Response to conspecific playback calls, distribution and habitat od Tyto Alba (Aves: Tytonidae) on la Gomera, Canary Islads

              Rev. Acad. Canar. Cienc, XI (Nums. 3-4), 213-222 (1999)
RESPONSE TO CONSPECIFIC PLAYBACK CALLS, DISTRIBUTION AND
HABITAT OF TYTOALBA (AVES: TYTONIDAE) ON LA GOMERA,
CANARY ISLANDS
F. Siverio*, R. Barone, M. Siverio, D. Trujillo & J. J. Ramos
*Ctra. General 20, San Vicente, E-38410 Los Realejos, Tenerife, Canary Islands
RESUMEN
En este trabajo se describe la aplicacion del metodo del playback para la
Lechuza Comun (Tyto alba) en La Gomera, se discute el resultado alcanzado y se dan a
conocer varias detecciones adicionales, permitiendo ofrecer una information preliminar
sobre la situation de esta especie en la isla. Los escasos contactos de T alba (n= 3; al
menos dos territorios) mediante las estaciones con playback (n= 65), reflejan el caracter
aleatorio a la hora de obtener respuestas asociado a la rarefaction de esta rapaz; aun asi,
la tecnica posibilita oir lechuzas que, debido a su localization, pasarian desapercibidas
si no son estimuladas. Hay un total de 14 registros (1993-1999) repartidos de forma
desigual por todas las vertientes de la isla, en altitudes comprendidas entre 10 y 900 m
(mediana, 95). La mayoria proceden de barrancos profundos (piso basal/termofilos), con
cauces anchos y parcialmente antropizados, demostrando todavia mas la relegation de
la especie. Aunque nuestras observaciones sugieren una cierta expansion a raiz de una
colonization relativamente reciente, la accidentada geomorfologia insular parece
constituir una barrera en este proceso.
Palabras clave: Respuesta al playback, distribution, habitat, Tyto alba, La
Gomera, islas Canarias.
ABSTRACT
In this paper the application of conspecific playback calls for the detection of
the Barn Owl (Tyto alba) on La Gomera is described, and the corresponding results are
discussed in addition to presenting several supplementary observations, all of which
allow a preliminary analysis of the species status on the island to be made. The very low
number of contacts of T. alba (n= 3; at least two territories) obtained from the playback
stations (n= 65) reflects the random character of the responses as a result of the rarity of
this bird of prey; in spite of this fact, the technique facilitates the audio-detection of barn
owls which otherwise, owing to their localization, would pass unnoticed if not
stimulated. During the period 1993-1999, a total of 14 registers have been accumulated,
213
distributed unevenly throughout the whole island at altitudes varying from 10 to 900 m
(median 95). The majority were recorded in partially humanized deep wide-bedded
ravines of the lower xerophytic and transitional vegetation zones. Although the
observations tend to indicate a slight expansion in range as a result of a relatively recent
colonization, the steep relief of the island appears to represent an impeding barrier.
Key words : Playback response, distribution, habitat, Tyto alba, La Gomera,
Canary Islands.
1. INTRODUCTION
With the detection of the Barn Owl {Tyto alba) on La Gomera in 1993 (v.
SIVERIO [15]), the species presumed absence from the island was refuted. Although
the playback technique has hardly been used with this species (v. e.g. ZUBEROGOITIA
& CAMPOS [20]), it has been stated that the excellent results obtained appear to be
associated with the increased territoriality shown in areas of high population density
(SORACE [18]; MORIMANDO et ai [13]), and thus, its use in regions where the
species is apparently very rare would, a priori, seems to be insubstantial. In the present
note, the application of the mentioned technique is described for T. alba on La Gomera,
together with a discussion of the corresponding results. Furthermore, details of
additional records are also provided.
2. STUDY AREA
The island of La Gomera (28° 06' N, 17° 12' W; 378.75 km2
) posesses a very
abrupt relief characterized by deep ravines and a highly precipitous coastline. The
zonification of the vegetation commences with xerophytic scrub in the coastal belt and
finalizes with dense formations of evergreen forest (laurisilva and tree-heath ["fayal-brezal"])
in the central summit region (max. altitude Garajonay, 1487 m). The majority
of these ecosystems have been partially modified due to the implantation of terraced
cultivations (coastal and mid-altitudinal zones) and human settlements (especially in
valley interiors and the mouth of certain ravines/watercourses ["barrancos"]). The
nearest island, Tenerife, is 25 km distant.
