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Trophic ecology of the Southern Grey Shrike (Lanius meridionalis) in insular environments: The influence of altitude and seasonality
Abstract
The seasonal diet and prey selection of the Southern Grey Shrike (Lanius meridionalis) was studied in two different insular habitats: shrub environments of the Canary Islands in coastal and high mountain zones.
We measured, in each season, food availability and prey size in order to determine prey size selection of shrikes along an
altitudinal gradient. Moreover, we compared the diet patterns observed with those documented on the continent, to determine
if Southern Grey Shrikes in the islands’ high mountain zone (which has a continental climate) showed seasonal diet variation
similar to those in northern continental areas. We analysed a total of 1,139 shrike pellets collected in 1 year and identified
10,179 prey items. Numerically arthropods (91%), and in terms of biomass lizards (70%) were the main prey consumed by the
shrikes. The proportions of the main prey items differed significantly between seasons and habitats. Diet in the coastal areas
was less variable than in the high mountain zone. The greater seasonal climatic variation in the high mountain zone was associated
with diet patterns similar to those found in some northern continental areas, such as the Iberian Peninsula and southern France.
Finally, shrikes selected the largest prey in the high mountain habitat. This suggests that foraging behaviour in this species
is related to climatic conditions, as the biggest and most profitable prey were consumed in the most harsh habitats.
... . 'Equilibrium theory' predicts that species richness is generally lower on smaller islands than on large islands (MacArthur & Wilson 1967). Accordingly, predators inhabiting islands in the same archipelago will show greater or lesser trophic diversity according to the particular biogeographic factors of the island that they inhabit (Padilla et al. 2009). Therefore, insular organisms are interesting models for studying ecological processes, underlying niche width expansion and dietary changes (Nunn 1994). ...
... In some habitats, especially subalpine scrub above 2000 m a.s.l., grasshoppers constituted the staple diet of Kestrels, which may exemplify opportunistic exploitation of a temporarily abundant resource (Oromí et al. 2002) with minimal energy expenditure. In subalpine scrub, the great open sandy landscapes with sparse vegetation favour the abundance of grasshoppers (Padilla et al. 2009), especially during the phases of nestling and fledgling development (Carrillo & González-Dávila 2005). On the oceanic island of Madeira, Kestrels showed unusual hunting behaviour in which they exploited temporarily abundant insect resources hidden in the grass. ...
... Interestingly, these insects predominate in the diet of other bird predators in this archipelago (e.g. Eurasian Stone-curlew Burhinus oedicnemus, Giannangeli et al. 2004; Southern Grey Shrike Lanius meridionalis, Padilla et al. 2009;Yellow-legged Gull Larus michahellis, López et al. 2016). We did not quantify prey availability in our study areas, but the Kestrel is considered a generalist predator in the southwestern Palaearctic regions with great capacity to adapt its diet according to prey availability (Village 1990). ...
The diet of Eurasian Kestrels Falco tinnunculus during the breeding season is well documented in continental populations, but not in their island-dwelling counterparts. The subspecies F. tinnunculus canariensis is a habitat-generalist on the oceanic island of Tenerife, one of its southernmost breeding sites in the Western Palaearctic. We analysed trophic diversity and diet in its main five habitat-types by identifying 5284 prey items in regurgitated pellets from nest sites and roosting sites collected during two consecutive breeding seasons. Secondly, we analysed 2585 prey remains collected from nest sites during 19 breeding seasons. In terms of numbers, insects (mainly Orthoptera and Coleoptera) predominated in pellets: 71% in chicks and 89% in adults, with variation in taxa according to habitat. This reflects opportunistic predation of insects, with grasshoppers being the staple diet in subalpine scrub (above 2000 m a.s.l). The endemic Gallot's Lizard Gallotia galloti provided the greatest biomass in pellets (58% in chicks, 55% in adults) and comprised the most abundant source of prey remains found at nest sites (92% in terms of numbers). Chick diet showed broader trophic diversity than adults. We found high overlap in diet composition between habitats in Tenerife (71% chicks; 66% adults). Kestrels inhabiting anthropogenic environments and northern scrub showed broader trophic diversity; those inhabiting subalpine scrub showed the lowest values. Kestrels inhabiting different habitats in Tenerife showed broad trophic diversity which suggests opportunistic and flexible foraging strategies, as expected of a generalist species. Regardless of habitat, trophic diversity showed high overlap in adult diet composition not only between the Canary Islands (87%), but also between insular and continental reference populations, especially the Iberian Peninsula (81%). Thus, the generalist habits of the Eurasian Kestrel seem to predominate over any expected ecological adaptations associated with island populations.
... Campos et al. (2007) also found similar inter-annual variation in breeding success in a population in northern Spain. In their study, however, breeding success in the bad years was below 60% (while in Padul was around 75%), and in the good years it was 70-84% Regarding nestling diet, arthropods were the main food (87%), especially Coleoptera (22%), which matches most previous studies on southern shrike nestling diet, indicating that the diet consists primarily of arthropods (Hernández, 1993b;Budden & Wright, 2000;Padilla et al., 2009;Campos et al., 2010). Nevertheless, Orthoptera were the main prey consumed by nestlings in Valladolid (Central Spain, Campos et al. 2010). ...
... In fact, Campos et al. (2010) reported that, in Valladolid, shrikes feed on Coleoptera less than expected by chance according to their availability; however, Coleoptera constitute the main prey of shrikes in the population studied here (Padul) and in León (near Valladolid;Hernández, 1993b). In general, the nestling diet matches well with the adult diet, i.e. mainly arthropods and occasionally small vertebrates (Hódar, 2006; see also Hernández et al., 1993;Lepley et al., 2004;Padilla et al., 2009). However, the portion of vertebrates in the diet should not be understated; although the frequency of items is low, they may represent a major portion of the biomass (which we failed to measure). ...
The southern shrike (Lanius meridionalis) is declining at the Spanish and European level. One cause of this decline could be low reproductive success due to low availability of prey in agricultural environments. To investigate this possibility we analysed the breeding ecology of a population of southern shrike in an agrosystem in Lomas de Padul (SE Spain). Our results suggestthe population is declining in this area. However, contrary to expectations, the population showed the highest reproductive success (% nests in which at least one egg produces a fledgling) reported for this species to date (83.3%), with a productivity of 4.04 fledglings per nest. Reproductive success varied throughout the years, ranging from 75% in the worst year to 92.9% in the best year. Similarly, productivity ranged from 3.25 to 5.0 fledglings per nest depending on the year. Other aspects of reproductive biology, such as clutch size, brood size, and nestling diet, were similar to those reported in other studies. Based on these results, we hypothesise that the determinant of population decline acts on the juvenile fraction, drastically reducing the recruitment rate, or affecting the dispersion of adults and recruits. Nevertheless, the exact factor or factors are unknown. This study shows that a high reproductive success does not guarantee good health status of the population.
... It's worth noting a scorpion, which is one of the preferred preys of the Great Grey Shrike, with a rate of 2.6%. Padilla et al. (2009) indicate that the house mouse presents a seasonal biomass ranging between 22.9% and 30% in different seasons. According to Taibi et al. (2011), Discoglos suspictus constitutes the highest biomass of 11.9% (in winter) in the Ramdhania station. ...
