barbary Falcon is still
not entirely clear, since it has been regarded both as
a subspecies of Peregrine Falcon (Falco peregrinus
pelegrinoides; Del Hoyo et al. 1994) and as a distinct
species (F. pelegrinoides; Cramp and Simmons 1980,
Ferguson-Lees and Christie 2001). With few exceptions,
resident falcons in the Canary Islands show Barbary
phenotypes (Delgado et al. 1999, Rodríguez et al. 2011).
A preliminary genetic study, in which three falcons from
the Canary Islands with Barbary phenotypes were used,
showed hybridisation with Peregrine Falcon F. p. brookei,
as two of these bird samples had identical haplotypes to
the latter (Amengual et al. 1996). Other preliminary studies
show a very close genetic relationship between Barbary
Falcon and Peregrine Falcon, which indicates that the
former ‘represents a subspecies rather than a true species’
(Wink and Seibold 1996, Wink et al. 2000).
Mainly as a consequence of the use of dichlorodiphenyltrichloroethane (DDT) and derivatives, populations of
Peregrine Falcon from different geographical areas
decreased dramatically during the 1950s and 1960s (Cade
et al. 1988, Ratcliffe 1993). After the prohibition of organochlorated pesticides and with increased legal protection, the
greater part of those populations has gradually recovered
naturally or by means of reintroduction programmes
(Castellanos et al. 1997, Horne and Fielding 2002, Powell
et al. 2002, Dzialak et al. 2005, Sielicki and Mizera 2009).
Although the precise causes are unknown, the falcons in
the Canary Islands were rarely seen at least from the
1950s to mid-1990s (Rodríguez et al. 2009). Indeed, in a
census carried out in 1987 and 1988 only seven pairs were
observed, and their distribution was limited to the eastern
islands and islets of this archipelago: Fuerteventura,
Lanzarote, Alegranza and Montaña Clara (Hernández et
al. 1991). Falcons are currently present on all the islands
and the number of pairs has been estimated to be about
144 (Siverio et al. 2009). Their presence on the island
of Tenerife has been known since the beginning of the
twentieth century (Thanner 1909), but the uncertainty
surrounding subsequent sightings led to their being considered extinct for many years (Hernández et al. 1991). Once
nesting was confirmed in 1991 (Hernández et al. 1992),
their numbers have increased until the present day, and 35
pairs is the latest population estimate for the entire area of
the island (Rodríguez et al. 2009).
In general, the density and breeding parameters of
many subspecies of Peregrine Falcon have been studied
in depth (Mearns and Newton 1988, Mendelsohn 1988,
Ratcliffe 1993, Zuberogoitia et al. 2002, Rizzolli et al. 2005,
Verdejo and López-López 2008). With the exception of the
Canaries, where their biology and ecology have received
some attention (Delgado et al. 1999, Rodríguez and Siverio
2006, Rodríguez et al. 2007), the acquired knowledge of the
Barbary Falcon throughout its distribution range is still scant
(Ferguson-Lees and Christie 2001). In this regard, many of
the data available regarding Morocco, its area of occupation closest to the Canaries, are confusing since they could
also pertain to other sympatric races of Peregrine Falcon
(Thévenot et al. 2003). In the present study we assess the
evolution of demographic and reproductive parameters in
an increasing Barbary Falcon population during 16 years.
Our main goals are to describe annual variation in density,
nest spacing and breeding rates, and evaluate the effect of
density on breeding performance.
Study area
Our study area is situated in the north-west sector of
Tenerife, the highest (3 718 m) and largest (2 034 km 2)
island of the Canarian archipelago, located some 100 km
off the north-west coast of Africa (Figure 1). This zone is a
very steep massif known as Teno, of which around 105 km²
have been studied (taking into account its coastal platforms)
within an altitude range of 0–1 350 m. The south-west
flank is characterised by the presence of a great sea cliff
(c. 12 km long and with several sectors almost 500 m high)
cut by a succession of perpendicular deep ravines. One of
the best-preserved laurel forests in Tenerife is to be found in
the middle altitude zones of the northern flanks. The human
population of the area is estimated at 10 770 inhabitants
(http://www.ine.es), distributed in villages (inland valleys)
and towns (particularly on the coastal platforms). A good
part of this surface area is included in the Canarian Network
of Protected Natural Areas (Government of the Canary
Islands): Parque Rural de Teno (80.63 km²) and Sitio de
Interés Científico de Interián (1.01 km2).
