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Abstract. A study of the
distribution, density and
habitat of the Corsican nuthatch, an endemic species to Corsica, suggests that
adjustments are needed to the present forest management in order to ensure the
conservation of this species. The survival of the Corsican nuthatch is dependent
on old Pinus lancio forests it’here there are lead and rotten trunks,
neededfor nesting. The Corsican nuthatch was recorded in almost 15 P. lancio
forests; the average density was 0.85 pairs 10 ha-1 (0-1.23); this density,
extrapolated to a total surface of 24 000 ha of suitable forests, suggests a
total population of about 2000 pairs. Apart from fires, snow-slips and predators,
which are important factors limiting this endemic species, the present forest
management, which includes the removal of dead trees suitable for nesting,
threatens the survival of the nuthatch. The third of theforest at the highest
altitude should be left unexploited; in the middle zone, only the dead trunks
below 3 m should he taken, in the lower third, where suitable trunks are scarce,
no dead trunks should be taken.
INTRODUCTION
Studies were conducted on the Corsican
nuthatch by Whitehead (1885) and by Jourdain (1911), in the years following its
discovery (Sharpe, 1884); no other substantial work was published until Lohrl’s
(1960) definitive separation of the species from Sitta canadensis. Chappuis
(1976) studied the vocalisations, and preliminary data on breeding were given by
Hobson (1964) and by Brichetti (1978, 1979). Other records, published in general
reports of the Corsican avifauna, are summarised by Thibault (1983).
From 1977 to 1984, and particularly from 1981 to 1984, we undertook research on
the distribution, breeding ecology and the population status of the Corsican
nuthatch Sitta whiteheadi. The species is stenoecius and is strictly tied to
Pinus laricio forests, where the present management does not ensure adequate
conservation (Brichetti & Di Capi, 1985).
Our results suggest that the population of the Corsican nuthatch totals about
2000 pairs; this estimate is based on the extrapolation of the average density
(0.85 pairs l0 ha-1), to the total surface of c. 24000 ha of suitable forests.
At the end of the 1950s, Lohrl (1960) estimated a population of about 3000 pairs
(one territory per 10-15 ha and 43750 ha of forest). We believe that this
nuthatch has decreased during the last two decades in some zones where forests
have been exploited, and that the reduction has been caused by the loss of
suitable breeding habitat.
The Corsican nuthatch is of special conservation concern in Europe (Parslow
& Everett, 1981), and fulfils at least 60% of the criteria proposed by
Adamus & Clough (1978) as guidelines towards evaluating species for
protection in natural areas. These criteria are discussed below.
Site fidelity
We could not test whether the same nests
are used year after year, but the same trees are occupied for long periods for example, in the Valdo Niello forest one tree had a nest for four breeding
seasons, until it was cut down.
Sedentariness
Observations during non-reproductive
periods (September 1980, October 1983, winter 1983-84) showed that a pair is
tied to its breeding territory all the year round, provided that the habitat is
not altered. The winter records of Corsican nuthatches below 700-800 m in low
valleys and near the west coast of Corsica (Thibault, 1983) probably reflect
non-territorial birds or the dispersive movement of fledgelings.
Habitat specialisation
The connection between the Corsican
nuthatch and the Pinus laricio is absolute, the pine fulfilling all the feeding
and breeding requirements of the bird.
Localised distribution, rarity, endemicity, and scientific value all result from
the peculiar biogeographical properties of the Corsican nuthatch.
STUDY AREA
The Corsican nuthatch
lives in at least 15 forests of the Pinus (nigra) laricio, spread along all the
mountain ridges of inland Corsica, from Tartagine in the north, to Ospedale in
the south (Fig. 1). The main forests lie around the Cinto, Rotondo, Renoso and
Incudine mountains. These forests characterise the summer dry area of the
mountain horizon, with precipitation chiefly concentrated in the autumn and
winter. They reach their most characteristic structural physionomy between
900-1000 and 1300-1400 m, while at lower altitudes they become associated with
Pinus pinaster. At higher altitudes they reach 1600 m and colonise the top of
certain rocky ridges in scattered groups, reaching the uppermost limit of high
arboreal vegetation at 1750-1800m. Among the other trees associated with the P.
laricio are Fagus sylvatica in very damp areas (e.g. Vizzavona forest), Abies
alba in the most rugged and least accessible areas (e.g. Aitone forest) and
Betula verrucosa, which forms groupings at the upper limit of forests and
occupies open and well-lit clearings (e.g. Valdo Niello forest).
