Research team:
Coordinator
Researcher´s:
Mgr. Klára Poláková
Quantity and surviving of young water birds in the conditions of fragmented pond and lake localities (ACTIVITY 5)
The goal of ACTIVITY 5 is an analysis of factors influencing the survival of youngsters in families led by individually marked females of diving ducks (mainly Common Pochard and Tufted Duck) with the use of data collected while observing the marked females of both mentioned kinds in 2002-2014 and during the ongoing project.
Research state:
The aim of the project was to evaluate the factors influencing the numbers and survival of chicks in water bird families. The collection of data in the field took place at 175 fishponds in the regions of Třeboň, Soběslav and Jindřichův Hradec and was thus a continuation of the monitoring of nesting populations of water birds that has been taking place in South Bohemia since 2005. The census was carried out at regular (10-14 day) intervals from April to August and consisted of nine surveys during which not only the total numbers of birds of each species but also the percentages of males and females were identified, including identification of individually marked birds. In addition to that, data on water bird families, i.e. the number and ages of chicks in each family, were recorded. This method was used not only to monitor the numbers of adult birds but also to monitor the numbers and survival of chicks in their families.
In the selected fishpond localities, nests were also sought out and females were caught in their nests. The females were measured, weighed and individually marked, which allowed for their future monitoring. In the years 2015 and 2016, a total of 10 red-crested pochard females, 9 common pochard females and tufted duck females were caught in the central part of the Třeboň basin. From among them, 10 red-crested pochard females, 7 common pochard females and 29 tufted duck females hatched their young. Subsequently, 80 % of red-crested pochard females, 57 % of common pochard females and 90 % of tufted duck females with or without chicks were caught.
The data obtained in this way were then used for the Analysis of the Impact of Distribution of Food Supply on the Reproduction Success Rate and Population Dynamics of Water Bird Nesting Populations. The impact of environmental and climatic conditions on the numbers of families and chicks of each water bird species was tested. Water transparency, which can be regarded as the indicator of the trophic situation in individual fishponds, had the most significant impact on the number of families. Its significance was highest particularly in June and July when there is a strong food consumption pressure, particularly from carps, due to high fish stock density, and this significantly reduces the supply of food for water bird chicks. The zooplankton density is also significantly reduced and as a result, zooplankton is then unable to prevent extensive growth of plankton algae. High levels of density of plankton algae are the very cause of decrease of water transparency. When there is low water transparency, duck chicks are unable to find suitable food. For some species, a positive impact of other environmental factors was also proven, such as the size of the water area, the average water depth, the number of islands and the portion of wetland and open landscape in the surroundings.
Based on individual marking, it was found out that the survival rate of chicks is the lowest from hatching to the 10th day of their lives; the overall survival probability was 0.395 for common pochard and 0.383 for tufted duck. The survival was negatively correlated with the distances travelled by the ducklings; the most frequent movement was within 2 km from the nesting pond, and the maximum distance covered by chicks was 5 km.
The impact of alternative fish stocking (without carps) with predominance of zander and additions of tench and other fish species, which was applied in the Rod fishpond in the Třeboňsko Protected Landscape Area (PLA), was also tested. In this fishpond, the zooplankton was not completely consumed and water transparency remained high throughout the nesting season. Understandably, water birds, particularly diving ducks, reacted to this favourable trophic situation and this fishpond became a key locality for them within the entire Třeboňsko PLA as well as the surrounding areas. The numbers of females leading chicks significantly exceeded the numbers in nearby fishponds. A higher survival rate of chicks was also identified here. It was also proven that the Rod fishpond is very important for herbivorous species of birds (such as Eurasian coot, gadwall and mute swan) whose numbers culminated (reaching the number of several hundred individuals) on the Rod fishpond in the post-nesting period and at the beginning of the autumn migration.
In both years mentioned above, the number of tufted duck females identified at the beginning of their nesting season as well as the total number of females leading chicks on the Rod fishpond significantly exceeded similar data for the last 10 years. It is also remarkable that in 2015, the number of females leading chicks on the Rod fishpond was significantly higher than the number of females identified at the beginning of the nesting season. This finding also suggests that females nesting at surrounding fishponds apparently relocated here.
Another remarkable finding was a proof that several individuals from among tufted ducks, common pochards and common goldeneyes spent the winter (January and February 2016) on this fishpond, including a female that was marked in her nest on the Naděje fishpond in July 2015 and was leading her chicks on the Rod fishpond later in July and August 2015. Apparently, there was enough food here for diving ducks even during winter.
The results of these analyses can be used to prepare the Proposal for Creating an Optimum Network of Localities Suitable in Terms of Food, Which Can Be Applied in the Conditions of a Fragmented Pond Landscape. Water birds, particularly diving duck females, are able to utilise fragmented food sources, if there are optimum food habitats (carp-free fish stock, high level of water transparency even in July) within a reachable distance from the nesting sites of individual species in our fishpond landscape.
Based on these findings, it is possible to propose the management for protection of the nesting populations of diving ducks in South Bohemia. It would be suitable to reduce the size of fish stock in some fishponds and to change the species structure of fish, which would reduce competition for food between ducks and carps and which would also ensure higher water transparency. This measure would support the growth of water macrophytic vegetation, which not only serves as the hiding place for bird families but there is also a number of invertebrate animals relying on this vegetation who are an important component of the food of diving ducks, especially their chicks. Water plants are also a part of the diet of some water birds, such as red-crested pochards. These extensively managed fishponds, combined with fingerling ponds, should be within a distance of 5 km from each other and should be interconnected with suitable bio-corridors (water streams, canals) so that water bird families are able to move between them without significant losses of their young. Furthermore, these suitable localities should not be at the edges of fishpond systems but preferably in their central part, surrounded by other wetlands. Islands should be maintained so that they are not overgrown with vegetation and can thus provide sufficient nesting opportunities for water birds.
Moreover, the results of the monitoring of individually marked common pochard females and particularly tufted duck females prove that appropriate management of even a single fishpond (the Rod fishpond in the years 2015 and 2016) can be of considerable significance for the trend in the nesting populations of these species within the entire Třeboňsko region, mainly due to a higher survival rate of chicks and a high degree of fidelity (inclination to return to the nesting site) on the part of the nesting females.
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