100 km
Canary Islands
Lanzarote
Fuerteventura
AFRICA
28° N
La Gomera
El Hierro
Gran Canaria
CANARY ISLANDS
15° W
AFRICA
Tenerife
study area
TENERIFE
ATLANTIC OCEAN
Methods
During the breeding season (February–May) from 1993 to
2008 the previously occupied territories of Barbary Falcon
were visited, in addition to potential breeding sites (all the
cliffs of the Teno massif, both coastal and inland, were
inspected). It was assumed that a territory was being
occupied when the birds frequently used perching sites,
courting behaviour was observed, or, in the majority of
cases, when the nest was located. In order to assess
breeding rates, nest surveillance was performed with
telescopes (20–60×) and binoculars at distances ranging
from 250 to 600 m. In order to estimate the distances to the
nearest neighbours a global positioning system receptor
and a geographic information system (ArcView 3.2) were
used. On those occasions when their position could not be
established, because of the difficult orographic conditions,
the point on the breeding cliff where the behaviour of the
birds pointed clearly to the existence of a nest (nestlings
calling, adults bringing prey and newly fledged young) was
taken into account. The age of breeding birds (adults or
young of 2 cy) and the sex of the fledglings were determined
by means of the coloration patterns and size, respectively
(Ferguson-Lees and Christie 2001).
In order to avoid bias in the reproductive parameters,
breeding pairs were closely monitored from an early date
(February), to determine which pairs laid eggs (taking turns
during incubation) or which failed. To define the reproductive parameters we consulted Steenhof and Newton (2007).
The population growth rate was calculated by means of the
formula r = Ln (Nt − No)/t, where Nt is the final number of pairs,
No the initial number of pairs, and t is the number of years
elapsed. The annual rate of multiplication was estimated with
La Palma
ATLANTIC OCEAN
Enlarged
area
Tenerife
N
E
W
S
Figure 1: Map of the Canarian archipelago showing the study area
for Barbary Falcon on the island of Tenerife
the formula λ = er, affording the annual growth rate (%) as
(λ − 1) × 100. The regularity of the spatial distribution of the
nesting sites was analysed by mean of the G-statistic (based
on the geometric mean/arithmetic mean ratio of the squared
distances between nearest nests), whose range of values
varies between 0 and 1, and would indicate a regular nest
spacing if it is greater than 0.65 (Brown 1975). To assess
the possible repercussion of pair density on reproduction, we have examined annually the size of the broods as
a function of the number of neighbour pairs (0, 1, 2, 3 and
4–5) in a radius of 5 km. The said surface area was chosen
after weighing up the mean distances and ranges between
neighbouring nests, both of Peregrine Falcon (Ratcliffe 1993,
Gainzarain et al. 2000, Zuberogoitia et al. 2002) and Barbary
Falcon in our study area. The data were analysed by means
of the Kruskal-Wallis non-parametric test and the chi-squared
test (Sokal and Rohlf 1981). The mean values are followed
by the standard error (±SE).
Results
Density
The population increased from two pairs in 1993 to 12
in 2008 (r = 0.119, λ = 12.7%; Figure 2a). Four out of 10
BREEDING PAIRS
(a)
10
8
6
4
2
(b)
7 000
6 000
5 000
4 000
3 000
2 000
1.0
(c)
G-STATISTIC
0.9
0.8
0.7
0.6
0.5
0.4
2
(d)
4
PRODUCTIVITY
Reproductive parameters
Reproductive parameters were analysed using 79 breeding
attempts. Taken together, 152 chicks fledged the nest,
the mean number of fledged young per territorial pair was
1.92 ± 0.12, that per laying pair was 2.0 ± 0.12 (n = 76),
and that per successful pair was 2.24 ± 0.10 (n = 68). The
nesting success (proportion of laying pairs that raise at least
one young) was 89.5%. Mean annual productivity (number
of fledged young/laying pairs) did not vary significantly
between years (H15 = 20.55, p > 0.05; Figure 2d), and no
differences were observed between productivity of pairs
with 0, 1, 2, 3 and 4–5 neighbouring territories in a radius
of 5 km (H4 = 5.792, p > 0.05). Two breeding attempts by
the mixed pairs (adult–young) proved successful, while the
remainder of these pairs (n = 3) did not lay eggs judging
by their behaviour. The breeding success observed in 74
breeding attempts of adult pairs was 89.2% (n = 66). All
the fledglings that left the nest did so during the month of
May, and the brood size varied between one and four: 14
(20.6%) had one fledging, 27 (39.7%) had two, 24 (35.3%)
had three, and three (4.4%) had four. Fledging sex could
be determined in 49 individuals belonging to 22 broods, of
which 61.2% were male (χ 21 = 12, p < 0.01).
12
MEAN DISTANCE BETWEEN
THE NEAREST NESTS (m)
new territories were occupied by young females. Their
subsequent mating, once adulthood was reached, involved
one adult and three young males. In another territory, also
occupied for the first time, the pair consisted of an adult
female and a young male. Birds of the remaining territories (63.6%) were adults. Moreover, a paired adult male
was replaced by a young male approximately one year after
disappearing from the territory.
