João E. Rabaça

Superpredation patterns in four large European raptors

Predatory interactions among top predators, like superpredation or intraguild predation (IGP), can influence community structure. Diurnal raptors occupy high trophic levels in terrestrial food webs, and thus can regulate the presence of mesopredators. We studied superpredation (the killing and eating of another predator) in four large European raptors. We gathered 121 dietary studies, totalling 161,456 prey for the Goshawk Accipiter gentilis L., Golden Eagle Aquila chrysaetos L., Bonelli’s Eagle Aquila fasciata Vieillot, and Eagle Owl Bubo bubo L. Results showed that superpredation: (1) is a widespread interaction in large raptors, but it can vary according to the top predator species; (2) is not an important energetic resource for large raptors, but rather seems mostly related to diet diversification when the main prey decreases; (3) is spatially clustered reflecting habitat heterogeneity, but shows no temporal or large-scale spatial trends; and (4) it is associated with lower breeding success of the top predator species. These findings support the food stress hypothesis as the main driving force behind increases in superpredation and IGP in raptors, with the decrease in breeding performance as a side effect. Superpredation by large raptors deserves future research to understand its effects on mesopredators, because on one hand it might contribute to promote biodiversity, while on the other hand, it can sometimes represent an additional risk for small populations of endangered mesopredators.

Birds like it Corky: the influence of habitat features and management of 'montados' in breeding bird communities

In the southwest part of the Iberian Peninsula the dominant land-use are the Portuguese montados and Spanish dehesas, parkland forested areas of anthropogenic origin dominated by cork oak. They form a wooded matrix with open areas, scattered woodlands and undisturbed patches of Mediterranean forest and scrublands. The montados are characterized by a rich bird community. We have focus our study in a multidisciplinary approach, evaluating how management and landscape patterns influence the bird community in order to identify potential threats to its conservation. The study was conducted in the Site of Community Importance of Serra de Monfurado were 70% of the area is cork and holm oaks. We used data from 120 10-min point counts. Using variation partitioning, we determined the independent and joint effects of Forest, Management and Habitat variables. The variation captured in bird assemblage was 65.06%. Most of the explained variation was related to the Habitat and Management variables. The explanatory variables that were high-lighted as important predictors were variables that reflected tree and shrub density and cork removal. Modelling for forest species through generalized linear models (GLM) emphasize that the management plays an important role in the species distribution. The most important variables selected in models reflected cattle grazing and cork removal. Our results point out that the type of management is crucial to maintain the equilibrium in bird community associated to montados. Farmland and forest species will benefit from areas with different tree densities, small patches of Mediterranean scrubs in the understory and correct livestock numbers.

Major roads have a negative impact on the Tawny Owl Strix aluco and the Little Owl Athene noctua populations

Abstract. The increasing road networks threaten ecosystems by direct effects such as increased mortality due to collision with vehicles and by various indirect effects leading to road avoidance. We censused Tawny Owls Strix aluco and Little Owls Athene noctua in 2005, 2007 and 2009 in a rural landscape in Southern Portugal in order to study the effects of roads and habitat characteristics on Tawny Owl density and Little Owl presence. The presence of both owl species in the 70 census locations was coherent among years. Our results showed that Tawny Owl density near major roads was lower, with the negative effects extending possibly up to 2 km. The probability of Little Owl presence was also negatively affected by the proximity to major roads. The negative effects of roads were significant even considering habitat preferences and spatial autocorrelation, which had the most marked effect on the density or presence of both owls. The reduced occupancy by Tawny Owls and Little Owls of habitats near major roads may be caused by several factors, including increased mortality, disturbance caused by high traffic density, and increased fragmentation. Traffic noise in particular may affect intra-specific communication and hunting efficiency. Consequently, habitat near roads may represent lower-quality territories for owls.

Breeding bird assemblages in riparian galleries of the Guadiana River basin (Portugal): the effect of spatial structure and habitat variables