This study investigates the diet of the Great Grey Shrike (Lanius excubitor elegans) in the Adrar region by comparing food availability and analysing 110 pellets collected across different seasons. A total of 194 prey species representing 7 classes and 21 orders were identified. Insects overwhelmingly dominated the diet, comprising 97.8% of the prey, with Coleoptera being the most consumed order (34.2%), followed by Hymenoptera (29.2%), and other orders making up the remaining 36%. The two most frequently eaten insect species were Messor sp. (9.61%) and Cataglyphis sp. (6.41%) from the Hymenoptera order (Formicidae) and a coleopteran species, Cicindela flexuosa (5.84%) (Cicindelidae). Additionally, the diet included notable pest species such as Gerbillus sp. (0.14%) and some grasshoppers like Eyprepocnemis plorans (1.28%). Statistical analyses indicated no significant seasonal differences in the distribution of prey classes, highlighting the consistently high presence of insects throughout the year. However, significant variations were observed within the Insecta class, particularly among the Coleoptera and Orthoptera orders. These findings suggest that the Great Grey Shrike plays a beneficial role in biocontrol and contributes to environmental balance.
... On many islands, the southern grey shrike remains in the same areas throughout all seasons of the year, probably because of the stable island climate and food availability; on Gran Canaria reptiles are active throughout all the year (Padilla et al., 2007(Padilla et al., & 2009. On the contrary, the mainland southern grey shrikes have to migrate during winter (Madroño et al., 2004). ...
This is a study focused on the importance of reptiles in the diet of the southern grey shrike (Lanius meridionalis). The two main objectives of the study are: the investigation of prey (reptiles) impaled by the southern grey shrike in the island of Gran Canaria (Spain), and a bibliographic review of the diet of this species, comparing the importance of reptiles as prey between mainland and island areas. The southern grey shrikes captured three native and one introduced reptile species found in Gran Canaria. The literature review revealed that there is a greater proportion of predated reptiles on islands than in continental areas.
... However, and according to the findings of Vicente et al. (1995), in spring females were the sex with the wider isotopic niche. Female lizards might favour energy-rich prey during spring, due to the high energetic requirements associated with reproduction (Galán, 1996;Padilla et al. 2009;Hierlihy et al. 2013;Sagonas et al. 2019). This prey selection can be sex-specific and might occur on the basis of prey size (e.g., by selecting larger arthropods with higher energy input) or be associated for instance with reductions in foraging movements (Díaz and Carrascal, 1990;Carretero, 2004). ...
Invasive species are a major threat to island biodiversity, and their eradications have substantially contributed to the conservation of island endemics. However, the consequences of eradications on the trophic ecology of native taxa are largely unexplored. Here, we used the eradication of invasive black rats Rattus rattus and European rabbits Oryctolagus cuniculus from the Berlenga Island, in the western coast of Portugal, as a whole-ecosystem experiment to investigate the effects of the eradication of invasive mammals on the trophic niche and body dimensions of the island-restricted Berlenga wall lizard Podarcis carbonelli berlengensis over a 2-year period. Our results suggest an expansion of the isotopic niche and an intensification of the sexual dimorphism of the lizard following mammal eradication. Additionally, we found considerable variability in isotopic niche across the island and detected evidence of sex-specific and season-modulated nutritional requirements of this threatened reptile. Our findings support that the eradication of two of the planet’s most problematic invasive vertebrates led to changes in the lizard trophic niche and sexual dimorphism in just two years. This suggests that the ecological pressures—e.g., prey availability and habitat structure—to which lizards are exposed have substantially changed post-eradication. Our study emphasizes the scientific value of island eradications as experiments to address a wide range of ecological questions and adds to the increasing body of evidence supporting substantial conservation gains associated with these restoration interventions.
... This renders most living animals very inconspicuous in their normal resting places, presumably reducing the risk of visual detection by predators. The snails are part of the diet of the Southern Grey Shrike, Lanius meridionalis Temminck (Aves, Passeriformes) (Padilla et al. 2009) and the omnivorous lizard, Gallotia atlantica (Peters & Doria), endemic to the eastern islands (Valido & Nogales 2003). Other species of Gallotia, which as a genus is present in all the islands of the Archipelago, probably also consume snails as part of their regular diet (Yanes et al. 2010). ...
Five new species of Napaeus are described, four from Gran Canaria and one from El Hierro (Canary Islands): Napaeus josei n. sp., N. venegueraensis n. sp., N. arinagaensis n. sp., N. validoi n. sp. and N. grohi n. sp. The main differences from the most similar species and data on distribution are presented. At least three of the new species disguise their shells with soil, presumably to avoid predation.
... Linhart & Feinsinger, 1980; Inoue, 1993; Olesen & Jordano, 2002; Pafilis et al., 2009) and abiotic factors (e.g. Grant et al., 2000; Illera et al., 2006; Padilla et al., 2009). In relation to biotic interactions occurring on oceanic islands such as the pollination of flowering plants by animals (or other kinds of mutualistic interaction among species, e.g. ...
... This renders most living animals very inconspicuous in their normal resting places, presumably reducing the risk of visual detection by predators. The snails are part of the diet of the Southern Grey Shrike, Lanius meridionalis Temminck (Aves, Passeriformes) (Padilla et al. 2009 ) and the omnivorous lizard, Gallotia atlantica (Peters & Doria), endemic to the eastern islands (Valido & Nogales 2003 ). Other species of Gallotia, which as a genus is present in all the islands of the Archipelago, probably also consume snails as part of their regular diet (Yanes et al. 2010). ...
Five new species of Napaeus are described, four from Gran Canaria and one from El Hierro (Canary Islands): Napaeus josei n. sp., N. venegueraensis n. sp., N. arinagaensis n. sp., N. validoi n. sp. and N. grohi n. sp. The main differences from the most similar species and data on distribution are presented. At least three of the new species disguise their shells with soil, presumably to avoid predation.
... All Gallotia lizard species present a marked omnivorous diet, with fleshy fruits being an important component year round. Thus, they legitimately disperse seeds of many fleshy-fruited plant species in different habitats and islands (Barquín-Díez and Wildpret, 1975;Machado, 1985b;Barquín-Díez et al., 1986;Naranjo et al., 1991;Valido, 1999;Pérez-Mellado et al., 1999;Martín et al., 2005;Carretero et al., 2006;Nogales et al., 2007;Padilla et al., 2009Carretero et al., 2014;. Nevertheless, the quantitative importance of the vegetarian components in the diet positively covary with lizard body sizes both withinand among species. ...
... All Gallotia lizard species present a marked omnivorous diet, with fleshy fruits being an important component year round. Thus, they legitimately disperse seeds of many fleshy-fruited plant species in different habitats and islands (Barquín-Díez and Wildpret, 1975;Machado, 1985b;Barquín-Díez et al., 1986;Naranjo et al., 1991;Valido, 1999;Pérez-Mellado et al., 1999;Martín et al., 2005;Carretero et al., 2006;Nogales et al., 2007;Padilla et al., 2009Carretero et al., 2014;. Nevertheless, the quantitative importance of the vegetarian components in the diet positively covary with lizard body sizes both withinand among species. ...
