Population status, reproduction and conservation of Osprey Pandion haliaetus in La Gomera and El Hierro, Canary Islands (2003-2004)

Abstract

Breeding population size and some reproductive aspects of Osprey Pandion haliaetus in La Gomera and El Hierro (Canary Islands) were studied during the 2003 and 2004 breeding seasons. All active nests were situated in the southern coasts of both islands. A total of five territorial pairs were detected in La Gomera and one in El Hierro, nesting three in La Gomera and one in El Hierro. These islands contain about 30% of the entire Canarian Osprey population and 16-17% of the Spanish population. Laying dates, estimated in ten day periods, were 21-31 March (n=3) and 1-10 May (n=2). Mean productivity (0.86) was lower than that estimated by some authors for stable populations. Human activities such as recreational sailing, trekking or camping near the nests could limit the establishment of new pairs and cause low productivity.

Full text

The Osprey Pandion haliaetus has a patchy
distribution in the Western Palearctic. Many local
populations, especially in the Mediterranean and
Macaronesian regions, have decreased dramatically
to or very close to extinction during the last decades
due to human persecution, habitat destruction and
tourism development (Schmidt 1998). Current
breeding populations of Osprey in the
Macaronesian islands are located only in the Cape
Verde (Palma et al. 2004) and Canary Islands. In the
latter case, the Osprey breeds in Lanzarote,
Tenerife, La Gomera and El Hierro, and in the islets
of Montaña Clara and Alegranza (Martín &
Lorenzo 2001). Single individuals or pairs can be
observed in the rest of the archipelago (Martín &
Lorenzo 2001). Recent population estimates in the
Canarian Archipelago vary between 15 and 20 pairs
(Triay et al. 2004). Because of its low numbers and
limited distribution range in Spain (Balearic,
Chafarinas and Canary Islands), the species has
been catalogued as Critically Endangered (Triay &
Siverio 2003).
POPULATION STATUS,REPRODUCTION AND
CONSERVATION OF OSPREY PANDION HALIAETUS IN
LAGOMERA AND ELHIERRO,CANARY ISLANDS
(2003-2004)
MANUEL SIVERIO1& BENEHARO RODRÍGUEZ2
Airo 15: 85-90 (2005)
1Los Barros 21, E-38410 Los Realejos, Tenerife, Canary Islands. / 2La Malecita s/n, E-38480 Buenavista del Norte,
Tenerife, Canary Islands. E-mail: benerguez@terra.es
SUMMARY - Breeding population size and some reproductive aspects of Osprey Pandion
haliaetus in La Gomera and El Hierro (Canary Islands) were studied during the 2003 and 2004
breeding seasons. All active nests were situated in the southern coasts of both islands. A total of five
territorial pairs were detected in La Gomera and one in El Hierro, nesting three in La Gomera and
one in El Hierro. These islands contain about 30% of the entire Canarian Osprey population and
16-17% of the Spanish population. Laying dates, estimated in ten day periods, were 21-31 March
(n=3) and 1-10 May (n=2). Mean productivity (0.86) was lower than that estimated by some authors
for stable populations. Human activities such as recreational sailing, trekking or camping near the nests
could limit the establishment of new pairs and cause low productivity.
ESTATUTO POPULACIONAL,REPRODUÇÃO E CONSERVAÇÃO DA ÁGUIA-PESQUEIRA
PANDION HALIAETUS NAS ILHAS DE LAGOMERA E ELHIERRO,CANÁRIAS (2003-2004).
Estimou-se a população reprodutora e estudaram-se alguns parâmetros reprodutores da Águia-
pesqueira Pandion haliateus nas ilhas de La Gomera e El Hierro (arquipélago das Canárias) em
2003 e 2004. Todos os ninhos activos na parte sul de cada uma das ilhas foram observados.
Detectaram-se cinco casais com comportamento territorial em La Gomera e um casal em El Hierro,
ilhas onde se encontraram três e um ninho, respectivamente. A Águia-pesqueira nestas duas ilhas
representa cerca de 30% da população total do arquipélago das Canárias e 16-17% do número de
efectivos em Espanha. As postura, avaliadas em períodos de 10 dias, ocorreram entre 21- 31 de Março
(n=3) e 1-10 Maio (n=2). A productividade (0,86 crias/casal onde ocorreu postura) foi mais baixa
do que os valores apresentados por alguns autores para populações estáveis. As actividades humanas
tais como vela de recreio, caminhadas ou campismo em áreas junto aos ninhos poderão limitar o
estabelecimento de novos casais e diminuir a produtividade.

