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DAILY SURVIVAL RATES OF EGGS IN ARTIFICIAL GROUND
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AND SHRUB BIRD NESTS ON SMALL ADRIATIC ISLANDS
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TASAS DIARIAS DE SUPERVIVENCIA DE HUEVOS DE AVES EN
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NIDOS ARTIFICIALES EN EL SUELO Y ARBUSTOS EN PEQUEÑAS
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ISLAS DEL ADRIÁTICO
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Jenő J. PURGER1*, Eduard KLETEČKI2, Balázs TRÓCSÁNYI3, Jasmina MUŽINIĆ4, Gabriella L.
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SZÉLES5 and József LANSZKI5
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Short title: DAILY SURVIVAL RATES OF EGGS IN ARTIFICIAL NESTS
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Hungary
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2
Croatian Natural History Museum, Demetrova 1, 10000 Zagreb, Croatia
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Duna-Drava National Park Directorate, Tettye tér 9, 7625 Pécs, Hungary
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Department of Ornithology CASA, Gundulićeva 24, 10000 Zagreb, Croatia
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Department of Nature Conservation, University of Kaposvár, P.O. Box 16. 7401 Kaposvár,
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Hungary
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* Corresponding author: purger@gamma.ttk.pte.hu
Department of Ecology, Institute of Biology, University of Pécs, Ifjúság útja 6, 7624 Pécs,
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SUMMARY.—Predation may cause important losses to the biodiversity of small islands.
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In this study we analysed the predation pressure on clutches of nesting birds in two islands of
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the Zadar archipelago, Olib and Silba, which belong to an Important Bird Area in Croatia. We
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paid special attention to introduced mammals: black rats Rattus rattus and feral domestic cats
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Felis silvestris catus. In May 2009, we carried out a study with artificial ground and bush
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nests. One quail egg and a plasticine egg of similar size were placed in each nest. On Olib, the
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daily survival rate of quail eggs in ground nests (0.92) was similar to that of nests located in
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shrubs (0.93), whereas on Silba the daily survival rates of quail eggs in ground (0.94) and
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shrub nests (0.86) were significantly different. We used the marks left on the plasticine eggs
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to identify nest predators. Many eggs in shrub nests (40% on Olib and 70% on Silba) were
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predated by hooded crows Corvus cornix. In ground nests on Olib, 43% of eggs were
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predated by hooded crows and small mammals, mainly black rats. On Silba Island, eggs in
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33% of the ground nests were damaged by the above predators and also by the Montpellier
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snake Malpolon monspessulanus. Contrary to our expectations, feral cats were not found to be
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egg predators on either of the islands.
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Key words: artificial nests, introduced mammals, predation, survival.
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RESUMEN.—La depredación puede causar pérdidas importantes de biodiversidad en
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pequeñas islas. En este estudio analizamos la presión de depredación en dos islas del
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archipiélago de Zadar, Olib y Silba, que están catalogadas como Áreas Importantes para las
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Aves en Croacia. Hemos prestado atención especial a las ratas Rattus rattus y gatos
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domésticos asilvestrados Felis silvestris catus. En mayo de 2009 realizamos un estudio
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utilizando nidos artificiales en el suelo y en arbustos. En cada nido se colocó un huevo de
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codorniz y otro de plastilina de tamaño similar. En Olib la tasa supervivencia diaria de los
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huevos de codorniz en nidos en el suelo (0,92) fue similar que la de los nidos en arbustos
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(0,93), mientras que en Silba la supervivencia de los huevos en el suelo (0,94) y arbustos
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(0,86) difirió significativamente. Para identificar a los predadores de los nidos utilizamos las
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marcas dejadas en los huevos de plastilina. La mayoría de los huevos depredados en arbustos
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(40% en Olib y 70% en Silba) lo fueron por cornejas cenicientas Corvus cornix. En los nidos
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en el suelo en Olib el 43% de los huevos fueron depredados por cornejas y pequeños
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mamíferos (principalmente ratas). En Silba el 33% de los los huevos en nidos en el suelo
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fueron dañados por los depredadores mencionados y también por culebras bastardas Malpolon
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monspessulanus. En contra de lo que esperábamos, en ninguna de las islas registramos
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depredación de los gatos sobre los huevos.
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Palabras clave: depredación, mamíferos introducidos, nidos artificiales,
supervivencia.
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In general, populations of plants and animals on small islands are limited and show
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restricted genetic diversity and narrow distribution ranges (Blondel, 1995; Underwood et al.,
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2009). Hence even minor environmental changes can considerably affect their persistence
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(Whittaker and Fernández-Palacios, 2007). The two major causes of the decline of island
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species are habitat degradation and the introduction of non-native predators. Data on
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predation rates is highly important for determining the most suitable management techniques
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for preserving island biodiversity. In this study we analysed the predation pressure on clutches
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of nesting birds on two islands of the Zadar archipelago, an Important Bird Area in Croatia.
