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The Breeding of the Kestrel, Falco Tinnunculus L., in the Reclaimed Area Oostelijk Flevoland

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For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

This paper is the report of a five-year study of the breeding of the Kestrel in relation to its food supply and other environmental factors in the IJsselmeer polder of Oostelijk Flevoland. The results are mainly based on breeding data obtained with nestboxes erected in three observation areas, each measuring 3 km X 3 km. The food of the Kestrel in Oostelijk Flevoland consists of 9 species of mammals, at least 28 species of birds, 18 species of beetles and some other insects. Their main prey is the Common Vole. At the end of the breeding season juvenile Starlings are also important. Data on the supply of Common Voles were obtained by breakneck captures; other food items are not considered. Kestrels breeding in Oostelijk Flevoland tend to winter mainly within short distances (10 km) of their breeding area. This made it possible to include winter circumstances in the analyses of breeding-bird density and the mean time of laying. It is concluded from regression analyses that the breeding-bird density is related positively to food supply (Common Voles) in winter and spring, negatively to precipitation in these seasons, and positively to temperature in spring. Although the effect of food supply is not significant at the 5% level, it may be assumed that food supply has an effect. The mean time of laying was also found to be related to food supply (Common Voles) in winter and spring, precipitation at that time of the year, and temperature in spring. The birds are early when voles are abundant in this period, when it is dry in winter and spring and when the spring is warm. These relations are significant at the 5% level. Since precipitation seems to impede the hunting activities of the bird and the above-ground activity of the Common Vole, it probably also has an influence on the amount of food obtained. Temperature has an effect on the amount of food needed, which means that at low temperatures in spring less energy is available for the formation of eggs. This conclusion was confirmed by histological examination of the ovaries, which showed that the oocytes start growth at least in the autumn. It was also demonstrated experimentally that scarcity of food retards the development of the oocytes in the ovary. With respect to clutch size, the findings show that late clutches are smaller than early ones, as in many species. As a result, the mean clutch size is related to the mean time of laying, and is therefore dependent on the same environmental conditions as those thought to influence the food supply. Desertion of the clutches by the parents is the most important mortality factor for the eggs. This desertion is considered to be due to the failure of the male to bring the female sufficient food, and is consequently related to food supply (Common Voles) during breeding. Precipitation during this time, in contrast to the situation in winter and spring, has a negligible effect. This lack of influence is attributed to the longer daily period suitable for hunting in this season. Mortality of the nestlings is highest in the first week after hatching. Since food consumption increases during the nestling period, it is not likely that this higher death rate of the young nestlings is due to lack of food. The mortality of the largest broods (6 to 7 nestlings) is higher than those of medium size (4 to 5 nestlings). Despite this, the largest number of young fledge from the largest broods and these young are of about the same weight. Between early and late broods there is no significant difference in mortality. For the different years and the different observation blocks, the mean survival in the nest is similar and is independent of food supply (Common Voles) and precipitation at that time of the year. Brood size also has a negligible effect. The mean final weights of the female nestlings are also similar for the different years of the study. The males, however, reached lower weights when voles were scarce. This difference between males and females is attributed to competition for food between the sexes in the nest. It has been stated above that the mortality of the nestlings is similar in years with a scarcity of voles and years with an abundance of voles. This implies that the parents are able to feed their young sufficiently when voles are scarce. This could be due to: 1. Smaller broods in such years. This is not the case, because no correlation was found between brood size and food supply during breeding. 2. A higher percentage of deserted clutches. This was found to be related to food supply, which means not only that there are fewer broods, but also that those that remain have a better chance of obtaining food. 3. Alternative food. In seasons in which voles are scarce the nestlings do not get a smaller amount of food, because they are brought other food-items, especially young Starlings. The mortality after fledging in the first year is estimated from recovery data to amount 51 %. For unknown reasons, however, this mortality is lower for early-fledged birds and higher for late-fledged ones. The annual mortality of the adult Kestrels is estimated to be 42%. Seasonal migration is not pronounced in Dutch Kestrels. However, in their first year they disperse over a large area. The trend to a south-western direction suggested by the recoveries is mainly the expression of a bias caused by the intense persecution in Belgium, France, and Spain. It is evident from these results that the influence of the environment weakens in the course of the breeding season. This is partly due to an increase in the amount of hunting time (increase of day-length), an increase in food supply (increase in vole densities and alternative preys later in the season), and selection of breeding pairs (only those pairs remain that can obtain food under the prevailing condition). Food is not the most important cause of death in the nest and there is no selection operating against the largest broods through an inadequate food supply, as suggested by LACK (1954, 1966) in his hypothesis concerning the evolution of clutch size. A number of alternative hypotheses are discussed.

Affiliations: 1: (Institute for Ecological Research, Arnhem, The Netherlands

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