Extinction-driven, body-size reduction of seed dispersers (i.e. an ecological downsizing resulting from severe defaunation) can entail the loss of unique ecological functions, and impair plant regeneration. However, the manner in which the downsizing of mutualistic animals affects seed dispersal and plant recruitment remains understudied. Here, we took advantage of a natural experiment in the Canarian archipelago to document the consequences of lizards body-size reduction (Gallotia, Lacertidae) on the recruitment of Neochamaelea pulverulenta (Rutaceae), which relies exclusively on these frugivores for seed dispersal. Subsequent to the arrival of humans (ca. 2000-2500 yr BP), the extinction of large-bodied lizards generated a gradient of increasing defaunation on the three islands inhabited by this plant. We hypothesized a significant reduction, and eventually collapse, of early seedling recruitment mirroring the defaunation intensity of the frugivores. We sampled 42 populations spanning the whole geographic range of the plant to examine the quantitative (age structure pattern) and qualitative components (proportion of seedlings growing outside the canopy, number of seedlings established outside the canopy relative to the number of adults -effective recruitment rate-, and seedling vigour) of plant regeneration. Our results show that the age structure patterns did not differ among the three contrasted insular scenarios. However, we found significant reductions in seedling recruitment outside the canopy, effective recruitment rate, and delayed negative effects on seedling vigour in populations hosting small- to medium-sized lizard species. Thus, extirpation of large seed-dispersers did not cause substantial reductions in quantitative components of seed dispersal, but determined declines in qualitative aspects impairing dispersal effectiveness. Our study highlights the importance of examining all components of the dispersal and recruitment process to properly document the regeneration outcomes of plants in defaunated, downsized ecological scenarios.
... exerting an important predation pressure, enough to induce behavioral changes (less mobility and changes in diet) in lizards (Hawlena & Pérez-Mellado, 2009;Padilla et al., 2009). Its role as a prominent predator of reptiles is also true for insular systems. ...
Spanish Abstract: El alcaudón sureño (Lanius meridionalis) es un depredador común de reptiles, cono-
cido por empalar sus capturas en arbustos espinosos como reserva. El 3 de Marzo de 2014, des-
pués de detectar un ejemplar de L. meridionalis alrededor de nuestro campamento en la reser-
va forestal de Firmhin (Socotra), encontramos unos restos de Trachylepis socotrana empalados
en una rama seca. Durante los diversos muestreos que tuvieron lugar ese año en Firmhin, ese
escincido no se detectó, lo que indica la posible importancia del examen de este tipo de restos
para detectar poblaciones de reptiles con poca densidad en ambientes insulares.
... This renders most living animals very inconspicuous in their normal resting places, presumably reducing the risk of visual detection by predators. The snails are part of the diet of the Southern Grey Shrike, Lanius meridionalis Temminck (Aves, Passeriformes) (Padilla et al. 2009) and the omnivorous lizard, Gallotia atlantica (Peters & Doria), endemic to the eastern islands (Valido & Nogales 2003). Other species of Gallotia, which as a genus is present in all the islands of the Archipelago, probably also consume snails as part of their regular diet (Yanes et al. 2010). ...
Five new species of Napaeus are described, four from Gran Canaria and one from El Hierro (Canary Islands): Napaeus josei n. sp., N. venegueraensis n. sp., N. arinagaensis n. sp., N. validoi n. sp. and N. grohi n. sp. The main differences from the most similar species and data on distribution are presented. At least three of the new species disguise their shells with soil, presumably to avoid predation.
... Brambilla & Ficetola 2012;Brambilla et al. 2009Brambilla et al. , 2010Ceresa et al. 2012;Goławski & Meissner 2008;Söderström & Karlsson 2011) and the two species Lanius excubitor and Lanius meridionalis (e.g. Karlsson 2002;Keynan & Yosef 2010;Kuczyński et al. 2010;Olsson et al. 2010;Padilla et al. 2009). The other two species, L. senator and L. minor, have been poorly investigated (but see Giralt & Valera 2007;Guerrieri & Castaldi 2000;Guerrieri & Castaldi 2010;Guerrieri et al. 1995;Hernandez 1994;Isenmann & Debout 2000;Moskát & Fuisz 2002). ...
Cats, introduced on thousands of islands worldwide, are one of the greatest threats to native wildlife. An analysis of 301 scats was performed in order to study the diet of cats in a National Park in the alpine ecosystem of an oceanic island (Tenerife, Canary Islands). The results were compared with those obtained 35 years ago. In this study, eight types of vertebrates were detected in the cat diet: two endemic reptiles, four introduced mammals, and two birds. Although introduced rabbits were the most important prey, accounting for 53.9% of the biomass in the diet, this figure is among the lowest recorded in the Canary Islands. Data show a shift from the diet 35 years ago, with a decrease in the percentage of rabbits consumed, from 73% of diet biomass in 1986 to 53.9% today, and an increase mainly not only in reptiles but also in native birds. This change is due to a decrease in the rabbit population, probably motivated by the incidence of haemorrhagic disease (RHDV2). Using a daily intake of 170 g, we estimated that a single cat could prey on 1331 vertebrates/year. Assuming a low cat density of 1 cat/km2, the total cat population in the National Park would kill 257,739 vertebrates, being native species particularly vulnerable to predation, with 166,249 reptiles and 5588 birds annually. Thus, with the aim of updating and improving management strategies for the conservation of native biodiversity, there is a need for new research on invasive predators on islands where diet may change over time.
Predation is one of the most important factors affecting biology, ecology and behaviour of the prey. We have studied predation of the common lizard (Zootoca vivipara) by the great grey shrike (Lanius excubitor) in farmland habitats in western Poland. Shrike caches were used as a source of information about preyed lizards. Shrikes hunt significantly more adult males than females, juveniles and sub-adults; the pattern was similar over all three study years. Male lizard had longer activity time than female; therefore, they seem to be under predation pressure for a longer time during breeding season. Capture and count transect data of common lizard populations living within and outside shrike territories showed significant seasonal differences: within shrike territories, there was lower proportion of males than females; moreover, in some territories, the number of males in the lizard population was negatively correlated with the number of males impaled by shrikes. Our findings suggest that in this particular predator-prey system, shrikes may be a strong selective force for lizards’ population dynamics. Male-biased predation could be caused by differences in the behaviour of adult male and female lizards, namely longer activity of males and differences in space and refuge use.
Although raptors are not usually seen as seed dispersers, we demonstrate their important ecological role as legitimate seed dispersers of weeds. On the oceanic island of Fuerteventura in the Canary Islands, up to 11 000 weed seeds from a total of 39 and 62 different plant species respectively were found in the pellets of two native predatory birds, the Common Buzzard Buteo buteo and the Eurasian Kestrel Falco tinnunculus, which consumed European Rabbits Oryctolagus cuniculus and Barbary Ground Squirrels Atlantoxerus getulus, both invasive mammals that carried these seeds in their guts. Four and seven weed species appeared with a frequency of occurrence greater than 10% in the pellets of Kestrels and Buzzards, respectively. Despite the time interval between pellet collection and germination experiments, almost 10% of planted seeds germinated, even after passing through two digestive processes, one inside the mammal and another inside the raptor. Our study encourages a change of perspective in this regard, as raptors may be involved in complex interactive dispersal processes on a wider geographical scale. This article is protected by copyright. All rights reserved.
1. Secondary seed dispersal is a multistep process with two or more phases, which involve different dispersers that usually extend the distance from the seed’s parent plant. This ecological process has been recorded in some subtropical oceanic islands, where predatory birds commonly consume frugivorous lizards and disperse seeds already consumed by the lizards.
2. We evaluated the overall importance of this type of secondary dispersal in the Canary Islands, the only place worldwide where it has been studied in depth. From an examination of all the islands and their suitable habitats, we found seeds from 78 plant species inside 2098 shrike pellets and 5304 kestrel pellets. A greater number of species were secondarily dispersed by kestrels (76; 97%) than by shrikes (26; 34%).