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The most widespread introduced mammalian predators on the Adriatic islands are
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black rats Rattus rattus and feral domestic cats Felis silvestris catus but their effects on the
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island fauna are not yet well documented (Barun et al., 2010; Purger et al., 2011). The black
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rat is known to be an egg predator (Martin et al., 2000; Prieto et al., 2003; Delgado-García et
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al., 2005) and its populations may be regulated by free-ranging feral cats, which are also
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regarded as nest predators (Bonnaud et al., 2011). Cats living on islands often take adult or
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young birds as prey (Nogales and Medina, 2009; Bonnaud et al., 2011; Purger et al., 2012)
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but egg consumption by predators is more difficult to prove (Reynolds and Aebischer, 1991).
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Nest predation studies, using artificial bird nests and eggs (Major and Kendal, 1996), are
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useful for detecting the effects of introduced mammal species or other predators on native
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birds. Such methods are widely used due to their simplicity (Moore and Robinson, 2004) and
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they offer additional advantages, such as potentially large sample sizes, the simultaneous
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assessment of all nests, and the non-disturbance of real nests with incubating birds (Major and
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Kendal, 1996). However, the results of studies with artificial nests may be useful to predict
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the predation risk to real nests only when the predator species are the same for the two nest
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types (Pärt and Wretenberg, 2002).
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Our study aimed to explore the survival rates of quail eggs in artificial nests located on
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the ground and on shrubs on two Adriatic islands. Imprints left on plasticine eggs serve to
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identify egg predators and enable the analysis of the role of introduced mammals as potential
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nest predators in damaging the clutches of native birds. This study tested egg survival of
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potential ground-nesting and shrub-nesting birds in general, rather of particular bird species.
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The northern part of the Zadar archipelago, between the island of Lošinj and Dugi
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otok is an Important Bird Area in Croatia (Radović et al., 2005). The island of Olib is 9.5 km
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long and up to 5.8 km wide, with a total area of 26.14 km2. Olib is the only settlement and
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harbour, existing since Roman times (Magaš and Faričić, 2002). The human population is
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low, with only 140 inhabitants in 2011 (Buršić, 2013). The island of Silba is smaller (14.27
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km2), located about 1.8 km west of Olib (Duplančić Leder et al., 2004). Its only settlement,
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Silba, had 292 inhabitants in 2011 (Buršić, 2013). The natural vegetation on both islands
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comprises Mediterranean forests of pubescent oak Quercus pubescens and holm oak Q. ilex
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(Horvat et al., 1974). Traditional horticulture is practised within the villages. The outer zones
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of the islands include olive groves and abandoned fields covered with different successional
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stages of the oak forests. Our study was performed on the village outskirts, where the most
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suitable habitat is available for birds nesting on ground (e.g. pheasants, nightjars and larks)
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and on shrubs (e.g. doves, shrikes, warblers, thrushes, flycatchers, finches and buntings).
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Among mammals that are possible egg predators, the northern white-breasted hedgehog
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Erinaceus roumanicus, black rat, wood mouse Apodemus sylvaticus and feral domestic cat are
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present on the islands (Tvrtković et al., 2013). Among avian egg predators, the hooded crow
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Corvus cornix is abundant, with an estimated population of 300-400 individuals on Silba in
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2006 (Mužinić and Purger, 2013). Crows have long been numerous on both islands
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(Maštrović, 1942; Mužinić and Purger, 2013). During our study in spring 2009, the crow
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population numbered at least 300 individuals on both islands (our unpublished data). The
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scattered occurrence of the jay Garrulus glandarius has been documented on Silba (Mužinić
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and Purger, 2013), but not on Olib.
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Groups of artificial ground and shrub nests were created in abandoned farmlands and
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gardens at six locations on the outskirts of settlements on each island. Nests were placed in
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sites as similar to natural locations as possible. One quail egg and a plasticine egg of similar
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size were placed in each nest. The quail (real) eggs were for the evaluation of nest predation
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rates, and the plasticine (artificial) eggs for predator identification from tooth and bill imprints
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(Niehaus et al., 2003). Artificial ground nests were formed by creating a heel depression in
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the soil (Fenske-Crawford and Niemi, 1997). The artificial bush nests were cup shapes (15 cm
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in diameter and 5 cm deep) made from fine wire mesh, attached to the foliage using wires and
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lined with grass and leaf litter (Bayne and Hobbson, 1999). Both egg types were stored
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outdoors for one week prior to the study to eliminate unnatural odours. Similarly, we
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thoroughly rubbed our hands with leaf litter taken from the ground before creating the
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artificial nests and positioning the eggs, to suppress human odours (Báldi, 1999). At all six
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localities we placed five ground nests and five shrub nests alternately, 20 m apart: hence nests
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of the same type were 40 m apart. In total 30 ground nests and 30 bush nests were placed on
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each of the two islands.