The pluviometric regime generally varies between 1000 and I300 mm yearly, with
extremes of 750-1000 mm in the lower parts of the valleys and 1200-1800 mm in
the upper, inner areas. The forests are exploited when the trees are 300 years
old (270-360), the trunk is 0.8-1 m wide and the height is 25-40 m (Debazac,
1964; Duplias et aI., 1965; Simi, 1981).
Fig. 1. The range of the Corsican nuthatch Sitta
whiteheadi overlaps almost completely with the range of Pinus
(nigra) laricio, shown in black. The Natural Regional Park is included
within the stippled line. The numbers indicate the forest localities of sample
censuses: 1. Tartagine-Melaya; 2. Carrozzica; 3. Valdo NielIo; 4. Aitone; 5.
Restonica; 6. Rospa-Sorba: 7. Ghisoni; 8. Marmano-CoI de
Verde; 9. Bavella; 10. Ospedale.
METHODS
From 1981 to 1984 we
carried out 10 strip censuses on linear transects 3000 m long (in three cases
the length was halved) and 200 m wide. Along each strip, we recorded all nests
found and every contact with territorial pairs which had the majority of their
territory within the transect. The total time needed to cover each strip was 6-8
h; about 200 h were taken for the research and 525 ha were censused. The
transects were planned so as to cover both old and pure stands of P. laricio and
less mature forestssubject to management or mixed with other tree species (Fig.
1).
RESULTS
Habitat requirements
Breeding Corsican
nuthatches were found from 800 to 1600 m above sea level (average 1260 m). We
recognised two main types of habitats, where the nuthatch densities differed.
On the other hand, the
lowest densities (0.24 pairs 10 ha-1, from 0 to 0-6) were recorded in the young
forests, in those mature stands under heavy management, or in those where the P.
laricio is mixed with P. pinaster, Fagus sylvatica or Abies alba. These forests
are usually below l000 m above sea level. Low densities were also found above c.
1500 m, where trees are scattered and often stunted, due to the harsh conditions.
No birds were found in recently burned forests (e.g. Restonica).
Fig. 2. Vegetation
profile recorded in a section of P. laricio forest (100 x l0 m) representing an example of optimal Corsican nuthatch
habitat: Average nest-to-nest distance 290 m (210-390 m); average density 1-13
pairs l0 ha-1 (0.92-1.50).
The Corsican nuthatch is believed to belong to the ‘Sittelles mésogéennes’
group, of pan-mediterranean origin, whose paleobiogeographic reconstruction was
discussed by Vielliard (1978) and by Ledant (1978) in relation to the surprising
discovery of the Kabylian nuthatch Sitta ledanti in 1975 in the Algerian Djebel
Babor.
According to Ledant (1978), the habitats of these two nuthatches are comparable
in the sense that they are found in the humid climate characteristic of the
Mediterranean mountains (annual precipitation over 2000 mm), although their
vegetational structure differs. The Kabylian nuthatch lives in the upper Djebel
Babor (maximum height 2004 m), and reaches its maximum density in a tight
Quercus-Abies woodland of 250 ha, lying above 1900 m; the species is scarcer or
irregular at lower altitudes, in the open, mixed forests, and in pure stands of
Quercus or Cedrus, totalling about 1000 ha (Vieiliard, 1980). According to
Gatter & Mattes (1979), the highest density (4 pairs 10 ha-1) is reached in
the mixed woodland of the summit area of the massif. The trees forming the main
stock are Cedrus atlantica, the endemic Abies numidica, Quercus faginea and Acer
obtusatum. The lower hill areas are more sparsely colonised (the lowest
territory was recorded at 1500 m).
Disturbance factors
and threats
Among the factors
limiting the local distribution and abundance of the Corsican nuthatch, forest
fires caused by man are certainly the most important, because fire destroys the
habitat needed by the nuthatch for decades or even centuries. In winters with
abundant snow, the vegetation is liable to erosion after snow-slips. These are
particularly detrimental because they open wide gaps in the woods and break off
many dead trunks which serve as nesting sites for the nuthatch.