3. Forty‐four (56%) of the total species detected in pellets were identified at the species level, comprising 73% native and 27% introduced species. Seventy per cent of these identified species were fleshy fruit‐bearing plants and 84% of the interactions took place in open habitats, close to coastal areas.
4. Germination experiments showed that seeds of at least 32 plant species were viable after being removed from the bird pellets. A similar pattern of seed germination was detected for seeds from the droppings of lizards and pellets of shrikes, showing both to be effective dispersers. However, the seeds dispersed by kestrels had different levels of success depending on the number of gut passages experienced. Seeds that had undergone double gut treatment (lizard and secondary ingestion by kestrel) had reduced germination rates of many small‐ and medium‐sized seeds compared with seeds ingested by lizards and discarded inside the lizard guts by kestrels.
5. We also studied the relationship between body length and gape width of lizards in order to assess limitations on the sizes and quantities of seeds available for secondary dispersal. Kestrels can disperse a greater number and variety of seeds because they predate larger lizards that potentially carry greater seed loads.
6. Synthesis . The current results show how these non‐standard long‐distance dispersal events produced by predatory birds can be considered as a regular and generalized process on all islands of the Canary archipelago.
On the diet of the Southern Grey Shrike Lanius meridionalis koenigi
Between 2001 and 2005, pellets from the Southern Grey Shrike Lanius meridionalis koenigi were analysed on 4 Canary Islands, particularly Fuerteventura.
As a whole 3085 prey items were found, among them 94 % Invertebrates (tab.1). With the help of 209 intact pellets it was possible to calculate the “Beutewert” (pray-value) according to NICOLAI (1992). The Beutewert for vertebrates (62 %) is nearly two times higher than for invertebrates (38 %; fig. 12). Most important prey are the endemic lizards Gallotia atlantica. They are available for the shrikes all year round. Among the Invertebrates hymenopterans predominate. Further orders of insects can reach seasonally and locally considerable prey-values. The Southern Grey Shrike from Canary Islands is a very opportunistic feeder and is able to use these only episodically or periodically available prey-source effectively. The most important prey-orders are discussed. Similarities and differences of feeding in comparison to the Great Grey Shrike L. excubitor are shown.
Habitat composition of breeding territories, habitat use during foraging, and nest-site selection of the Great Grey Shrike Lanius excubitor were examined in western Poland during years 1998-1999. Meadows and spring crops occurred within territories more frequently than expected by their availability. Habitat utilisation during foraging was significantly non-random and shrikes appeared to prefer low vegetation. All of 27 nests were located along tree lines and edges of small woodlots (maximum 7 m), and neither deciduous nor coniferous trees were preferred (78% and 22%, respectively). Preferred habitats of Great Grey Shrikes are relatively uncommon in modern agricultural landscape. Lack of suitable habitats in intensively used farmland may be the most important reasons for strike population decline over Europe. Wielkopolska region offers good breeding habitats and this species has more dense and stable population over here, than in other parts of Europe.
Hen Harriers Circus cyaneus and Montagu's Harriers Circus pygargus are medium-sized raptors that differ in size (Hen Harrier being slightly bigger than Montagu's Harrier) and breeding system (Montagu's Harriers are semi-colonial and Hen Harriers defend nesting-hunting territories). In contrast, the diets of the two species when in sympatry are very similar. We evaluated food-niche differentiation among these coexisting raptor species and how between-species differences in body size and social system influence interspecific relationships. We present data from a study conducted in 1997 and 1998 in northeastern Madrid province (central Spain). Diet of the two species largely overlapped (55-95%) during the breeding season, but Hen Harriers preyed more often on larger species. This segregation was observed both in the average size of the primary prey (lagomorphs) and in the alternative prey (birds for Hen Harriers vs. insects for Montagu's Harriers), and was particularly apparent late in the season. Accordingly, feeding frequency of Montagu's Harriers, but not of Hen Harriers, increased later in the season. Size differences between species in prey brought to the nest were apparent for both males and females. Foraging behaviour also differed, as Hen Harriers spent more time hunting close to the nest than did Montagu's Harriers. This implies that segregation in foraging areas may also exist. Observed niche partitioning may relax the potential for competition between these species.
We observed color-banded and/or radio-tagged Northern Shrikes (Lanius excubitor) wintering in southwest Idaho and determined that foraging success of these shrikes was over 69%. Foraging success was dependent upon the type of prey attacked. Predation success upon arthropods was greater than 90%, whereas predation upon vertebrates (small mammals and passerines) was substantially lower (56% and 19%, respectively). We collected 237 pellets from 12 shrikes and identified 671 individual prey items contained in these pellets. Arthropods and small mammals were the most important prey items as measured by number (63.9% and 29.8%, respectively) and Index of Relative Importance (38.9% and 59.6%, respectively), whereas small mammals were the most important components of shrike diet by biomass contributing 83.1% of the total prey biomass. Passerines were of lesser importance in the winter diet of shrikes accounting for 11.8% of the biomass but only 1.7% of the Index of Relative Importance.
This paper analyzes factors determining the extent of density compensation on islands: i.e., is the summed population density of individuals of all species on islands equal to the summed mainland density as a result of niche expansions and higher abundances of island species compensating for the absence of many mainland species? In addition, a method is described for estimating bird population densities based on analysis of the time dependence or mist-netting yields. Puercos Island in the Pearl Archipelago off Panama has less than one-third as many resident birds species as comparable mainland habitats. Analysis of the Pearl avifauna suggests that about one-quarter of the island species may be relicts of the Pleistocene land bridge and that the remainder are subsequent over-water colonists. The successful colonists are a highly nonrandom sample of the mainland avifauna in such respects as family composition, social structure, and second-growth habitat origin. Song-based censuses and analysis of mist-netting show that Puercos has a slightly density of individuals than the mainland. Niche shifts between islands and mainland, or among different islands, include habitat expansions, wider ranges of vertical foraging strata, abundance increases checkerboard distribution patterns, and decreased morphological variability. Comparison of the present study with other studies shows that summed population densities on islands may be higher than, comparable to, or less than mainland levels, depending upon the particular island, habitat, and group of animals studied. Among factors affecting the extent of density colonists less appropriate to the vacated habitat, tending to lower island densities; and underrepresentation of large species on islands, tending to increase island population densities for a given biomass.
Capsule: In this region the diet is mainly cold-blooded prey, mostly insects such as beetles. Aims: To describe the diet of this newly separated, poorly documented and endangered species. Method: Diet was inferred from pellet analysis, collected during a single breeding and winter period in the steppe of the Crau. Results: A total of 5409 prey were identified from 257 pellets. Vertebrates were seldom taken, except by adults (small passerines) during the fledgling period. High seasonal differences were found. Hymenoptera were largely consumed in autumn, Arachnida in autumn and winter, Orthoptera in summer and autumn and Lepidoptera larvae in winter and spring and by fledglings. Nevertheless, Coleoptera were ingested in large proportions all year round. Carabidae were the main prey in winter and Melolonthidae were especially important for adults during the nestling period, as were Cetoniidae for the fledglings. Conclusion: Small mammals and small birds were less exploited in France and Spain (L. m. meridionalis) than in Israel (L. m. elegans or L. m. aucheri), whereas the opposite might be expected, following a north-south climatic gradient. Thus, the nominate subspecies L. m. meridionalis differed in diet from L. m. elegans or L m. aucheri.