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The study started on 19May 2009 and nest contents were checked one day (20 May),
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four days (23 May) and seven days (26 May) after placement. The duration of similar nest
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studies is usually seven days (e.g. Wilcove 1985, Purger et al., 2011), since predation cases
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within this period are often sufficient for calculating daily survival rates. Artificial nests were
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considered to be predated if the quail egg had disappeared or had been damaged in some way
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(Bayne and Hobbson, 1999). Nest predators were identified from the marks left by them on
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the plasticine eggs (Major, 1991). We used Mayfield's (1975) method for estimating the daily
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survival of a sample of nests or eggs using exposure days (the cumulative number of days that
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the nests in the sample were monitored) and the number of known losses. Mayfield estimated
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daily nest survival as 1  (number of nest losses / total exposure days). The daily nest or egg
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survival rate is the probability that a nest or egg will survive a single day. These values are
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comparable using the test proposed by Johnson (1979). For comparing the proportions of
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plasticine eggs left intact, taken away (disappeared) or marked by different predators
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(mammals, birds, snakes) 4 × 2 and 5 × 2 contingency tables were used (Zar, 1999). A
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minimum tail probability level of P < 0.05 (i.e. Type I error rate of 5%) was accepted for all
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the statistical tests, and all Pvalues were two-tailed.
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On Olib island predators damaged quail eggs in 43% of the artificial ground nests and
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40% of shrub nests. The daily survival rate of quail eggs in ground nests was 0.92 (95%
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confidence limits: 87.82  96.28), similar (Z = 0.232 P = 0.847) to eggs in shrub nests at 0.93
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(95% c. l.: 88.68  96.77) (fig. 1). The results obtained on Silba were somewhat different,
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quail eggs having been predated in only 33% of ground nests and 70% of shrub nests. The
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daily survival rates of quail eggs in ground nests on Silba was 0.94 (95% c. l.: 90.83  97.81)
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which was significantly higher (Z = 2.466 P = 0.014) than in shrub nests 0.86 (95% c. l.:
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80.59  91.78) (fig. 1). The daily survival rates of quail eggs in ground nests on Olib and
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Silba were similar (Z = 0.827 P = 0.408) and the differences in daily survival rates of quail
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eggs in shrub nests between the two islands were not significant (Z = 1.895 P = 0.058) (fig.
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1).
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Predators managed to break only four and two quail eggs in ground nests on Olib and
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Silba respectively, while the remaining quail eggs in shrub nests on both islands were left
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intact. The number of quail eggs that disappeared from shrub nests on Olib (40%, n = 12) and
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Silba (70%, n = 21) was higher than the number of eggs that disappeared from ground nests,
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30% (n = 9) and 27% (n = 8), respectively.
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The proportions of plasticine eggs left intact, taken away or marked by different
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predators were significantly different between ground- and shrub nests on the islands: Olib (χ2
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= 23.13, df = 3, P < 0.001) and Silba (χ2 = 21.30, df = 4, P < 0.001) (fig. 2). According to
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marks left on plasticine eggs in ground nests, the egg predator communities differed between
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the two islands (χ2 = 18.38, df = 4, P < 0.001) (fig. 2). On Olib, marks on plasticine eggs left
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by small mammals dominated (n = 18), with only two cases of beak marks, originating from
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hooded crows. On Silba, however, plasticine eggs had marks from small birds (n = 2) and
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small mammals (n = 7), and there were five instances of snake bite impressions (fig. 2). On
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both islands, the small mammal marks on plasticine eggs in ground nests corresponded to
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tooth marks of black rats, suggesting that these introduced animals may have damaged at least
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17% of eggs in ground nests on both Olib and Silba.
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Based on the marks found on plasticine eggs, the predator communities robbing the
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shrub nests in the two islands were similar (χ2 = 1.37, df = 3, P > 0.05) (fig. 2). Most of the
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plasticine eggs disappeared from shrub nests but some of the remaining ones showed the beak
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marks of large birds, probably hooded crows (fig. 2). Small mammal tooth marks on
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plasticine eggs in shrub nests were recorded in one case on Olib and also in one case on Silba.
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Based on the size of these tooth marks, the predator could be the wood mouse, common on
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both islands (fig. 2).
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Different nesting sites (i.e. ground and shrub nests) are probably vulnerable to
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different predators due to differences in predator activity, search tactics and sensory inputs
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(Ricklefs, 1989; Rangen et al., 2000). As in our study, a previous review of predation rates on
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artificial ground and shrub nests in the temperate zone (Söderström et al., 1998) also indicated
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that shrub nests were subject to higher predation rates. Our results indicate that the most
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important predators of eggs in shrub nests on both islands were birds. Avian predators mainly
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use visual cues to search for food from the air (Rangen et al., 2000) and they can easily access
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the contents of nests located both on the ground and in the shrubs. Crows typically remove
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entire eggs (e.g., Olsen and Schmidt, 2004). On these islands crows commonly crush the
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stolen eggs on hard substrates before eating them (Purger et al., 2011). This behaviour
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probably explains why we did not find any broken eggs in the shrub nests.