214
3. MATERIAL AND METHODS
During the course of annual visits (one to three from 1985 to 1997), visual and
auditive nocturnal explorations were carried out in practically all parts of the island with
the aim of detecting T. alba or for other purposes. Furthermore, in 1997 (28th June- 1st
July) and in 1998 (24th-26th April) the playback method was employed (v. GALEOTTI
[7]) utilizing a conspecific territorial call, "the screech" (BUHLER & EPPLE [3];
BUNN et al. [2]; etc.), reproduced on a conventional tape-recorder (10W). In both
years, playback emissions commenced at dusk (20.30 and 20.00 hr solar time,
respectively) in accordance with the initiation of activity on the neighbouring island of
Tenerife during June-July (X= 20.28 ± 0.05 SD; n= 16 registers) and April (x= 19.57 ±
0.03; n= 16) (pers. obs.) and finalized at variable times after midnight (max. 03.57-
04.12 hr). Sampling periods lasted 15 minutes and were divided into two consecutive
intervals consisting of 2.5 minutes of stimulation followed by 5 minutes of silence.
The altitudinal distribution of the sampling stations ranged from 10 to 1050 m
(median 175; n= 65). The sites (ravines, cliffs, mountain slopes, etc.) were selected on
the basis of the species habitat preferences in the Canary Islands (MARTIN [10];
DELGADO et al [6]; SIVERIO & CARRILLO [16]), and their precise location (Fig. 1)
was determined with the aid of a GPS ("Global Positioning System") and a UTM
proyection 1: 25000 scale map. As regards to specific itineraries (e.g. following the
course of ravines), the average distance between stations was 922.58 ± 281.32 m (range
500-1500; w=31).
The method was complemented by diurnal prospections and interviewing local
inhabitants in several areas as well as utilizing information obtained by various
naturalists during 1993-1999.
4. RESULTS AND DISCUSSION
From 1985 onwards, all attempts to detect T. alba were fruitless until 7th
February 1997 when, a bird showing plumage characteristics attributable to T. a. alba,
was observed near the top of a cliff close to Playa Santiago ( Alajero) in the south of the
215
island (Fig. 1); however, there are earlier records from the N, W and E (Appendix).
The results of the playback stimulations carried out in 1997 (n= 50) and 1998
(/?= 15), only produced three responses (4.6%) during the first year (Fig. 1): 28-VI,
barranco de Santiago (San Sabastian/Alajero), 30-VI, barranco del Valle
(Vallehermoso) and 30-VI, Valle Abajo (Vallehermoso). The last two contacts could
well correspond to the same individual (or members of the same pair), judging by the
distance (1000 m) and the time elapsed (42 min.) between them. Behavioural responses
were not recorded (v. SORACE [18]), only territorial calls (once or twice) (v. BUNN et
al. [2]) emitted, at less than 200 m, during the first (n= 1) or second (n= 4) silence
period, though one cannot rule out that they may have passed unnoticed during the
stimulation intervals.
Although this data points to at least two breeding territories, the results of both
diurnal searches and interviews with local inhabitants turned out to be totally negative
at these and other localities.
Territory size in the Barn Owl (v. MICHELAT & GIRAUDOUX [11] and cited
references) appears to be dependant upon food availability (MIKKOLA [12]; CRAMP
[4]) and, although the species is not markedly territorial (SHAWYER [14]; MARTI [9];
TAYLOR [19]; etc.), it could occasionally be limited by intraspecific competition
(BUNN et al. [2]). On the assumption that the pairs are extremely localized on La
Gomera, one would expect the birds to hold large territories, a fact which together with
their rarity, would account for the very low number of contacts obtained employing
conspecific playback calls. Consequently, reply responses will be random and subjected
to the presence or absence of the species in the sector where the stimulation is
performed, provided that it coincides with a territory. Contrary to what has been
confirmed by ZUBEROGOITIA & CAMPOS [20] in the north of the Iberian
Penninsula, the results of the present study have not been influenced by the weather
conditions. All sampling was carried out under very mild temperatures (x= 19.0 ± 2.5
°C; n= 47 [April, June & July]) and calm conditions or, at the most, in the presence of a
light breeze. Although the way in which the method has been applied does not allow
an evaluation of its efficiency to be made (v. GALEOTTI [7]; ZUBEROGOITIA &
216
CAMPOS [20]), it has clearly aided in the detection of barn owls that under normal
circumstances (without being stimulated), would otherwise be very difficult to detect
due to their reduced vocal activity product of the very low population density (v. BUNN
et al. [2] for other regions). This assertion does not appear to completely coincide with
what has been suggested by SORACE [18].