Size-related selection of insular lizards by two sympatric predatory bird species (Southern Grey Shrike and
Eurasian Kestrel) was studied in an arid insular environment. The endemic lizard genus Gallotia was a key resource in the diet of both predators, constituting more than 50% of the total biomass. Shrikes captured smaller lizards than kestrels during all seasons (mean snout - vent length (SVL): 7.4 ± 1.9 vs. 9.4 ± 2.1 cm respectively), presenting a sequential use of lizard sizes and avoiding potential competition. On the other hand, shrikes and kestrels showed the same seasonal variation pattern, capturing the largest lizards during their breeding periods (spring). Considering lizard availability, shrikes displayed less selective predation than kestrels in all seasons. Shrikes positively selected the medium lizard size (SVL: 5-10 cm) during the nestling period, but negatively selected the small lizard size (SVL: < 5 cm) in autumn and winter, probably due to an explosion of juvenile lizards. Lastly, kestrels appeared to be more selective, negatively choosing the small lizard size but positively selecting the largest ones all the year round (SVL: > 10 cm).
Budden A. E., Wright J. 2000. Nestling diet, chick growth and breeding success in the Southern Grey Shrike (Lanius meridionalis). Ring 22, 1: 165-172. Data were collected on a population of Southern Grey Shrikes breeding in a 25 km 2 area of desert within the Syrian-African rift system, southern Israel. Parental provisioning behaviour to offspring was recorded with a video camera. The mass and body size of the chicks was re-corded daily to assess chick growth. Prey delivered to the nest consisted primarily of inverte-brates, as well as some vertebrate prey, and the size of the prey items increased with chick age. Age had no significant effect on parental visit rate although prey types delivered ap-peared to shift with increasing chick age. Curiously, there were no differences in the mean visit rate and size or type of prey delivered to nests that fledged young and those that failed. A logistic growth equation fitted to the data on chick weight yielded similar results to those recorded for Southern Grey Shrikes breeding in another desert region in Southern Israel.
Diet composition of the Great Grey Shrike (Lanius excubitor) was studied on the basis of cached prey, pellets and food remains collected beneath perches, from November to March for 5 years (19982002). The ratio of vertebrates and invertebrates (by numbers) was nearly 1:9. Pellet dimensions were on average 25.4(±5.45)mm×10.8(±1.13)mm. A total of 104 caches were found, including 95 with Gryllus campestris. Most caches were fixed on plants (91.3%) and the rest on barbed wire. Stored prey items were placed at an average height of 1.05±0.29 m (range 0.571.9 m) above ground, most commonly in the middle part of the plants. Storage height was significantly correlated with plant height. Patterns of impaling were also studied about half of the specimens of G. campestris were impaled through the forepart of the abdomen. A latitudinal cline in the proportion of vertebrates in the Great Grey Shrike diet during winter in Europe proved to be statistically significant.
Recent reviews of plant–pollinator mutualistic networks showed that gen-eralization is a common pattern in this type of interaction. Here we examine the ecological correlates of generalization patterns in plant–pollinator networks, especially how interaction patterns covary with latitude, elevation, and insularity. We review the few published anal-yses of whole networks and include unpublished material, analyzing 29 complete plant– pollinator networks that encompass arctic, alpine, temperate, Mediterranean, and subtrop-ical–tropical areas. The number of interactions observed (I) was a linear function of network size (M) the maximum number of interactions: ln I 0.575 0.61 ln M; R 2 0.946. The connectance (C), the fraction of observed interactions relative to the total possible, decreased exponentially with species richness, the sum of animal and plant species in each community (A P): C 13.83 exp[0.003(A P)]. After controlling for species richness, the residual connectance was significantly lower in highland (1500 m elevation) than in lowland networks and differed marginally among biogeographic regions, with both alpine and trop-ical networks showing a trend for lower residual connectance. The two Mediterranean networks showed the highest residual connectance. After correcting for variation in network size, plant species were shown to be more generalized at higher latitude and lowland habitats, but showed increased specialization on islands. Oceanic island networks showed an im-poverishment of potential animal pollinators (lower ratio of animal to plant species, A : P, compared to mainland networks) associated with this trend of increased specialization. Plants, but not their flower-visiting animals, supported the often-repeated statements about higher specificity in the tropics than at higher latitudes. The pattern of interaction build-up as diversity increases in pollination networks does not differ appreciably from other mutualisms, such as plant–seed disperser networks or more complex food webs.
We studied the distribution of a common Mediterranean lacertid lizard, Psammodromus algirus (L.) 1758, on nineteen sites within a regional gradient of homogeneous yet contrasted habitats. This scale was large enough to allow line-transect estimates of lizard abundance, which were related to quantitative (and when possible multivariate) measurements of the structure and floristic composition of vegetation, the abundance of arthropod prey, the relative density of other lizard species, and the climatic data obtained from nearby meteorological stations. Neither the climate not the abundance of other lizards seemed to condition the quantitative distribution of the species. The positive influence of broad-leaved forests on the abundance of P. algirus appeared to be a consequence of structure attributes more directly related to the ecology of lizards than floristic composition per se. Thus, population levels were most highly correlated with the cover of shrubs over 20 cm in height, and once this structural requirement was met, they increased with the abundance of potential prey (itself conditioned by vegetation cover at the ground level and litter cover). We suggest that our results should be interpreted in the context of thermoregulatory, predator avoidance and movement minimization strategies whose influence on survival, and hence abundance, could probably be applied to other insectivorous lizards from temperate zones.
Theory predicts that generalist predators will switch to alternative prey when preferred foods are not readily available.
Studies on the feeding ecology of the American marten (Martes americana) throughout North America suggest that this mustelid is a generalist predator feeding largely on voles (Microtus sp.; Clethrionomys sp.). We investigated seasonal and annual changes in diets of martens in response to the changing abundance of small rodents
(Peromyscus keeni, and Microtus longicaudus) on Chichagof Island, Southeast Alaska, using stable isotope analysis. We hypothesized that martens would feed primarily
on small rodents during years with high abundance of these prey species, whereas during years of low abundance of prey, martens
would switch to feed primarily on the seasonally available carcasses of salmon. We also hypothesized that home-range location
on the landscape (i.e., access to salmon streams) would determine the type of food consumed by martens, and martens feeding
on preferred prey would exhibit better body condition than those feeding on other foods. We live-captured 75 martens repeatedly,
from mid-February to mid-December 1992–1994. We also obtained marten carcasses from trappers during late autumn 1991 and 1992,
from which we randomly sub-sampled 165 individuals. Using stable isotope ratios and a multiple-source mixing model, we inferred
that salmon carcasses composed a large portion of the diet of martens in autumn during years of low abundance of rodents (1991
and 1992). When small rodents were available in high numbers (1993 and 1994), they composed the bulk of the diet of martens
in autumn, despite salmon carcasses being equally available in all years. Selection for small rodents occurred only in seasons
in which abundance of small rodents was low. Logistic regression revealed that individuals with access to salmon streams were
more likely to incorporate salmon carcasses in their diet during years of low abundance of small rodents. Using stable isotope
analysis on repeated samples from the same individuals, we explored some of the factors underlying feeding habits of individuals
under variable ecological conditions. We were unable to demonstrate that body weights of live-captured male and female martens
differed significantly between individuals feeding on marine-derived or terrestrial diets. Therefore, martens, as true generalist
predators, switched to alternative prey when their principal food was not readily available on a seasonal or annual basis.
Although salmon carcasses were not a preferred food for martens, they provided a suitable alternative to maintain body condition
during years when small rodents were not readily available.