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Although the daily survival rates of quail eggs in ground nests were similar on Olib
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and Silba, marks left on plasticine eggs suggested that the abundance or activity of small
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mammals in the study areas on Silba was less than on Olib. Plasticine eggs may indicate the
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presence of certain predator species but their presence may not necessarily indicate their real
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influence on egg predation rates (Fulton and Ford, 2003; Maier and DeGraaf, 2001). The
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primary predators of ground nests on Olib, as revealed by tooth marks on plasticine eggs,
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were wood mice and black rats, both commonly found all around the island. These small
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mammals can easily damage the eggs of small birds (e.g. larks). Black rats, as well as avian
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predators, are also able to break larger eggs (Latorre et al., 2013). The damage done by
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introduced rats to island biotas is globally recognised (e.g. Delgado-García et al., 2005; Igual
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et al., 2007; Jones et al., 2008). Predation by snakes and crows was probably more
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pronounced on Silba than Olib Island. Snakes are well known for their predation on bird eggs
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(e.g. Weatherhead and Blouin-Demers, 2004). The Montpellier snake Malpolon
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monspessulanus is present on Silba (Kletečki and Kuljerić, 2013) and thus is possibly an
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important predator of ground nests. Similar bite marks have also been found on plasticine
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eggs on ground nests in a study in the island of Šolta (Purger et al., 2011).
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Overall, the daily survival rates of quail eggs and the marks on plasticine eggs together
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suggest that shrub nests were robbed almost exclusively by hooded crows, but that the eggs
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located in ground nests were accessible to more types of predators. We found no indication of
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predation by domestic feral cats. Studies on other islands have shown that free-ranging
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domestic cats are responsible for a substantial degree of predation on birds (e.g. Bonnaud et
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al., 2007; Nogales and Medina, 2009; Rayner et al., 2007). Cats could possibly have
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contributed to egg losses in the 'unknown predator' category (missing eggs) in addition to
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other predators, but their involvement was found to be much more limited than expected. Egg
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predation by cats and other potential predators could be proved by using trail cameras.
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There are at least 1,246 small land masses in the coastal waters of the Republic of
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Croatia: 79 islands, 525 islets and 642 rocks (Duplančić Leder et al., 2004). Most of the
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islands lack the larger natural predators, such as the stone marten Martes foina, but black rats
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are present on almost all of them. Cats exist mostly on the 66 inhabited islands but their
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numbers are quite substantial. The diverse wildlife of the Croatian coastal islands could be
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more effectively protected and maintained not only through the protected status given to a
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large number of species and sites (e.g. Radović et al., 2005, 2006; Tutiš et al., 2013), but also
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by estimating the predation roles of introduced species and by intervening if necessary,
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through pest control or cat eradication for example.
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ACKNOWLEDGEMENTS.—We would like to thank Dragica Purger for useful comments
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on the previous version of manuscript, and for Alberto Velando, Alejandro Martinez-Abraín
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and an anonymous reviewer for suggestions and corrections. Mobility for the research was
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facilitated by the Hungarian-Croatian Intergovernmental S&T Co-operation Programme
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(project No. CRO-17/2006).
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Received: 19 February 2015
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Accepted: 17 June 2015
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Editor: Alberto Velando
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LEGENDS TO FIGURES
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FIG. 1.—Daily survival rates of quail eggs (+SE) in ground-sited and shrub-sited artificial
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nests on the islands of Olib and Silba. The upper figures within columns indicate the total
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number of exposure days, while the lower figures show egg losses (NS – non significant, * P
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< 0.05).
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[Tasas de supervivencia diaria de huevos de codorniz (+ error estándar) en nidos artificiales
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en el suelo y arbustos en las islas de Olib y Silba. Los números superiores en las barras
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indican los números totales de días de exposición, mientras que los números inferiores
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muestran los huevos perdidos (NS – no significativo, * P < 0.05).]
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FIG. 2.—Identification of predators by using plasticine eggs in ground-sited and shrub-sited
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artificial nests on the islands of Olib and Silba. Proportions of eggs found intact, taken away
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(removed) or marked by different predators.
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[Identificación de depredadores utilizando huevos de plastilina en nidos artificiales en el
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suelo y arbustos en las islas de Olib y Silba. Se indican las proporciones de huevos intactos,
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desaparecidos o con marcas de diferentes depredadores.]
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Figure 1
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Figure 2