Habitat structure is a major determinant of bird community composition and a link between riparian vegetation and bird communities in riparian zones, suggesting that changes in bird community composition can be predicted from changes in habitats. Our study aimed to identify the relationships between the structure of breeding bird communities and environmental drivers, using fine-resolution habitat measurements. We examined two bird assemblages (Passerines, P; and Aquatic/Riparian species, AR) and their spatial structure and relationships to environmental river variables in riparian galleries in southern Portugal. The study was conducted on the ca. 230xa0km of riparian corridors located in the flooded area of the Alqueva reservoir, before closure of the dam. We used data from 134 10-min point counts. Using variation partitioning, we determined the independent and joint effects of river edge, river bed, and spatial variables on our selected bird assemblages. The variation in bird assemblages captured was 27.5% for P and 55.3% for AR. The explanatory variables that were highlighted as important predictors for both bird groups were variables that reflected the complexity of the riparian gallery vegetation: rivers with high vegetation cover of riparian species, and riverbeds and riversides dominated by rock and scarce emergent rooted vegetation. Most of the explained variation was related to environmental variables: river-edge, river-bed, and their joint effect. Riparian areas can be considered a specific habitat and the variables analyzed are habitat descriptors at a fine scale, so that we believe that the species–environment relationships revealed in this study could be good indicators of the wealth of riparian ecosystems in Mediterranean rivers.

Time to rethink the management intensity in a Mediterranean oak woodland: the response of insectivorous birds and leaf-chewing defoliators as key groups in the forest ecosystem

Abstract• ContextThe Iberian cork oak Quercus suber montados are dynamic agro-silvo-pastoral systems, contrasting with the abandonment trend of other Mediterranean forested areas.• AimsWe aimed to identify the effect of management type and vegetation features on breeding insectivorous birds and leaf-chewing defoliator insects.• MethodsIn central Portugal, we selected two groups of 20 sites: dense montados (DM, high cover of cork oaks and low cattle impact) and sparse montados (SM, low cover of oaks and high cattle impact). We collected variables associated with vegetation features, bird community and chewing defoliators.• ResultsThe two systems differed significantly in oak trunk diameter, oak and shrub cover, tree and shrub diversity. Ground bird densities were higher in SM with fragmented shrub cover. In DM, higher woody vegetation cover and diversity can promote shrub birds and leaf warblers. Oak defoliation was mainly attributed to the sawfly Periclista andrei. Sawfly defoliation was significantly higher in SM and negatively correlated with shrub cover.• ConclusionsOur results highlight (1) the relevance of management activities conducted at the shrubby layer, (2) the need of an increase in tree diversity and density and (3) the maintenance of a mosaic landscape in order to improve the bird community richness and reduce the defoliation impact in montado system.

Kill before being killed: an experimental approach supports the predator-removal hypothesis as a determinant of intraguild predation in top predators

Intraguild predation (IGP) has been explained in terms of competitor-removal, food-stress and predator-removal hypotheses. Only the first two hypotheses have been fairly well studied. To test the predator-removal hypothesis as a force determining IGP in avian predators, we performed a field experiment to simulate the presence of an IG predator (eagle owl Bubo bubo dummy) in the surrounding of the nests of four potential IG prey (black kite Milvus migrans, red kite Milvus milvus, booted eagle Aquila pennata and common buzzard Buteo buteo). To discard the possibility that an aggressive reaction towards the eagle owl was not related to the presence of the IG predator, we also presented a stuffed tawny owl Strix aluco, which is a potential competitor but cannot be considered an IG predator of the studied diurnal raptors considered in the experiment. While almost always ignoring the tawny owl, raptors chiefly showed an interspecific aggressive behaviour towards their IG predator. Our results seem to support the predator-removal hypothesis, as the IG prey may take advantage of the diurnal inactivity of the IG predator to remove it from their territory. However, the recorded behaviour may be also considered as a special variety of mobbing (i.e. a prey’s counter-strategy against its predator), where the mobber is sufficiently powerful to escalate predator harassment into deliberate killing attempts. In their turn, eagle owls can respond with an IG predatory behaviour aimed at removing IG prey species which are highly aggressive mobbers.

Can human-made saltpans represent an alternative habitat for shorebirds? Implications for a predictable loss of estuarine sediment flats.

AbstractEstuarine areas worldwide are under intense pressure due to human activities such as upstream dam building. Shorebirds strongly depend on estuarine intertidal flats during migration and wintering periods and so are particularly vulnerable to such impacts, whose magnitude will depend on the availability of alternative feeding habitats. In this study we analyze if man-made saltpans can represent an alternative habitat for wintering and migrating shorebirds in the Guadiana estuary, a wetland that is already experiencing environmental changes due to the building of the Alqueva reservoir, the largest in Western Europe. We compared the use of mudflats and saltpans as feeding areas by several shorebird species before the construction of the dam. A dataset with 26xa0years of counts data was also analyzed in order to detect any long-term trend in shorebirds abundance. We concluded that saltpans, in particular the fully mechanized, can be used as an alternative habitat by larger species during winter and southward migration, thus playing a major role in minimizing the possible effects of sediment loss due to dam building. In contrast, smaller species were particularly dependent on mudflats to feed. A significant change in population trends, from positive to negative, was detected for two species. Although we still have no evidence that this is directly linked to dam building, this result and documented changes that limit primary productivity justifies the implementation of a long-term monitoring scheme of shorebird populations in this estuary. We also reinforce the need to manage the saltpans as key habitats for shorebirds.n