Although it is well established that many insects, birds and mammals serve as important pollinators and seed dispersers of flowering plants, the role of lizards in these processes has traditionally been considered as rare and less important. However, recent work shows both that their role as mutualistic agents has been underestimated and also reveals a striking pattern – that pollination and seed dispersal by lizards is most common on islands. We argue that this island phenomenon occurs because island lizards reach very high densities (density compensation) and experience a lower predation risk than do those on the mainland and, consequently, can expand their diet to include nectar, pollen and fruit. Although further empirical evidence is needed to confirm this explanation, such relationships could be ideal systems with which to study fundamental ecological problems, such as niche shifts, ecological release and competition.
First published in 1983 to celebrate the centennial of the American Ornithologists' Union, Perspectives in Ornithology collects together a series of essays and commentaries by leading authorities about especially active areas of research on the biology of birds. Readers will find in this collection a useful overview of many major concepts and controversies in ornithology.
Field observations on Great Grey Shrikes Lanius excubitor were performed in four study areas in SW Finland (60°N, 22°E). The relation between habitat use, perch selection, prey type and the presence of a snow layer was evaluated. The mean yearly (± SE) territory size was 144 ha ± 14.3. The habitat selection depended on if snow cover was present, reflecting the favouring of vegetation-scarce arable fields when hunting invertebrates and vegetation-rich habitats when mainly vertebrates were available. Consistent with theories on optimal perch height, the shrikes chose higher perches when mainly vertebrates were hunted and also increased the time/perch, probably reflecting that vertebrate prey occur less frequently than invertebrate prey. The selection of lower perches when hunting invertebrates was evident in mild winter periods but not in the autumn;the reasons for this is discussed. The appearance of snow increased the distance to mammalian prey but the average capture frequency of mammals remained unchanged. In accordance with optimal foraging theory the shrikes preferred to hunt invertebrates when these were available rather than to adopt a hunting strategy that would optimise the encounter rate with vertebrates. However, the average hunting rate of ground-living prey decreased as the snow depth increased whereas energetically costly types of hunting behaviour, like rate of movements, hovering and hunting of birds remained unchanged.
Aims: The study of the diet and prey selection of the Southern grey shrike. Location: Two shrubsteppes of South-eastern Spain. Methods: The diet is determined by pellet analysis, and then compared with samples of prey availability recorded by means of pitfall traps, on a monthly basis during an annual cycle. Results: The Southern grey shrike consumes several beetle types, grasshoppers, and, during the breeding period, lizards. Birds and small mammals are captured incidentally. It consistently rejects prey smaller than 10 mm long, and prey are larger during the breeding period and summer. Compared with other populations of Southern grey shrike and also with Northern grey shrike, the population studied here shows a greater importance of lizards in the diet. Birds and mammals, which are progressively more frequent in diet as latitude increases, both in breeding and wintering periods, are here at low frequency. Conclusions: The diet of the Southern grey shrike in South Spain combines arthropods (mainly between 10 and 30 mm in length) and lizards. The changes in the taxonomic groups captured along an annual cycle and their sizes suggest that, apart of the weak limits of prey size described, the Southern grey shrike is quite opportunistic when feeding, and captures any prey of the adequate size available, regardless of their taxonomy. This also corresponds with the changes in diet showed along the European continent by both the Southern and the Northern grey shrike.
Diet composition of Great Grey Shrikes Lanius excubitor was studied during nine winter seasons (October-March) in southern Finland. In pellets, the main preys were Microtus-voles (35.4% of vertebrates by number), the harvest mouse Micromys minutus (27.5%), the common shrew Sorex araneus (14.8%), the house mouse Mus musculus (7.0%) and birds (7.0%). Invertebrates constituted about 0.1-1.7% of the total prey biomass. The shrikes hunted birds in an opportunistic manner, no preference for larger species within the suitable size-range was evident but tits dominated in the diet (61.2%). Compared to other species this was not due to favourable availability or prey selection but rather a result of higher vulnerability. The relationship between diet diversity and prey availability was studied using snow cover as an indicator for reduced availability of small mammals. Accordingly, the use of voles decreased in midwinter and during periods with snow, whereas birds and especially mice showed a corresponding increase. However, the overall vertebrate prey diversity was only slightly increased. A comparison between studies in northern and central Europe revealed a higher vertebrate prey breadth in the north. These observations are discussed in the light of optimal foraging theories predicting increased prey diversity in conditions with lower availability.
The local habitat use and diet of Red-backed Shrikes were studied in a main study area of 1.4 km2 in SW Finland. Based on pellets (n=688), Carabidae beetles constituted 29.1% of the prey (n=3,388) and Apidae (especially bumble bees), 18.3%. The rest was mainly representative of other beetle families (21.6%), other hymenopterons (12.6%) and Orthoptera (8.3%). In the diet, beetles decreased whereas Apidae and Orthoptera increased as the season progressed, reflecting alterations in abundance and vegetation-dependent availability of these prey. Breeding shrikes were found in natural unmowed meadows (n=14 pairs and in areas created by forestry (n=11). The start of incubation (average June 9th), territory size, breeding success (3.4-3.7 large nestlings/pair) and the abundance and percentage of diet of the main prey were similar in both habitats. However, in the post-breeding season, shrikes avoided habitats created by forestry but showed a particular preference for meadows. This was, in part, related to a high abundance of Orthoptera insects in this habitat type, which was supported by diet analyses. In addition, factors minimizing the risk of predation, i.e. the openness of meadows, may have contributed to the observed habitat selection. The role of suitable post-breeding habitats is discussed based on the observed population decline of Red-backed Shrikes.
1. In North America and México there is a strong tendency for island birds to have a longer tarsus and bill than their mainland counterparts; but there is no such tendency for them to have a longer wing and tail. 2. Although climate and body-size are known to influence the size of tarsus and bill the larger dimensions on islands are considered to be adaptations primarily to ecological conditions. The bill is longer because it deals with a greater range of food-sizes, and the tarsus is longer because a greater variety of perches is used. 3. The differences in usage have arisen as a result of the absence, on the islands, of species with similar ecological requirements, which has permitted some of those present to extend their activities and occupy at least part of the vacant niches, sometimes in new habitats. 4. It is suggested that the same ecological conditions and responses have influenced the evolution of large body-size of island rodents in particular, and of mainland animals of several phyla in general.
Fluctuations of some aspects of the predatory behavior of the Loggerhead Shrike were examined relative to cycles in prey availability and other physical and biological environmental factors during the nonbreeding period (September 1972 to April 1973) near Davis, California. The attack rate of shrikes declined significantly after a minimum temperature threshold was reached, apparently because of the reduced availability of poikilothermic prey at low temperatures. Attack rate stabilized when prey density was high. Average prey size taken was constant with time and well below the maximum that shrikes are capable of capturing. The capture efficiency of shrikes was higher than that reported for other predators. Capture efficiency increased at very low prey density (availability), tending to balance the decreased attack rate. Handling time, though probably a significant variable in predation by shrikes, did not reduce time available for other activities during nonbreeding months. Handling time was highly correlated with length of prey squared.