Of the potential predators of the nests (small rodents, reptiles or birds), the
great spotted woodpecker Picoides major seems the most effective, as proposed by
earlier authors, and as recorded by ourselves in 1984 in the Bavella forest (a
case of predation of chicks from the nest). Breeding by the Corsican nuthatch
occurs synchronously (the eggs hatch at the end of April or in the first ten
days of May), so that the reproductive success of the entire population is
exposed to the detrimental effects of adverse weather.
The present management of the Pinus laricio forests act as a further,
man-induced limiting factor, because dead and rotten trees are felled and the
suitable habitat is thus reduced. All the nests we found were in trees 200-300
years old at the time of death (estimated from the average diameter, at chest
height, of 0.7 m in 66% of the trunks, range 0.4-1.2 m). These old trunks had
become more suitable to the nuthatches by a long process of decomposition (Fig.
3).
In some sectors of the forests, we observed a decrease of 40-60% in breeding
Corsican nuthatches, depending upon the management and exploitation of the
forests. In a 20 ha plot on the top of Valdo Niello forest, the density
decreased from 1.5 to 0.6 pairs 10 ha-1, and in the lower part of
Aitone forest the decrease was from 1.4 to 0.9 pairs. However, these decreases
cannot be extrapolated to the entire range of the species, which includes large
surfaces of non-accessible forests which are not exploited.
Fig. 3. Types of Pinus laricio trunks
used by the Corsican nuthatch for nesting. For type A (583% of the cases) and
type B (22.2%), the graphs show, on the left, the relation between trunk height
(triangle) and nest entrance height (dot). Trunks of the type C occur rarely,
and are used in 83% of cases; type D is used in 55% of cases. Other types, which
total 5.5%, are not shown. Types A and B are drawn in scale with the average
height of the trunks and of the nests (A, trunk 14.6 m, range 7-22, nest 9.7,
range 4-17: B, trunk 7.5, range 3.5-15, nest 5.6, range 2.7-10).
PROPOSALS FOR FOREST
MANAGEMENT
The above information suggests that
limited adjustment to forest management (listed below) could effectively enhance
the conservation of the Corsican nuthatch, allowing at the same time the
continued exploitation of the forests.
The upper third of the old P. laricio forests should be left unexploited. These
forests hold the highest number of dead trees suitable for nesting (15-20 trees
l0 ha-1), although natural limiting factors, like snow-slips, are severe.
The central third of the forest could be managed and exploited for wood, but
only dead trunks shorter than 3 m should be felled. Only 5.5% of the nests are
below 3 m; the average height of the nests is 10 m (1.6-26.0 m, n = 36). Trunks
higher than 3 m should be removed only in particular situations avoided by the
nuthatch (slopes exposed to snow-slips, burned areas) where there are few living
trees and many dead trunks.
The territory of one pair extends over 7-10 ha; therefore in each 10 ha of
forest at least 5-15 suitable dead trees shouìd be left, preferably evenly
dispersed. Some living pines with rotting top (type D in Fig. 3) should also be
left, because they may be used for nesting, and provide a turnover for the old
trunks which are gradually lost. Some of the dead trees of a clump may be
removed, particularly those shorter than 3 m and those that are unsteady. When
breeding pairs were dense, the average inter-nest distance was 290 m (210-390
m).
In the lower third of the forests, which is often mixed, suitable trees are
extremely scarce and none should be removed. In these forests the average
inter-nest distance is 470 m (400-500 m), and we found a relation between the
low density of breeding pairs (0.3 10 ha-1) and the scarcity of suitable dead
trees (from 0 to 4 10 ha-1).
Many details of the ecology of this interesting endemic species are still to be
studied (e.g. reproductive biology, population dynamics, feeding); however, we
believe that our preliminary proposals on forest management could ensure the
survival of the species, whose range lies almost totally within the boundaries
of the Corsican Natural Regional Park.
ACKNOWLEDGEMENTS
For information and
advice received we thank Mauro Fasola, Ugo F. Foschi, Paul Géroudet, Jean
Quiriconi, Jean-Claude Thibault and Hartmut Walter.
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