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J. Raptor Res. 44(2):159–161
E2010 The Raptor Research Foundation, Inc.
OSPREY (PANDION HALIAETUS)USE OF RHIZOLITHS IN NEST BUILDING
KEY WORDS:Osprey; Pandion haliaetus; nest material;rhizoliths.
The type of material used by Ospreys (Pandion haliaetus)
for nest building or repair appears to depend on availabil-
ity in the surrounding area. Natural material (bush or tree
branches, grasses, algae, etc.) is today complemented by
an endless range of human refuse (plastic bags, pieces of
fishing net, cloth, string, etc.), which can occasionally
prove hazardous both for broods (Poole 1989, Thibault
et al. 2001, R. Triay pers. comm.) and for adult birds
(Palma 2001). The disproportionate size of many nests
(Cramp and Simmons 1980) may be a response to bad
weather (e.g., high winds), as large nests tend to be more
stable (Poole 1989). We here describe the manner in
which some pairs of Osprey on the island of Boavista, Cape
Verde, use unconventional nesting materials not previous-
ly reported.
The island of Boavista (16u059N, 23u509W; 620 km
2
),
situated 570 km from the coast of Senegal (west Africa),
is the easternmost and one of the most arid of the islands
that make up the Cape Verde archipelago. Topography
consists primarily of stony tablelands and dune systems,
with a few somewhat rugged regions (eroded mountains
and low coastal cliffs). Although the Osprey population of
Boavista was estimated at 5–8 pairs in the 1960s (Naurois
1987), 8 pairs (11 nests detected) in 1999 (Ontiveros 2003)
and 11 pairs (25 nests detected) in 1998–2001 (Palma et al.
2004), our most recent and more systematic surveys (2005–
2007) revealed numbers of nests (both active and inactive)
and pairs that exceeded previous estimates (78 nests and
about 15 pairs; P. Lo´pez unpubl. data).
While visiting the south coast of Boavista in November
2001, we discovered an old nest on the smooth surface of a
rocky point, ca. 2–3 m above sea level. A good part of the
material still remaining consisted of rhizoliths (Fig. 1a),
the original dimensions of which, perhaps measuring
.0.5 m, had been reduced by erosion. These concretions,
which may be attributed to Tamarix (Tamaricaceae; Nørre-
vang and Hartog 1984, W. Lobin pers. comm.), are fre-
quently found in the dune systems of the island, where
they occasionally outcrop vertically and can be mistaken
for dry branches or sticks (Fig. 2). Subsequent surveys on
Boavista revealed the existence of more material of this
type in nests of other coastal areas (Ponta do Sol-Rotch-
inha NW, Abrolhal N-NE, and Varandinha W), which were
characterized, as was the region near the first nest, by the
presence of active dunes and adjacent fossil dunes. For
example, six of the twelve active and old nests known at
Ponta do Sol had rhizoliths incorporated into their struc-
ture. The masses and lengths (mean 6SD) of 50 rhizoliths
from one of these nests were respectively 185 6100.9 g
(range 37–437 g) and 195.6 656.9 mm (range 114–
353 mm).
Although it is likely that the appearance of the concre-
tions, simulating dry sticks and branches, is the sole
reason Ospreys collect them, the use of this material
reinforces the nest. Even if the rest of nest material van-
ishes, the rigidity of these rhizoliths ensures their durabil-
ity over time, and such mounds may provide a stimulus for
future Osprey pairs when nest building (Fig. 1b). More-
over, thanks to the presence of these characteristic re-
mains accumulated on cliff ledges, coastal platforms,
etc., breeding sites can be identified, thus providing a
means by which to estimate past population size and dis-
tribution.
We would like to thank Pedro Felipe (Alas Cinematogra-
fı´a S.L.) for facilitating the journey to Cape Verde in 2001
and Rube´n Barone for his constant assistance. Thanks are
also due to Manuel Siverio, Luı´s Palma, and Rafel Triay for
their helpful review of the initial manuscript and to Pau-
line Agnew for translating the original into English. Cheryl
Dykstra and three anonymous reviewers also made useful
comments.—Felipe Siverio (e-mail address: felipe.siverio@
telefonica.net), c/ Los Barros 21, E-38410 Los Realejos, Te-
nerife, Canary Islands, Spain; Pedro Lo´pez, C.P. 100, Sal
Rei, Boavista, Republic of Cape Verde; and Domingo Tru-
jillo, c/ Jose´ Betancor Cabrera 20, 38410 Los Realejos, Ca-
nary Islands, Spain.
LITERATURE CITED
CRAMP,S.AND K.E.L. SIMMONS.[EDS.]. 1980. The birds of
the western palearctic. Vol. 2. Oxford Univ. Press, Ox-
ford, U.K.
NAUROIS, R. de. 1987. Le Balbuzard (Pandion haliaetus) aux
ıˆles du Cap Vert. Annali del Museo Civico di Storia Natur-
ale 86:143–218.
NØRREVANG,A.AND J.C. HARTOG. 1984. Bird observations in
the Cape Verde Islands (4–22 June 1982). Courier For-
schungsinstitut Senckenberg 68:107–134.
ONTIVEROS, D. 2003. Nesting distribution, food habits, and
conservation of Osprey on Boavista Island (Archipela-
go of Cape Verde). Journal of Raptor Research 37:67–70.
PALMA, L. 2001. The Osprey Pandion haliaetus on the Por-
tuguese coast: past, present and recovery potential. Vo-
gelwelt 122:179–190.
———, J. FERREIRA,R.CANGARATO,AND P. VAZ PINTO 2004.
Current status of the Osprey in the Cape Verde Islands.
Journal of Raptor Research 38:141–147.
JUNE 2010 LETTERS 159
Figure 1. Osprey (Pandion haliaetus) nest on a coastal platform in south Boavista, Cape Verde: (a) probably inactive for
several years, the nest would have been completely dismantled had it not been for abundant rhizoliths and some plant
remains (November 2001), and (b) new construction over the layer of rhizoliths between December 2002 and May 2003
(photograph taken in January 2006). Photos by P. Lo´pez.
160 LETTERS VOL. 44, NO.2
POOLE, A.F. 1989. Ospreys: a natural and unnatural history.
Cambridge University Press, Cambridge, U.K.
THIBAULT, J.-C., V. BRETAGNOLLE,AND J.-M. DOMINICI. 2001.
Le Balbuzard peˆcheur en Corse. Du martyre au
symbole de la protection de la nature. E
´ditions Alain Piaz-
zola, Ajaccio, France.
Received 12 August 2009; accepted 15 February 2010
Figure 2. Rhizoliths outcropping in fossil dunes, northwestern Boavista, Cape Verde. Photo by P. Lo´pez.
JUNE 2010 LETTERS 161
