David Hořák

Factors Determining Pochard Nest Predation Along a Wetland Gradient

Abstract Waterfowl management on breeding grounds focuses on improving nest success, but few studies have compared waterfowl nest success and factors affecting nest survival along a wetland gradient and simultaneously identified nest predators. We monitored nests (n = 195) of common pochards (Aythya ferina) in Trebon Basin Biosphere Reserve, Czech Republic, during 1999–2002. Daily nest survival rates (DSRs, logistic-exposure) declined from island (0.985, 95% confidence interval, 0.978–0.991) to overwater (0.962, 0.950–0.971) and terrestrial (0.844, 0.759–0.904) nests. The most parsimonious model for DSRs included habitat class (DSRs: island > overwater > terrestrial) and nest visibility. Nest survival was improved by reduced nest visibility, increased water depth, and increased distance from the nest to habitat edge in littoral habitats. On islands, nest success increased with advancing date and increased distance to open water. A model of constant nest survival best explained the data for terrestrial nests. There were no observer effects on DSRs in any habitat. In 2003, artificial nests (n = 180; 120 contained a wax-filled egg) were deployed on study plots. The model that best explained variation in DSRs for artificial nests included only 1 variable: habitat class (DSRs: island ≥ overwater > terrestrial). Mammalian predation of artificial nests (by foxes [Vulpes vulpes] and martens [Martes spp.]) was more likely in terrestrial habitats than in littoral habitats or on islands. By contrast, corvids and marsh harriers (Circus aeruginosus) prevailed among predators of overwater and island nests. Our data indicate that artificial islands and wide strips of littoral vegetation may represent secure breeding habitats for waterfowl because those habitats allow nests to be placed in areas that are not accessible to, or that are avoided by, mammalian predators. Management actions should be aimed at preserving these habitats. This, along with creation of new artificial islands, could help to enhance breeding productivity of pochards and possibly other waterfowl species inhabiting man-made ponds.

Habitat preferences of birds in a montane forest mosaic in the Bamenda Highlands, Cameroon

Although the high species richness and endemism of birds in the Bamenda Highlands has attracted ornithological research for decades, most studies have been restricted to bird communities of continuous montane forests. Instead, we focused on a mosaic landscape with montane forest remnants, where the habitat preferences of birds remain unknown. We performed an assessment of habitat associations of birds in the Bamenda Highlands in the Cameroon Mountains. Using a point count census method, we detected 71 species within the study area. The most abundant species were the Northern Double-collared Sunbird Cynniris reichenowi, the Oriole Finch Linurgus olivaceus, the Common Stonechat Saxicola torquata, the Thick-billed Seed-eater Serinus burtoni, the Black-crowned Waxbill Estrilda nonnula, the Brown-backed Cisticola Cisticola chubbi and the Yellow-breasted Boubou Laniarius atroflavus. Canonical correspondence analysis revealed that the most important environmental gradient structuring the bird community follows the forest coverage. We found that both endemic and non-endemic montane species are more closely associated with montane forest remnants, compared to widespread species. Endemic species are most closely dependent on continuous forest cover. However, some montane species did not show any clear habitat associations and thus can be viewed as local habitat generalists. This study shows that many restricted-range species (including endangered endemics) are able to live in fragmented landscapes, which cover a substantial part of the Bamenda Highlands. Therefore, conservation programmes should focus their action plans on these landscapes.

Sperm competition in tropical versus temperate zone birds

Sperm competition represents an important component of post-copulatory sexual selection. It has been argued that the level of sperm competition declines in birds towards the equator. However, to date, sperm competition estimates have been available mainly for avian species inhabiting the northern temperate zone. Here we apply a novel approach, using the coefficient of between-male variation (CVbm) in sperm size as an index for sperm competition risk, in a comparative analysis of 31 Afrotropical and 99 northern temperate zone passerine species. We found no difference in sperm competition risk between the two groups, nor any relationship with migration distance. However, a multivariate model indicated that sperm competition risk was highest in species with a combination of low body mass and few eggs per clutch. The effect of clutch size was most pronounced in tropical species, which indicates that sperm competition risk in tropical and temperate species is differently associated with particular life-history traits. Although tropical species had lower sperm competition risk than temperate zone species for overlapping clutch sizes, the idea of a generally reduced risk of sperm competition in tropical birds was not supported by our analysis.