Superpredation increases mercury levels in a generalist top predator, the eagle owl

Superpredation can increase the length of the food chain and potentially lead to mercury (Hg) bioaccumulation in top predators. We analysed the relationship of Hg concentrations in eagle owls Bubo bubo to diet composition and the percentage of mesopredators in the diet. Hg levels were measured in the adult feathers of eagle owls from 33 owl territories in the south-western Iberian Peninsula, and in three trophic levels of their prey: primary consumers, secondary consumers and mesopredators. In addition, we studied 6,181 prey in the eagle owl diet. Hg concentrations increased along the food chain, but the concentrations in eagle owls showed considerable variation. The Hg concentration in eagle owls increased when the percentage of mesopredators in the diet increased and the percentage of primary consumers decreased. Superpredation is often related to food stress, and the associated increase in accumulation of Hg may cause additional negative effects on vertebrate top predators. Hg levels in these eagle owl populations are relatively low, but future monitoring is recommended.

Waterbird-mediated passive dispersal is a viable process for crayfish (Procambarus clarkii)

Human transport and active dispersal of the red swamp crayfish (Procambarus clarkii) contribute to its rapid spread. However, some small aquatic organisms can be transported by birds. We made two hypotheses related to waterbird-mediated passive dispersal of juvenile crayfish. The first is that, depending on water depth, recently hatched crayfish can attach to ducks, initiating passive external transport (i.e., ectozoochory). The second is that recently hatched crayfish can survive bird flight, being affected by crayfish features, flight distance, and environmental conditions. A first experiment tested the attachment of juvenile crayfish to ducks at different water depths by using a freshly dead duck and tanks with crayfish. Another set of three experiments tested crayfish survival during air transportation. To simulate bird flight, we first used a vehicle moving at bird flight speed, and we then used trained pigeons. Several flight distances, environmental conditions, and crayfish sizes were tested. Our results showed that juvenile crayfish were capable of clinging to duck feathers and were transported when ducks were removed from the water. Furthermore, some juveniles of P. clarkii were able to survive long-distance transport when suspended outside a moving vehicle or when transported by birds. The probability of success was affected by water depth, crayfish size, distance travelled, and relative humidity. Our results support the occurrence of passive transportation of this invader by means of attachment to birds. These findings indicate that waterbird-mediated passive dispersal should be taken into account to explain P. clarkii’s rapid spread and should be considered when managing its invasions.

The importance of the surroundings: are bird communities of riparian galleries influenced by agroforestry matrices in SW Iberian Peninsula?

ContextThe remaining riparian stretches are often the unique suitable habitats for forest breeding birds in Mediterranean landscapes undergoing long-term changes. Understanding the interactions between riparian zones and their surroundings is critical to establish successful management actions.AimsWe assessed the influence of surrounding matrix on riparian bird communities and the use of riparian galleries as surrogate habitats for the forest passerine communities of southwestern Iberia.MethodsWe used point counts in three simultaneous sampling stations, one in the riparian gallery and two in the adjacent matrix. Three matrix types were selected with a decreasing tree density gradient: dense montado, sparse montado, and open agricultural areas. Data were analysed with redundancy analysis and differences in species’ occurrences were tested with one-way ANOVA.ResultsIn riparian galleries, we found bird species belong to three ecological guilds: riparian, woodland, and edge guilds. The richness of bird guilds and the occurrence of some species depended on the surrounding matrix type. Riparian bird richness was constant in all surrounding matrices, woodland bird guild was richer in galleries embedded in dense montados, and edge guild in riparian galleries surrounded by sparse montados. Five among 19 assessed species were influenced by matrix type, within particular a few strictly riparian species. Species richness increased close to riparian galleries, due to the increase in habitat heterogeneity and resource availability for birds in densely vegetated riversides. However, the occurrence of some bird species differed according to the type of surrounding matrix.ConclusionThe matrix type explained most of the variance in riparian bird assemblages. Some woodland birds have used riparian galleries as surrogate habitat. Our study suggests that land use in the surrounding matrices must be taking into account for the management and rehabilitation of watercourses and bird conservation actions.