We describe a new species of Gallotia from Tenerife (Canary Islands). This lizard is intermediate in size between the two smaller (G. atlantica and G. galloti) and the two larger (G. simonyi and G. stehlini) species. Furthermore, it differs from the two similar larger Gallotia in the number of supratemporal scales (mode 4 instead of mode 2), usually 16 longitudinal ventral scale rows, and having distinctive dorsal yellow spots, and small lateral yellow or blue spots. Individuals from a small population located in La Habiga show a pale gray reticulation on dorsum and absence of ocelli in the lateral region. Univariate and multivariate analyses of nine meristic variables of each extant species within the genus Gallotia revealed significant variation. Principal component analyses support the existence of three main morphological clusters within the genus, the new lizard being nearest to G. simonyi and G. stehlini. Molecular data from mtDNA sequences (cytochrome b and 12S ribosomal RNA) indicate that G. intermedia is closely related to G. simonyi.
We describe the external morphology of a new giant lizard found alive on La Gomera Island (Canarian Archipelago), which had been previously described from subfossil bones. Adult size (SVL 135-190 mm) is comparable with the two largest living species (G. simonyi and G. stehlini) of the genus. It differs from the other species of Gallotia in the low number of temporal scales (21-2), presence of one elongate interprefrontal scale (linearly depressed in center) that is very infrequent in most of its congeners (small when it is present), having two distinctive lateral lines of small blue spots, and in having a blackish-brown dorsum and ventral parts ivory white. According to subfossil data, based on tooth morphology and body size, this lizard seems to correspond to the putatively extinct subspecies (G. simonyi gomerana). This taxon is sufficiently distinct to be treated as a full species, G. gomerana. Molecular data analysis from mtDNA sequences (cytochrome b and 12S rRNA) indicate that G. gomerana is closely related to G. simonyi and G. intermedia. After prospecting 70 localities in La Gomera, we only found a very small and threatened population of this species m Valle Gran Rey, in the western part of the island and living in an area of less than 1 ha. A small population size (only six individuals are known) and a large number of feral cats in their habitat makes this lizard the most endangered vertebrate of the Canaries and Europe and one of the most threatened in all the world. The species is suspected to be on the brink of extinction, so conservation measures are urgently needed, including a captive breeding plan at Valle Gran Rey, close to the natural habitat.
The early human occupation of South America was favored by an unusual set of geological and climatic factors which channeled population movements in a north to south direction. Because of the exposure of the continental shelf and the ameliorating effect of the oceanic climate, the area between 30⚬ and 50⚬ south latitude may have featured favorable environmental zones for human habitation. Therefore, it is no accident that the earliest dated cultures are known from the southern end of the continent. This factor is a logical result of the climatic and geographic factors influencing human migrations in South America, influences which may have resulted in this north to south migration requiring only a brief span of time. The exact chronology of this migration is inadequately known. However, comparison of the geographical location of the early sites with the presumed location of the late Pleistocene ice masses suggests that a majority of these sites are of post-glacial age. The presence of any sites of glacial...
A model which relates optimal food preference relationships and caloric yield per unit time of potential food sources is derived. It is suggested, on the basis of this model, that: 1) Food preferences can be adequately described only if a number of factors other than relative frequencies in the diet and relative abundances of the food types are known. 2) Animals should be more selective in their choice of foods when satiated or when food is common, more indiscriminate when starved or when food is scarce. 3) Animals may eat one food type with greater frequency, relative to its abundance, than another even if the other food is richer and more efficiently exploited. This occurs in situations of high relative abundance of the first food type. 4) The extent to which predators tend to pass by potential food items may be used to evaluate the role of food in the population limitation of a predator species. 5) Food preferences appear to change readily and appropriately to changes in the environment. preferences a ...
We tested the morphological trait utility in Great Grey Shrike Lanius excubitor. The food composition was used as an indicator of phenotype performance advantage in utilising of particular dietary niche. Mounts and accompanying data (measurements and stomach contents) from the Slovak extensive collection, involving adult birds obtained from April to October, were selected for analysis (n = 48). Ordination methods (redundancy analysis) and Generalized Linear Model regression show that the composition of the bird's food was non-random in respect to body characteristics. Tarsus and wing lengths were the most important morphometric predictors and ex- plained 12.7% of overall variance in food data. Smaller birds foraged mainly on Carabidae. Individuals with longer wings and tails more frequently preyed on flying and plant-dwelling insects. We conclude that phenotypic characteristics of an animal at least partially determine its diet.
Omnivorous endemic Canarian lacertids (Gallotia atlantica and G. galloti) do not present any specific digestive and physiological adaptations to herbivorous diet, compared to species and populations with a different degree of herbivory in the Canarian archipelago. The only characteristics that could be related to the type of diet were the number of cusps per tooth (between species) and the number of small stones contained in droppings (between species and populations). The rest of measured traits were correlated with lizard size and for this reason G. galloti has longer intestines, heavier stomachs and livers, more teeth and cusps, and longer gut passage. These data suggest that body size is a major determinant of the reliance on plant food (mainly fleshy fruits) in these lizards and facilitates mutualistic interactions with fleshy-fruited plant species.
The crayfish Procambarus
clarkii is an introduced species in Portugal, becoming a new available resource that has possibly changed local trophic relationships. The purpose of the present study was to understand better how a naïve assembly of predators reacted to a recent invader. Predation on P.
clarkii by nine species of mammals (Order: Carnivora) and six species of birds (Order: Ciconiiformes) in a marsh area in Portugal, was surveyed through the collection of faeces and pellets over a 24-month period. Seasonal variation in the consumption of this prey by different predators was evaluated, and estimation made of size and weight of P.
clarkii consumed by them. Procambarus
clarkii was readily preyed upon by four species of mammalian carnivores and five species of ciconiiform birds. The consumption of P.
clarkii varied seasonally in both years with a trend between seasonal exploitation and its population structure and availability. Generally, the use of P.
clarkii by all predators was more intense in spring, summer and autumn than in winter, and was directed towards larger and heavier individuals. These findings result from the population dynamics presented by P.
clarkii in the marsh area studied here, since larger and heavier individuals were more available in spring, summer and autumn. Procambarus
clarkii is an important food resource for mammals and birds and it seems to play a key role in the trophic interactions of the riparian and terrestrial communities of the marsh.
A method is described for the identification of arthropod fragments in the faeces of gamebird chicks. The method of collection and preparation of faecal samples and the identification of commonly occurring arthropod parts is described. The use of diagnostic parts, specific to certain groups, enables the method to be used quantitatively as well as qualitatively. Photographs of the major diagnostic features are provided.
1. Secondary seed dispersal occurs whenever a seed is dispersed in two or more different dispersal events, so that different dispersal agents (e.g. animal frugivores or inverte-brates) contribute to different events. Three secondary seed dispersal systems, in which lizards and predatory birds participate, are studied in Lanzarote (Canary Islands). 2. Seeds from all three plant species studied (Lycium intricatum , Rubia fruticosa and Asparagus nesiotes) were found with the remains of lizards (Gallotia atlantica) when they appeared inside shrike (Lanius meridionalis) and kestrel (Falco tinnunculus) pellets. This suggests that these two avian predators might secondarily disperse these plants by incidentally ingesting seeds when they prey upon the frugivorous lizards. 3. Lycium and Rubia seeds dispersed by Falco had significantly thinner seed coats com-pared to the other treatments (Gallotia and Lanius) and to uningested seeds. Correlated with this result, seed hardness increased from Lycium (the softest coat), to Rubia (inter-mediate values), to Asparagus. 4. Seed viability was high in all three species, although both viability and germination were significantly reduced in seeds of Lycium and Rubia dispersed by Falco . 5. The seed distribution was markedly different, Gallotia being an important disperser in the open ground microhabitats, Lanius in hillocks and Falco in hills. Because all three plant species were present in all these microhabitats, our observations suggest that the dispersal of these plants might be associated with the differential use of the habitat by the different dispersers. 6. Synthesis: Contrary to some long-distance seed dispersal (LDD) paradigms, the complex seed dispersal systems we describe are common and affect an important number of seeds that are moved each year to particular microhabitats. Finally, the simultaneous ecological factors analysed in this study support the hypotheses that diplochory by double endozoochory could have played a more important role in LDD events than is currently recognized, both in recent volcanic areas (lowlands) and also probably in the colonization of other subtropical islands.