The only specimens examined from La Gomera originate from barranco de la
Villa (June 14th 1997) (P. Romero & V. Quilis, pers. comm.) and La Lomada (June
1999) (J. J. Hervas & J. Abad, pers. comm.), both localities situated in San Sabastian,
and are deposited in the Natural History Museum of Tenerife (TFMC VA: 444 [max.
age, 2nd calender year] and 445 [min. age, 4th calender year]). They correspond to the
typical subspecies (Fig. 2) which also inhabits Gran Canaria, Tenerife, El Hierro
(MARTIN [10]; DELGADO et al. [6]) and probably La Palma (D. Conception, pers.
comm.). JAUME et al. [8] assign the fossil remains found on Gran Canaria (v.
ALCOVER & FLORIT[l]) and in the south of La Gomera to T. (a.) gracilirostris
(actually endemic to Fuerteventura, Lanzarote and adjacent islets). Furthermore, the
mentioned authors suggest that after having analysed the fossil and present day
distributions of both subspecies, the nominal form has replaced the endemic one on
various islands. If our data from La Gomera are indicative of a relatively recent
colonization by the present population, this fact naturally presupposes that its
establishment and the extinction of T. (a.) gracilirostris (JAUME et al. [8]) occurred
asynchronously.
The altitudinal distribution of all the contacts considered varied from 10 (San
Sabastian) to 900 m (La Cerpa) (median 95; n- 14), the majority having been made in
ravines within the lower xerophytic and transitional vegetation zones. In contrast to
what has been observed on Tenerife (SIVERIO & CARRILLO [16]; SIVERIO [15]),
the bed of these ravines is relatively wide -with cultivations, small human dwellings,
palm groves of Phoenix canadensis, etc.- and bordered by high rocky margins (e.g. >
300 m in barranco de Santiago). The occupation of this type of ravines could well be
limited to the lower parts of the margins where numerous potential breeding cavities are
217
to be found close to the dry river-bed (pers. obs.), which constitutes the only ideal
hunting habitat (Fig. 3), judging by its requirements in other latitudes (SMITH &
MARTI [17]; BUNN et al. [2]; TAYLOR [19]; etc.). The species presence at 900 m
a.s.l. corresponded to a humid area (ravine with tree-heath and cultivations) and may
constitute the upper limit to its distribution range, bearing in mind that at higher
elevations the laurel forest appears (Garajonay National Park). Here the species is
totally absent (pers. obs.), a fact which once again, serves to corroborate the species
indifference towards this (SIVERIO [15]) and other types of forest biotopes (CRAMP
[4]; MARTI [9]; TAYLOR [19]; etc.).
The scarcity of T. alba on La Gomera (perhaps less than 15 pairs) could well be
related to the restrictive role exercised by the abrupt orography in the establishment and,
perhaps, in the dispersion of individuals, being probable that earlier colonizations (v.
JAUME et al. [8]) never prospered. On Tenerife, SIVERIO [15] has shown that
despite the fact that the species can be considered to be common (min. 161 pairs), it is
locally rare in the Teno and Anaga massifs, two areas with geomorphological
characteristics that bear a close resemblance to those of La Gomera.
5. ACKNOWLEDGEMENTS
Dr. Aurelio Martin kindly revised and criticized the original manuscript and
Guillermo Delgado facilitated the examination of the specimens in the Tenerife Natural
History Museum. Angel Fernandez (Director of Garajonay National Park) allowed us to
use the installations at the Juego de Bolas Visitor's Centre during the course of our
visits to the island. Finally, we must acknowledge the help received at all times from
Jacinto Leralta. The present study was carried out without any financial support.
6.BIBLIOGRAPHY
[1] ALCOVER, J. A. & X. FLORIT (1989): Els ocells del jaciment arqueologic de La
Aldea, Gran Canaria. Butll Inst. Cat. Hist. Nat. 56 (Sec. Geol., 5): 47-55.
[2] BUNN, D. S., A. S. WARBURTON & R. D. S. WILSON (1982): The Barn Owl. T
& AD Poyser. Calton.
[3] BUHLER, P. & W. EPPLE (1980): Die Lautauferungen der Schleireule (Tyto alba).
J. Orn. 121:36-70.
[4] CRAMP, S. (ed.) (1985): The Birds of the Western Palearctic. Vol. IV. Oxford
University Press. Oxford.
218
[5] DELGADO, G., J. CARRILLO & D. TRUJILLO (1992): Sobre la presencia y
distribucion de la Lechuza Comun (Tyto alba) (Scopoli, 1769) en las islas
orientates de Archipielago Canario. Vieraea 21: 145-148.