Mean density for the entire study period was
5.48 ± 0.64 pairs per 100 km2 (range 1.90–11.43). Mean
distance between nearest-neighbour nests varied significantly between years (H15 = 50.81, p < 0.001; Figure 2b) and
the overall mean was 3 119 ± 332 m (n = 16). In most years
(n = 12) the spatial distribution pattern was regular (Figure 2c).
The number of nests recorded per territory fluctuated between
one and five (mean 2.27 ± 0.38). In those territories with more
than one nest, these were always very closely placed on the
same rock face (<200 m), with the exceptions of six over
1 000 m distant from the last nest used.
3
5
4
3
6
3
6
4
2
2
2
8
7
6
7
4
7
1
Discussion
0
Density
In the mid-1980s, none of the present territories of the
study area was occupied (FS and MS pers. obs.), and the
scant sightings of falcons both in the remainder of Tenerife
and in other western islands of the Canarian archipelago
were sketchy (Hernández et al. 1991). After the lack of
information since the beginning of the last century (see
Introduction), the first reliable observations of Barbary
Falcons in Tenerife took place relatively frequently during
1989/90 at Teno (Rodríguez et al. 2009), where two pairs
were confirmed to have bred one year later (Hernández et
al. 1992). All the above points to a recolonisation of Teno
(and, by extension, of Tenerife also), beginning in the years
immediately preceding the study period, probably from the
eastern Canaries (Siverio and Concepción 2004) or from
YEAR
Figure 2: Annual variation (1993–2008) in (a) number of pairs,
(b) mean distance between neighbouring nests, (c) spatial distribution
pattern, and (d) mean productivity of the Barbary Falcon population at
Teno Massif (Tenerife), Canary Islands. Productivity was calculated as
the number of fledged young per laying pair (the annual sample size
does not refer to the total of pairs, but pairs that could be monitored)
north-west Africa. This apparent expansion, which began
likewise to be observed in the remaining central and western
islands (Delgado et al. 1999), might have also taken place
from North Africa towards the Sahel, where observations of
the birds have continued to increase (Brouwer and Mullié
2000). The huge population increase in our study area, as in
the Canary Islands in general (Siverio et al. 2009), could be
because of coincidence of several factors including, among
others, legal protection of the species and high availability of
food resources (nowadays, feral pigeons constitute a plague
in many natural sites of the islands; MS pers. obs.).
If population dynamics during the Teno study are taken
into account, the cases of young territorial females and
mixed pairs (adult–young) detected can not be taken to
indicate a negative tendency (e.g. replacement of birds
induced by human persecution), as reported for stable
populations of falcons and other birds of prey (Ferrer et al.
2003, Pandolfi et al. 2004). On the contrary, our examples
(sighted throughout the monitoring period) appear to be
more related to population growth in an enclave with a high
load capacity, where the young produced occupy optimal
vacant sites, at times on an individual basis (females). On
the other hand, the use of new nests within a territory could
not be attributed to substitutions of breeding females (see
Zuberogoitia et al. 2002 for the Peregrine Falcon) owing to
the fact that the birds were unmarked.
For the entire island of Tenerife, Rodríguez et al. (2007)
found a density of 1.27 pairs 100 km2 and a mean distance
between neighbouring nests of 5 868 m. In this study, the
importance of Teno is already noteworthy (given the high
availability of suitable cliffs and other factors) as being one
of the areas of the island with a greater density of Barbary
Falcon. Thus, the availability of these cliffs favours the
abundance of falcons and other rupicolous species, both
here (Rodríguez et al. 2007, Siverio et al. 2007, Rodríguez
et al. 2010, present study) and in continental environments
(Newton 1988, Gainzarain et al. 2000, Jenkins and van Zyl
2005, Rizzolli et al. 2005). On the other hand, annual differences in distances between neighbouring nests detected
in Teno are because they were greater when there were
fewer pairs at the beginning of the study. When their
number subsequently increased, the abundance of trophic
resources may have minimised intraspecific conflicts and,
consequently, favoured the regularity of the spatial distribution pattern recorded in most years (see e.g. Solonen 1993).
On the other hand, the irregularity of the spatial distribution
pattern observed between 2005 and 2007 (Figure 2c) was
caused by a new pair established at a much greater than
average distance (>5 000 m). Considering that Teno still
offers potential nesting territories, it is foreseeable that the
population will continue to increase and when it stabilises
(c. 15 pairs) spatial regularity will be maintained. Indeed,
in the Italian Alps, the spacing of a stable population of
Peregrine Falcon was always regular in each of the six
years studied (Rizzolli et al. 2005), although in the Cape
Peninsula (South Africa) observed spacing was irregular,
probably because of the equally irregular distribution of cliff
sites (Jenkins and van Zyl 2005).