Overall, very little information has been
published on the biology of Osprey in the Canaries
(cf. Martín & Lorenzo 2001). Up to now, studies
conducted at La Gomera and El Hierro involved
only extensive censuses, that were also carried out in
the whole Canarian Archipelago (Díaz et al. 1986,
Hernández et al. 1987, Delgado et al. 1988, González
et al. 1992). According to these censuses, the species
maintained three breeding pairs in both islands
during the period 1983-1991. Recently, Martín &
Lorenzo (2001) estimated five pairs in each island.
Little information is available on its breeding
biology; available data refers mainly to isolated
observations of non-fledged chicks in their nests.
In the present paper, we present new
information on breeding numbers and repro-
duction (especially productivity) of Osprey in La
Gomera and El Hierro. Threats that could affect
the population status and breeding success are
assessed.
STUDY AREA AND METHODS
The Canary Islands are a volcanic archipelago
that is located 100km off the north-west African
coast (27º37’-29º25’N and 13º20’-18º19’W) and is
comprised of seven major islands and several small
islets and rocks. The islands of La Gomera and El
Hierro are situated in the south-west of the Canarian
Archipelago (Figure 1), and are the smallest (370 and
269 km2, respectively). Their coastlines are
predominantly rocky with boulder shore, and cliffs
up to 300 m high. Sea conditions are usually good
throughout the year in the sheltered coast sectors,
mainly with southern or western orientations.
Human settlements are scattered along the coast, and
a total of five major ports are present in these
islands. Around 33% and 58% of the territory of La
Gomera and El Hierro, respectively, are protected by
Canarian laws and the south-west marine sector of
El Hierro (Mar de Las Calmas) has been declared a
marine reserve by the Canarian government.
Osprey populations in La Gomera and El
Hierro were studied during the breeding seasons of
2003 and 2004 by three to four researchers. In the
first year, one visit was made to each island (during
the fledging period); La Gomera was surveyed at
the end of June (4 days) and El Hierro at the
beginning of the same month (4 days). During
2004, several visits (during the mating, incubation
and fledging periods) were made to both islands
from February to August (23 and 17 days to La
Gomera and El Hierro, respectively).
In both islands and years, all known territories
and recognizable nests (Delgado et al. 1988,
González et al. 1992, Martín & Lorenzo 2001) were
inspected and the remaining potential nesting areas
were searched carefully for new pairs. Coastal
transects by boat and observation from vantage
points were carried out, using binoculars and
Figure 1. Map of the Canary Islands and the position (in black) of La Gomera and El Hierro. / Figura 1. Mapa das
ilhas Canárias assinalando a posição (a negro) das ilhas La Gomera e El Hierro.
OSPREY IN LAGOMERA AND ELHIERRO,CANARY ISLANDS
86