[6] DELGADO, G., J. CARRILLO, E. HERNANDEZ, N. TRUJILLO, F. SANTANA,
V. QUILIS, M. NOGALES, O. TRUJILLO & K. EMMERSON (1988): Censo de
las Aves Rapaces del Archipielago Canario. Museo Insular de Ciencias Naturales
de Santa Cruz de Tenerife. Informe no publicado.
[7] GALEOTTI, P. (1991): Tavola rotonda: metodi di censimento per gli strigiformi.
Suppl Ric. Biol. Selv. 16: 437-445.
[8] JAUME, D., M. McMINN & J. A. ALCOVER (1993): Fossil birds from the Bujero
del Silo, La Gomera (Canary Islands), with a description of a new species of Quail
(Galliformes: Phasianidae). Bol. Mus. Mun. Funchal, Sup. No. 2: 147-165.
[9] MARTI, C. D. (1992): Barn Owl. In The Birds of North America, No. 1 (A. Poole,
P. Stettenheim & F. Gill, Eds.). The Academy of Natural Sciences, Philadelphia;
The American Ornithologists' Union.
[10] MARTIN, A. (1987): Atlas de las aves nidificantes en la isla de Tenerife. Instituto
de Estudios Canarios, Monografia XXXII. Tenerife.
[11] MICHELAT, D. & P. GIRAUDOUX (1991): Dimension du domaine vital de la
Chouette effraie Tyto alba pendant la nidification. Alauda 59 (3): 137-142.
[12] MIKKOLA, H. (1983): Owls of Europe. T & A D Poyser. Calton.
[13] MORIMANDO, F., F. PEZZO & A. DRAGHI (1995): II censimento degli
Strigiformi nella citta di Siena: dati preliminari. Avocetta 19: 117.
[14] SHAWYER, C. R. (1987): The Barn Owl In The British Isles: Its Past, Present and
Future. The Hawk Trust. London.
[15] SIVERIO, F. (1998): Distribucion y estatus de Tyto alba (Scopoli, 1769) en
Tenerife, islas Canarias (Aves, Tytonidae). Vieraea 26 (1997): 121-131.
[16] SIVERIO, F. & J. CARRILLO (1993): Habitat y reproduction de la Lechuza
Comun (Tyto alba) en Tenerife (Islas Canarias). Contribution a su estudio. Alytes
6:231-240.
[17] SMITH, D. G. & C. D. MARTI (1976): Distributional status and ecology of Barn
Owls in Utah. Raptor Research 10 (2): 33-44.
[18] SORACE, A. (1987): Note sul canto territoriale del Barbagianni, Tyto alba. Riv.
ital. Orn. 57 (1-2): 144-145.
219
[19] TAYLOR, I. (1994): Barn Owls: predator-prey relationships and conservation.
Cambridge University Press. Cambridge.
[20] ZUBEROGOITIA, I. & L. F. CAMPOS (1998): Censusing owls in large areas: a
comparison between methods. Ardeola 45 (1): 47-53.
Appendix
Localities, UTM coordinates (lxl km) and dates of supplementary detections of the
Barn Owl (Tyto alba) on the island of La Gomera. All records refer to individual birds
observed or heard fortuitously.
Locality UTM Month/year Source
Barranco de Monteforte BS8517 3/1993 Siverio, 1998; E. Hernandez
(Hermigua) & G. Delgado, in litt.
BS7008 8/1993 P. Romero, pers. comm.
Vueltas (Valle Gran Rey) BS9208 1995 A. Valido, pers. comm.
Public square of San Sebastian BS8219 1/1996 J.G. Hernandez, pers.
Barranco de las Rosas (Agulo) BS9208 4/1996 comm.
San Sebastian BS9109 5/1996 A. Garcia, pers. comm.
Barranco de la Villa (S. Sebastian) BS8117 5/1998 A. Valido, pers. comm.
La Cerpa (Agulo) BS6910 10/1999 J.G. Hernandez, pers.
La Playa Calera (Valle Gran Rey) comm.
220
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Figure 1. Geographical distribution of the playback stations used for the detection of the
Barn Owl {Tyto alba) during 1997-1998 (0= no response; •= positive reponse) and
supplementary registers of the species made between 1993-1999 (A)
on the island ofLa Gomera.
221
Figure 2. Variation in the degree of underbody coloration and flecking in the specimens
of the Barn Owl (Tyto alba) from La Gomera (marked with G), El Hierro and Tenerife
deposited in TFMC (Photo: S. Socorro).
Figure 3. View of barranco de Santiago (San Sebastian/Alajero). In the deep ravines
where the Barn Owl (Tyto alba) has been detected on La Gomera, the lower parts of the
margins and the broad bed of the dry watercourses constitute the species most suitable
breeding and hunting habitats (Photo: M. Siverio).
222