A comparison of point counts with a new acoustic sampling method: a case study of a bird community from the montane forests of Mount Cameroon

Acoustic signals are frequently used for estimating avian species richness, abundance and community composition. However, sampling by traditional methods of bird surveys is often limited by availability of experienced researchers in the field, especially in the tropics. New bioacoustic approaches offer some solutions to such limitations and provide opportunities for more extensive spatial and temporal sampling. In our study, we compared results of traditional point counts with simultaneous acoustic samples obtained by automated soundscape recording units in the montane forest of Mount Cameroon. We showed that the estimates of species richness, abundance and community composition based on point counts and post-hoc laboratory listening to acoustic samples are very similar, especially for a distance limited up to 50 m. Species that were frequently missed during both point counts and listening to acoustic samples were typically those with relatively quiet songs. Abundances were rather underestimated by listening to acoustic samples in the most abundant species, including those occurring in flocks and species with low singing activity. Despite some possible biases, we demonstrated that the method based on listening to acoustic samples is relatively effective and offers a useful alternative approach for surveying Afromontane bird communities.

Geographic variation in avian clutch size and nest predation risk along a productivity gradient in South Africa

Geographic variation in avian clutch size is thought to be related to the variation in nest predation rate and food availability. We studied predation on artificial ground nests along a large-scale geographic gradient in South Africa characterised by increasing productivity from the deserts in the west to humid savannas in the east, and calculated mean clutch sizes of birds occurring in atlas quadrates surrounding our study sites. Clutch sizes generally increased with increasing productivity and seasonality. The least productive desert site was characterised also by the highest predation rate, whereas all the other sites located in savannas revealed much lower and more or less constant predation rate. We found no evidence for relationship between nest predation rates and clutch sizes of ground-nesting birds. This indicates that food availability is the major factor responsible for geographical variation in bird clutch sizes across South Africa, though high predation rate might also contribute to low clutch size in least productive arid environments.

Food niche differentiation in two syntopic sunbird species: a case study from the Cameroon Mountains

Closely related bird species can coexist in areas of range overlap due to differentiation of their ecological niches. If coexisting species have similar habitat requirements, separation of food niches presumably plays a crucial role. Theoretically, two possible food niche separation scenarios are possible: (1) use of different food resources or (2) temporal differences in feeding activity. We examined these mechanisms by investigating the feeding habits of two short-billed sunbirds (Cinnyris reichenowi and C. bouvieri) coexisting locally in the Bamenda Highlands, NW Cameroon. Daily feeding activity in both species showed a similar pattern, with two peaks (0800–0900 hours and 1500–1600 hours, respectively) and a prominent decrease in activity between 1100 and 1400 hours. However, the studied species clearly differed in their exploitation of plant resources. C. bouvieri mostly visited Lobelia columnaris, while C. reichenowi regularly visited three plant species, namely, Hypericum revolutum, Hypoestes aristata and Impatiens sakerana, with the most time spent at the second species. Such resource partitioning was probably caused by interspecific aggressive behaviour, since C. bouvieri actively repelled C. reichenowi from Lobelia columnaris plants, forcing C. reichenowi to feed on other plant species.ZusammenfassungNahe verwandte Vogelarten können in Gegenden, in denen sich ihre Vorkommen überschneiden, aufgrund einer Differenzierung ihrer ökologischen Nischen koexistieren. Falls koexistente Arten ähnliche Ansprüche an den Lebensraum aufweisen, kommt einer Trennung der Nahrungsnischen höchst wahrscheinlich die Schlüsselrolle zu. Theoretisch sind zwei mögliche Szenarien der Nahrungsnischentrennung denkbar: (1) die Nutzung unterschiedlicher Nahrungsressourcen, oder (2) zeitliche Unterschiede in der Nahrungsaufnahme-Aktivität. Wir erforschten diese Mechanismen indem wir die Ernährungsgewohnheiten zweier kurzschnäbliger Nektarvogelarten (Cinnyris reichenowi and C. bouvieri), die im Bamenda Hochland, Nordwestkamerun lokal gemeinsam vorkommen, untersuchten. Die tägliche Nahrungsaufnahme-Aktivität folgte bei beiden Arten einem ähnlichen Muster mit zwei Gipfeln (zwischen 08:00–09:00 bzw. 15:00–16:00 Uhr) und einem deutlichen Aktivitätsabfall zwischen 11:00 und 14:00 Uhr. Die beiden untersuchten Arten unterschieden sich aber klar in der Ausnutzung der Pflanzenressourcen. C. bouvieri besuchte überwiegend Lobelia columnaris, wohingegen C. reichenowi drei Pflanzenarten regelmäßig besuchte: Hypericum revolutum, Hypoestes aristata und Impatiens sakerana, dabei verbrachte sie an der zweiten Art die meiste Zeit. Diese Art der Ressourcenaufteilung hatte ihre Ursache wahrscheinlich in interspezifischem Aggressionsverhalten, da C. bouvieri und C. reichenowi aktiv von Pflanzen der Art Lobelia columnaris vertrieb und C. reichenowi dadurch zwang an anderen Pflanzenarten zu fressen.

