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For a study of long-distance migrants in sub-Saharan Africa, a census method was developed that combined precision and accuracy regarding bird numbers and tree choice. The number of birds present in trees and shrubs can be counted accurately, although it is time-consuming. We describe how much time is needed to detect all birds present in trees, using data collected in over 2000 plots across West Africa during the dry season (October–March in 2007–2015). The observation time per tree depended on tree size, number of birds present and the opacity of the crown. The giving-up time of the observers increased with canopy volume, but was independent of the number of birds in a tree. Detection probabilities of bird species differed relative to microhabitat choice and feeding techniques. Species-specific detectabilities hardly varied during the day or the season. All foraging birds and immobile birds (save a few percent in dense canopies) were detected using the individual-tree-approach. Bird density is expressed as number per canopy volume, but little information is lost when density is given as number per canopy surface. The variation in bird density was large and differed per tree species. Within tree species, bird density was related to the opacity of the crown, the abundance of insects and whether there were berries or flowers. These findings suggest that, to collect biologically relevant information, the density of tree-dwelling birds should be measured at the level of the individual tree, and not per surface area, habitat type or tree species (as is typical in published studies).
In West Africa, tree preferences of wintering migratory birds (and African residents) were quantified in order to assess the importance of wintering conditions on distribution, abundance and trends of insectivorous woodland birds. This study encompassed 2000 plots between 10–18°N and 0–17°W, visited in October-March 2007–2015, and covered 183 woody species and 59 bird species. Canopy surface (measured in a horizontal plane) and birds present were determined in 308,000 trees and shrubs. Absolute bird density amounted to 13 birds/ha canopy, on average, varying for the different woody species between 0 and 130 birds/ha canopy. Birds were highly selective in their tree choice, with no insectivorous birds at all in 65% of the woody species. Bird density was four times higher in acacias and other thorny species than in non-thorny trees, and seven times higher in trees with leaves having a low crude fibre content than in trees with high crude fibre foliage. Salvadora persica shrubs, but only when carrying berries, were even more attractive. Overall, densities of migratory woodland birds were highest in the (thorny) trees of the Sahelian vegetation zone. This counterintuitive finding, with highest numbers of wintering birds in the driest and most desiccated parts of West Africa (short of the Sahara), also known as Moreau's Paradox, can be explained by the foliage palatability hypothesis. The Sahelian vegetation zone has always been subject to heavy grazing from large herbivores, and as a consequence woody species have evolved mechanical defences (thorns) to withstand grazing of large herbivores, at the expense of chemical defence against arthropods. South of the Sahel, with a much lower grazing pressure, thorny trees (rich in arthropods) are replaced by (usually non-thorny) trees with less palatable foliage and a higher crude fibre content, and hence with less arthropod food for insectivorous birds.
The survival probability of adult Scops Owls was studied in NW Italy, near the northern limits of the European breeding range. The species reappeared in the study area at the beginning of this century, after more than 30 years of absence as a breeding bird. Scops Owls were captured at a bird-ringing station that operated continuously from 1990, and apart from three sporadic captures in the nineties, regular presence was detected only from 2002. Survival estimates were based on 190 captures of adult birds obtained in twelve years (2003–2014). After allowing for the presence of transients, the model with constant survival and constant recapture probabilities suggested an adult apparent survival probability of 0.58 ± 0.07 (SE), with no sex differences. This is the first Scops Owl survival estimate, and it puts the species within the lower range of owl survival estimates, as partially expected by its small size and its migratory habits.
Edge effects on nesting success have been documented in breeding birds in a variety of contexts, but there is still uncertainty in how edge type and spatial scale determine the magnitude and detectability of edge effects. Habitat edges are often viewed as predator corridors that surround or penetrate core habitat and increase the risk of predation for nearby nests. We studied the effects of three different types of potential predator corridors (main perimeter roads, field boundaries, and ATV trails within fields) on waterfowl nest survival in California. We measured the distance from duck nests to the nearest edge of each type, and used distance as a covariate in a logistic exposure analysis of nest survival. We found only weak evidence for edge effects due to predation. The best supported model of nest survival included all three distance categories, and while all coefficient estimates were positive (indicating that survival increased with distance from edge), 85% coefficient confidence intervals approached or bounded zero indicating an overall weak effect of habitat edges on nest success. We suggest that given the configuration of edges at our site, there may be few areas far enough from hard edges to be considered ‘core’ habitat, making edge effects on nest survival particularly difficult to detect.