Breeding parameters
The values of the breeding parameters of our study are within
the range of those recorded in populations of Peregrine
Falcon (see reviews in Zuberogoitia et al. 2002, Rizzolli et
al. 2005). For F. p. babylonicus, a very similar race to that
dealt with here, the scant data published give the most
usual brood size as two (Dementiev 1957). According to
the available information on the few populations of Barbary
Falcon monitored, particularly in the Canaries, no disparity is
observed between the mean values found in the islands and
those of the study area (Table 1). The sole exception is the
considerably higher value of the Teno fledging rate (fledged
young/successful pairs) versus that of the island of Tenerife
(Teno included), which may well be caused by a greater
sample in the former case (Table 1).
It is highly probable that the very scant rainfall in our
study area (mean values for March–April, 2000–2008:
37.68 ± 6.5 mm, range 11.2–74.8; http://www.agrocabildo.
com) does not limit productivity, as occurs in other regions
where rainfall is abundant (Mearns and Newton 1988, Olsen
and Olsen 1989, Zuberogoitia et al. 2002). Moreover, if we
consider that a great number of nest sites in Teno are on
cliff ledges under an overhang and in cavities (MS pers.
obs.), the absence of differences between years is not
surprising. A further factor limiting productivity in birds of
prey is the availability of prey species (Newton 1979). In
our study area, both wild and feral pigeons Columba livia
are abundant and may represent a substantial contribution to the diet during chick rearing (93.3%, n = 45 nest
prey delivery observations). It is noteworthy that in some
populations of Peregrine Falcon the continuous availability
of large prey, such as pigeons, implies high productivity
(López-López et al. 2009).
In birds of prey, population increase can also involve
decreased productivity, as the new pairs occupy lowerquality territories (presence and absence of floating population, trophic resources, etc.), and narrowing of territories
(Ferrer and Donázar 1996, Carrete et al 2006). However,
in Teno, even taking into account the gradual decrease
in distance between neighbour territories (Figure 2c),
the good quality of territories could minimise the densitydependent effect on productivity. The causes of breeding
failure (10.5%) may have been related to the disappearance of a male during incubation, the apparent nest robbing
or depredation of a brood at early-stage and the probable
infertility of the eggs of six clutches (incubation was verified,
but no chicks were subsequently observed).
Conservation
Both the information contained in the present study and
that provided by Rodríguez et al. (2007) and Delgado et
al. (1999) for all of Tenerife and the archipelago, respectively, provide evidence that Teno is one of the best areas
for Barbary Falcon in the Canaries. Despite the fact that
its population has, in general, increased in the Canarian
archipelago (Rodríguez et al. 2009), its conservation
status (IUCN criteria) remains Endangered (Siverio and
Concepción 2004). The greater part of the territories are
located in Protected Natural Areas of the Canary Islands,
and it is thus improbable that its nesting habitat will be
significantly modified, at least in the short and medium
term. Despìte all the above, many people practise outdoor
sports such as rock climbing, abseiling, and hang-gliding
in Teno and other sites, implying a potential risk for the
falcons, as well as other birds of prey, during the breeding
season. Indeed, it is known that the productivity of the
Table 1: Productivity estimates for populations of the Barbary Falcon across its distribution range. Sample size is shown in brackets followed
by the number of years monitored
Region
Morocco
Lanzarote
Tenerife
Teno
Territorial pair
–
–
1.55 (37; 2)
1.92 (79; 16)
Mean number of fledging young per pair
Laying pair
Successful pair
–
2.30 (10; ?)
2.53 (54; 7)
2.48 (37; 5)
–
1.76 (30; 2)
2.00 (76; 16)
2.24 (68; 16)
Peregrine Falcon is lower in nests situated on rock faces
where climbing is practised, unlike sites that remain
undisturbed by such activity (Brambilla et al. 2004). On the
other hand, since the Barbary Falcon population began to
increase in Teno and in the Canaries in general, problems
have arisen between certain pigeon fanciers (because of
pigeon capture) and hunters (for alleged disloyal competition) on the one hand, and the appropriate authorities on the
other. Although such conflicts should be resolved, the most
pressing issue at present is the efficient management by the
relevant authorities of the above-mentioned sporting activities where necessary. Likewise, before implementation of
conservation measures such as modifying the legal conservation status of the species, we recommend a census be
undertaken of the entire Canarian archipelago in order to
assess the current status correctly.
Acknowledgements — All expenses incurred during this study
were defrayed by the authors. We thank Pauline Agnew and Luis
Cadahía for assisting with the English translation, Francisco M
González for logistic support, and Pilar Bello for preparing Figure
1. Thanks are also due to Iñigo Zuberogoitia for his critical review
of the initial manuscript, and two anonymous referees for their
valuable comments and corrections.
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Received July 2010, accepted August 2011
Editor: M Virani