telescopes. To avoid disturbance, all observations
were made more than 150 meters away from nests.
Each nest was categorized according to Van
Daele & Van Daele (1982): active (defended by
adults and containing eggs), occupied (defended
but no egg laying) or inactive (no adults in the
surrounding and no new nesting material). The egg
laying dates (estimated in ten day periods) in
successful pairs were obtained from the chick
estimated ages, using 38 days as incubation period
(Triay 1995) and 53-54 days as nestling period
(Stotts & Henny 1975, Bustamante 1995).
Furthermore, the number of fledged young per
active pair (with clutch), breeding success
(percentage of pairs with young) and fledging rate
(number of fledged young per successful pair) were
calculated (Thibault et al. 2001, Triay 1995).
RESULTS AND DISCUSSION
Nests
A total of ten nests, at different stages of
preservation, were located on sea cliffs around La
Gomera, one in the north coast and the rest in the
south. Occupied (n=1) and active (n=4) nests were
concentrated in 7.64 km (lineal distance) of the
south coast. Mean distance between nests of both
categories was 1,910 m (range 1,100-3,080 m). In El
Hierro, we located seven nests on coastal cliffs (two
of them were very deteriorated, consisting only of
some branches). None of them presented signs of
occupation during 2003, while in the following year
one was active in the southern sector.
In 1991, González et al. (1992) counted eight
nest structures in La Gomera (three on the north
coast and five on the south) and four in El Hierro.
Presumably some nests went unnoticed by these
authors and also by us, due to their bad state of
conservation; however, it is possible that two nests
were built on each island during the period 1992-
2002. In Tenerife, only one new nest has been build
during the last fifteen years and the nests/pair ratio
was 2.2 in 2003 (Siverio 2003).
Although a similar degree of human activity
seems to occur on both the north and south coasts
of La Gomera, breeding pairs occupied
preferentially the southern coast, where sea
conditions are usually good during all seasons, and
foraging activities are more successful (Grubbs
1977). In fact, the three active nests detected in La
Gomera in 1991 were also located in this coastal
sector (González et al. 1992).
Population status
As shown in Table 1, notable differences in
number of breeding pairs exist between the two
island. Two new pairs were counted in La Gomera,
while in El Hierro, two pairs apparently disappeared
since previous censuses, which mentioned three
pairs in 1983, 1984, 1987 and 1991 in both islands
(see González et al. 1992). However, our data agrees
with estimates made by Martín & Lorenzo (2001) in
the case of La Gomera but not in El Hierro (4-5
pairs according to these authors). The current
breeding individuals of La Gomera and El Hierro
represents 30% and 16-17% of the entire Canarian
and Spanish populations, respectively (Triay et al.
2004).
Although all known nests were inspected for
signs of occupation (new nesting material,
dropping, etc.), some pairs could have been left
undetected during 2003. For example, a single
individual observed in El Hierro in June could
belong to an unsuccessful pair.
Our data does not allow us to explain the
current status of the species in El Hierro. However,
the low or zero productivity (see Reproduction
section below), high adult mortality and/or human
disturbance during the breeding season should have
influenced the present critical situation.
Accordingly, in surveys carried out during 2000, no
successfully pairs were detected on the island (de la
Puente et al. 2003).
Due to the philopatric behaviour of the species
(Poole 1989a, Thibault et al. 2001), it is supposed that
the fledging rate of previous years (M. Siverio pers.
obs.) has contributed to stability and increase in
breeding numbers at La Gomera. It is known that
distance between hatching and breeding sites of the
majority of individuals (mainly males) of some
migratory populations is lower than 50 km (Poole
1989a). The same behaviour has been observed in the
resident osprey population in Corsica (West
Mediterranean), where median distance between natal
and breeding sites of both sexes is around 15 km
(Thibault et al. 2001). Distances between La Gomera
and Tenerife (around 29 km), and between La
Gomera and El Hierro (61 km), could influence the
M. SIVERIO & B. RODRÍGUEZ 87

interbreeding rates of Ospreys between those islands,
the rate being higher within closer islands. A similar
situation was suggested by Ferreira et al. (1999) for the
Cape Verde Archipelago, where the southern and
peripheral Sotavento (leeward) group of islands
(more dispersed) show a decrease in breeding
numbers, which may depend on immigration from
the Barlavento (windward) clustered group, with
stable or locally increasing numbers.
Reproduction
During the two years a total of seven active
pairs were counted, six in La Gomera (three per
year) and one in El Hierro. Laying dates of
successful pairs (n=5) were: three during 21-31
March and two during 1-10 May. Mean productivity
(young fledged/laying pair) and mean fledging rate
(fledged young/successful pair) for each year and
island are presented in Table 1. In La Gomera
average productivity and fledging rate in both years
were 0.83 and 1.25 (n=4), respectively; values of
these parameters for the two islands were 0.86 and
1.20 (n=5), respectively.
Previously, laying dates in the Canary Islands
were registered in the first fortnight of April
(González et al. 1992, Siverio & Siverio 1997, Siverio
2003). Therefore, laying dates in this archipelago
range from the first fortnight of February to the
first fortnight of May. Overall, pairs in their first or
second year of reproduction lay later (Triay 1995,
Siverio 2003), but it is impossible to know if this is
the case at La Gomera and El Hierro, because birds
were not individually marked (Bretagnolle et al.
1994).
Mean productivity at La Gomera (0.83) was
very low when compared to other nearby sedentary
populations: 1.32 in Tenerife (Siverio 2003), 1.30 in
Minorca, West Mediterranean (Triay 2002) or 1.43
in Corsica, West Mediterranean (Thibault &
Bretagnolle 2001). Some studies (based on
mortality and production) carried out in the United
States suggest that the mean productivity needed to
maintain a stable population must be 0.8 (Poole
1989b) or between 0.95 and 1.30 (Henny & Wight
1969) fledged young per active pair.
In our study area, low productivity coupled with
a probable high adult mortality rate will surely
influence population stability. However, depending
on the immigration rate from other islands (with
stable productivity), this situation might improve.
However, in Tenerife (the nearest island to La
Gomera), high productivity (>1.5) during recent
years has decreased to 0.5 in 2003 (Siverio 2003). In
Minorca, decreasing productivity is correlated with
a reduction in the number of pairs (Triay 2002,
Triay et al. 2004). It is probable that productivity in
La Gomera during recent years has been higher
than that observed in the present study. For
example, if we consider that all young observed by
González et al. (1992) in this island had fledged,
mean productivity would have been 2.0 (active
pairs, n=3).
Threats and conservation
Conservation problems in the islands studied
seem to be related with different human activities.
In La Gomera, camping and recreational sailing
probably caused the loss of one breeding pair in
1999 (M. Siverio pers. obs.). Habitat (nesting cliff
tops) modification or destruction could seriously
affect the population. In fact, in La Gomera, one
pair abandoned its eyrie while an airport was being
Table 1. Number of territorial pairs, active pairs (with
clutch) and some breeding parameters of Osprey (Pandion
haliaetus) in La Gomera and El Hierro (Canary Islands)
during 2003-2004. / Tabela 1. Número de casais de Águia-
pesqueira (Pandion haliateus) com comportamento territorial e
com ninho, e parâmetros reprodutores destes casais em La Gomera
e el Hierro, ilhas Canárias em 2003-2004.
La Gomera El Hierro
2003 2004 2003 2004
Territorial pairs 4 5 0 1
Active pairs 3 3 0 1
Fledged young 3 2 0 1
Mean productivity
(young fledged/
laying pair)
1 0.66 0 1
Breeding success
(% pairs with
young)
50 40 0 100
Fledging rate
(fledged
young/successful
pair)
1.5 1 0 1
88 OSPREY IN LAGOMERA AND ELHIERRO,CANARY ISLANDS