The focus of most research on the influence of recent climate change on birds has been on the northern hemisphere. Climate change has been different in the southern hemisphere, prohibiting extrapolation from northern research findings — and inference regarding future climate change — to species living there. We investigated the correlation between climatic conditions and survival of a migratory population of African Reed Warblers Acrocephlaus baeticatus in Paarl, South Africa. We used temperature and rainfall in its breeding area, and Normalized Difference Vegetation Index (NDVI) in its wintering area, Central Africa around the Congo Basin. We fitted capture-mark-recapture models for open populations to a 12-year ringing dataset (1998–2010). After accounting for transience — this species exhibits high breeding site fidelity — with a ‘Time-Since-Marking’ model we found a mean survival probability of 0.79 ± 0.04 SE. Rainfall and NDVI did not influence survival in this dataset. Mean temperature (Aug–Apr) had a positive effect on survival: an increase of 1.6°C was associated with an increase of annual survival from 0.69 ± 0.05 to 0.88 ± 0.03. Higher temperatures could have increased local survival by providing more food and breeding habitat, thereby increasing adult body condition and reducing foraging costs, predation and territorial conflicts. Even though we would need data on abundance and reproduction to quantify the effects of climatic conditions on population growth, we found a clear effect of climatic variation on a key demographic parameter, adult survival.
Along several migratory corridors in Europe, the Common Crane Grus grus exhibits a variety of migratory and roost site selection strategies. In this study we determined which environmental factors explained roost-site usage of Common Cranes in 64 fishponds and 25 marshes suitable for Crane roosting inside the Hortobágy National Park (HNP, Hungary) between 1995 and 2007. HNP has become the largest stop-over site of Cranes in Europe. Despite the relatively high number of potential roost sites, Cranes chose only 44% of suitable wetlands for roosting. Roost site selection of Common Cranes was primarily governed by conservation management measures, wetland type and size, as well as disturbance-related characteristics of the roost-site. Artificially flooded marshes, as well as large fishponds, actively managed through drainage, far from human settlements, are of critical importance for staging Common Cranes. The timing of controlled floods in marshes as well as the matching of drainage for fish-farming to migratory periods of Cranes, serve as suitable management measures for waterbird conservation. In summary, our study emphasizes the importance of managing large, undisturbed wetlands with species-specific management prescriptions in maintaining mass migrations of wetland birds.
Adult sex ratio in ducks is male-biased, probably due to higher mortality of incubating females. In an introduced population of Mandarin Ducks Aix galericulata in Berlin and Potsdam, adult sex ratio (proportion of males) was 0.65 (95% confidence interval 0.64–0.67). Annual apparent survival of 163 colour-ringed birds during 2003–2012 was 0.65 (0.55–0.76) for first year and 0.66 (0.60–0.72) for adult males, and 0.47 (0.25–0.69) for first year and 0.57 (0.48–0.66) for adult females. Female mortality but not male mortality peaked during the breeding season. A Leslie matrix model based on productivity and survival rates in the study population estimated an adult sex ratio of 0.64, or 0.67 when first-year birds were excluded. The similarity between observed adult sex ratio and model estimates based on survival suggests that the male-bias might be fully explained by sex differences in survival.
Lindström A., Green M., Husby M., Kalas J.A. & Lehikoinen A. 2015. Largescale monitoring of waders on their boreal and arctic breeding grounds in northern Europe. Ardea 103: 3–15. doi:10.5253/arde.v103i1.a1
In Figure 2, the first mentioning of Common Redshank, in the fifth panel from the top, should read Common Greenshank.