Indirect seed dispersal in the system Lycium intricatum (Solanaceae)–lizards (Gallotia atlantica)–shrikes (Lanius excubitor) was studied in Alegranza, a xerophytic small island of the Canarian archipelago.
A total of 835 seeds (224 obtained from lizard droppings and 611 from shrike pellets) was found by analysing 123 droppings and 146 pellets. Lycium fruit remains (including seeds) were observed in 31.7% of the lizard droppings and 50% of the shrike pellets.
As would be expected for legitimate seed dispersers, external seed damage produced by each species was negligible. Lycium seeds were significantly matched with the presence of lizard remains in shrike pellets. Seeds in shrike pellets have come from fruit consumed by lizards that have been predated by the shrikes, and not directly from bird frugivory.
Seeds from shrike pellets showed significantly higher germination rates than those from uneaten fruits and lizard droppings.
It seems probable that different treatments in the guts of each species and retention time are two main factors influencing the germination process. While gut pass time in G. atlantica has been estimated to be 2.42 days on average, the retention time of a seed inside a shrike gizzard is much shorter (45–55 min).
This is a new case of the scarcely known phenomenon of indirect seed dispersal, which seems to acquire a relatively important role in small islands where the animal–plant interaction is very intense and all the elements of the system are native.
This paper presents the first data on the diet of the Raven Corvus corax in the Canary Islands, based on analysis of 2315 pellets collected from all of the islands and islets in the archipelago. The Ravens on three groups of island showed differences in diet, but in all cases the diet was clearly omnivorous, both in its qualitative and quantitative composition. It is evident that a broadening of the Raven's trophic niche has occurred in comparison with those living in continental areas. While the diet has a strong plant component on the wetter islands (frequency of occurrence > 87%), an appreciable animal component in the diet (principally vertebrates) is more frequent on the drier islands of the archipelago (frequency of occurrence > 96%). The composition of the plant component of the diet varied markedly from one island to another. Remains of refuse are more commonly found in the Raven's diet on the central and western islands. We concluded that while the animal diet is essential from the bioenergetic point of view, the vegetation diet is only a complementary food source. The Raven plays an important role in seed dispersal among the different habitats within each island.
We have studied the unusual phenomenon of secondary seed dispersal of Lycium intricatum seeds on a small oceanic Atlantic island (Alegranza, Canarian Archipelago) in which a small frugivorous lizard (Gallotia atlantica) and two different predatory birds participate, a shrike (Lanius excubitor) and a kestrel (Falco tinnunculus). Endemic lizards that are common prey of both bird species consume Lycium fruits. Lizard remains were significantly matched with the presence of Lycium fruits in the regurgitation pellets of the two predatory birds. Seeds were found in 7.3% of the lizard droppings, 31.0% of kestrel pellets and 55.7% of shrike regurgitations. The mean number of seeds per dropping or pellet was 4.8 ±4 in lizard, 20.2 ±34.5 in shrike and 6.7 ±8.1 in kestrel. The percentage of viable seeds showed significant differences among all four treatments, decreasing in the following direction: seeds collected directly from plants (98.0%), shrikes (88.0%), lizards (72.3%), and kestrels (31.7%). Seeds from Lycium fresh fruits and shrike pellets showed significantly higher germination rates than those from lizard droppings and kestrel pellets. While lizards and shrikes are effective seed dispersers, kestrel gut treatment decreases seed viability. Seed viability is always higher than seed germination in each of the four treatments. In this island environment, Lycium seeds are under an important random influence during the seed dispersal process. Secondary seed dispersal seems to acquire a relevant dimension in small and remote insular environments or isolated continental systems where interactions among the different elements involved are intense, all of them are abundant native residents, and they have been coexisting for a long time. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 75, 345–352.
Hen Harriers Circus cyaneus and Montagu's Harriers Circus pygargus are medium-sized raptors that differ in size (Hen Harrier being slightly bigger than Montagu's Harrier) and breeding system (Montagu's Harriers are semi-colonial and Hen Harriers defend nesting–hunting territories). In contrast, the diets of the two species when in sympatry are very similar. We evaluated food-niche differentiation among these coexisting raptor species and how between-species differences in body size and social system influence interspecific relationships. We present data from a study conducted in 1997 and 1998 in northeastern Madrid province (central Spain). Diet of the two species largely overlapped (55–95%) during the breeding season, but Hen Harriers preyed more often on larger species. This segregation was observed both in the average size of the primary prey (lagomorphs) and in the alternative prey (birds for Hen Harriers vs. insects for Montagu's Harriers), and was particularly apparent late in the season. Accordingly, feeding frequency of Montagu's Harriers, but not of Hen Harriers, increased later in the season. Size differences between species in prey brought to the nest were apparent for both males and females. Foraging behaviour also differed, as Hen Harriers spent more time hunting close to the nest than did Montagu's Harriers. This implies that segregation in foraging areas may also exist. Observed niche partitioning may relax the potential for competition between these species.
A graphical method is discussed which allows a specification of the optimal diet of a predator in terms of the net amount of energy gained from a capture of prey as compared to the energy expended in searching for the prey. The method allows several predictions about changes in the degree of specialization of the diet as the numbers of different prey organisms change. For example, a more productive environment should lead to more restricted diet in numbers of different species eaten. In a patchy environment, however, this will not apply to predators that spend most of their time searching. Moreover, larger patches are used in a more specialized way than smaller patches.
Phylogenetic relationships among species and subspecies of the Canary Island endemic lizard genus Gallotia are inferred based on nucleotide sequences of fragments of 12S ribosomal RNA and cytochrome b mitochondrial genes. The four morphologically established species have also been recognized at the molecular level. Relative affinities among species follow an eastern-western geographic transect. The nearly extinct species Gallotia simonyi from the most western island of El Hierro is closely related to the common western species Gallotia galloti, the nearest branch to this pair is Gallotia stehlini from the central island of Gram Canaria, and finally, Gallotia atlantica from the two eastern and geologically oldest islands appears as the most distantly related species of the group. At the statistical level, four subspecies can be recognized in G. galloti, but only two in G. atlantica.
Insectos de Canarias. Cabildo Insular de Gran Canaria Phylogenetic relationships of the Canary Islands endemic lizard genus Gallotia (Sauria: Lacertidae), inferred from mitochondrial DNA sequences
- Ortega R Becerra
- Pé G Muñ
- Gran Sá Jm
- Canaria P Gonzá
- Nogales F M Pinto
- Jimé-Asensio Jj
- Herná
- Cabrera
Becerra R, Ortega Muñ G, Pé Sá JM (1992) Insectos de Canarias. Cabildo Insular de Gran Canaria, Las Palmas de Gran Canaria Gonzá P, Pinto F, Nogales M, Jimé-Asensio JJ, Herná, Cabrera VM (1996) Phylogenetic relationships of the Canary Islands endemic lizard genus Gallotia (Sauria: Lacertidae), inferred from mitochondrial DNA sequences. Mol Phyl Evol 6:63–71