Using stable isotopes to trace resource acquisition and trophic position in four Afrotropical birds with different diets

(2010). Using stable isotopes to trace resource acquisition and trophic position in four Afrotropical birds with different diets. Ostrich: Vol. 81, No. 3, pp. 273-275.

Alien Egg Retrieval in Common Pochard: Do Females Discriminate between Conspecific and Heterospecific Eggs?

Incorporation of unrelated eggs into a clutch by incubating females (egg retrieval), which has an obvious adaptive value when female retrieves her own egg, seems to be also a part of the reproductive tactics related to brood parasitism. In open nesting waterfowl, the parasitic egg remains frequently outside the nest bowl after the parasitic event. Using time-lapse video recorders, we described experimentally the behavioural reaction of the common pochard (Aythya ferina) females towards an egg lying beside the nest. We tested whether the females discriminate between conspecific and heterospecific eggs (brown chicken eggs). All 16 experimental females retrieved both conspecific and heterospecific eggs. We found no apparent differences in female responses towards either egg type. The retrieval of alien eggs can be a result of an imperfect recognition ability of the female, anti-predation defence, or sophisticated tactic related to the brood parasitism. The last explanation seems to be less likely due to imperfect egg recognition abilities in the species.

Evidence for an Edge Effect on Avian Nest Predation in Fragmented Afromontane Forests in the Bamenda-Banso Highlands, NW Cameroon

Habitat loss and fragmentation in tropical areas have long been presumed to reduce avian nesting success due to increased predation rates. Nevertheless, this prediction remains largely untested in tropical areas, because empirical data on the impacts of forest fragmentation on nest predation at both the landscape and patch scales in the tropics are still scarce, especially in West Africa. In this study, we examined the edge effects on nest predation rates in a large montane forest block and small forest fragments. Artificial nests used for our experiments mimicked the real nests of passerines confined to montane forest undergrowth in the Bamenda-Banso Highlands, the Northwest region of Cameroon, an endemic bird area of high conservation priority. We found equal overall predation rates in the landscape dominated by the large forest block as well as in the landscape consisting of small forest fragments, implying that the probability of nest failure was not significantly affected by habitat fragmentation on a landscape scale. However, predation rates were higher close to forest edges in the small forest remnants. Since such remnants represent the majority of local montane forests, this result suggests that the edge effect on bird nest predation may reduce nest survival and the population viability of many range-restricted bird species confined to the remaining natural habitats of the Bamenda-Banso Highlands.

Incubation temperature influences trade-off between structural size and energy reserves in mallard hatchlings.

The reproductive success of precocial birds depends on investments in clutch formation and incubation. Egg quality strongly affects the phenotypic traits correlated with survival of the hatchling, but parental ability to maintain incubation temperature can also influence hatchling outcomes. The effect of incubation temperature on hatchling phenotype has been widely studied in reptiles but not in birds. The aim of this study was to explore the effects of egg mass and incubation temperature on the incubation period, hatchability, and hatchling phenotype of the mallard (Anas platyrhynchos). Mallard eggs were incubated under six constant incubation temperatures (ranging from 35.0° to 39.0°C). Hatchlings were weighed, and their structural size was measured. Some hatchlings were used for an examination of residual yolk sac mass and basic chemical composition of the yolk-free body. All investigated phenotypic traits except for chemical composition were positively correlated with egg mass. Incubation temperature did not affect hatchling body mass, but increased temperatures led to a decreased yolk-free body mass and structural size of hatchlings and to increased yolk sac mass. Our results suggest that there is a trade-off between the yolk-free body size and energetic reserves in the form of the yolk sac and that this trade-off is modulated by incubation temperature.