constructed during the 1990s. When it was finished,
one pair occupied the area again but recent tourist
development in its proximities caused the
disappearance of the birds. Other activities
potentially negative for Ospreys detected in the
eyries’ surroundings are sport fishing and wildlife
photography. In the vicinity of two nests in El
Hierro, one of them occupied at least until 2000 (de
la Puente et al. 2003) and the other until 2002 (F.
Siverio & P. Felipe pers. comm.), sightseeing tracks
were obvious.
Studies carried out in other populations in the
United States (Ames & Mersereau 1964, Swenson
1979, Van Daele & Van Daele 1982, Levenson &
Koplin 1984), Europe (Odsjö & Sondell 2001,
Thibault et al. 2001) and even Tenerife (Triay et al.
2004) have demonstrated that these activities are
important negative factors for the breeding success
of Ospreys. It has been speculated that repeated
human presence during the incubation or the early
chick rearing periods keep birds off their nests.
Such presence is related to low egg hatchability, due
to the overheating of eggs and subsequent
embryonic death, or low nestling survival (Swenson
1979, Van Daele & Van Daele 1982, Levenson &
Koplin 1984). Furthermore, in these situations
potential predation of eggs and chicks by the
Yellow-legged Gull Larus cachinnans (common
breeder in the studied area) may occur.
Action is needed to reduce the negative impact
of the aforementioned activities. Firstly, an annual
monitoring plan of the breeding population and the
identification of particular threats to each eyrie is
very important to take precise conservation
measures. During the breeding season, nest and
perch surroundings must be restricted to sailing and
anchoring. Approximately 150 m are recommended
by Rodgers & Schwikert (2002) as the minimum
buffer-zone to prevent Osprey flushing caused by
vessels and personal watercraft in the foraging and
feeding areas of the west coast of Florida.
Although no precise studies have been made of the
Canaries, we tentatively propose a buffer-zone of
more than 200 m for breeding territories and nests.
Furthermore, activities such as camping, abseiling
or trekking in the proximities of nests must be
prohibited, because it has caused the loss of active
nests and the low productivity in eyries less than
one km apart (Swenson 1979). Finally, information
on natal philopatry and mortality, based on a
ringing program of young birds would be very
useful for Osprey conservation in the Canary
Islands.
Acknowledgements: Some visits to La
Gomera and El Hierro were supported by the
SEO/BirdLife and ICIAC, S.L. We are in dept to
Francisco M. González and David Acuña for the
loan of their boats, which allowed us to conduct the
coastal transects of La Gomera and El Hierro,
respectively. Special thanks also to Felipe Siverio,
Airam Rodríguez and Alejandro Padrón (and his
family) for their help during part of the fieldwork.
In La Gomera, Juana María Darias (Cabildo Insular
de La Gomera) facilitated our visits to this island
and, Juan González and Longino Lima
(Vicenconsejería de Pesca del Gobierno de
Canarias) help us very much during sea
prospections in 2003 in El Hierro. The location
map was produced by Jesús Alonso and valuable
comments and corrections on the earlier draft of
this manuscript were given by Felipe Siverio, Rubén
Barone and two anonymous referees.
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