As an homage to all the interesting bird species which I find on my bird surveys through Reading, I have decided to make a sort of fact sheet of the most interesting species which can be spotted right here in Reading. This blog entry, unlike the previous, contains scientific research on the species.
Female raptor birds are usually larger in size compared to their male counter-parts. In the case of the Sparrowhawk (Accipiter nisus), this dimorphism is extreme with females (280-300g) being twice as heavy as males (150 g). Hatched from eggs of similar weight, the sexes diverge in size and weight while in the nest, and fledge at week 4. There is no real sex ratio skew towards females, but there is massive dimorphism in terms of size. Such good survival of males cannot be attributed to a consistently good food supply during the nestling period, and to lack of competition among siblings. Great variation in growth rates occur both between and within broods, and starvation of at least one young in a brood is seen to be fairly frequent irrespective of the young’s sex. Within broods, males are seen to eat just as much as females throughout the nestling period but, while females continued to gain weight, males developed rapidly in other respects, such as they become more advanced in behavioural development and more agile than the females and are ready to leave the nest 3-4 days earlier (Newton and Marquiss 1979).
Sparrowhawks nest only in woodland of a certain structure – thick enough to provide good cover, yet open enough to permit easy flight between trucks and branches of trees. Theses structural conditions are more often provided by coniferous rather than broad-leaved woodland. More than 90% of food which is consumed in the nesting season for Sparrowhawks consists of woodland birds of all species, up to and including Woodpigeon (Columba palumbus). Densities of nesting territories in Sparrowhawk nesting areas depend on the quantity, quality, size and ease of the capture of prey in their habitat (Newton et al. 1977).
Sparrowhawks have seen some decline throughout Britain, which started primarily during 1957-1963, following the introduction of aldrin, dieldrin and other cyclodiene pesticides through agriculture. Recovery of the Sparrowhawk population has occurred however, due to the successful restrictions on the use of aldrin and dieldrin. The population recovery of Sparrowhawks has occurred in a wave-like, west to east pattern, occurring first in areas with the least tilled land. Both the decline and the recovery of the British Sparrowhawk population are primarily due to an increase and then subsequent decrease in adult mortality, caused by changes in the use of cyclodiene in agriculture (Newton and Hass 1984).
- Newton.I., & Hass.M.B., (1984) The return of the Sparrowhawk. British Birds, 72, 47-70.
- Newton.I., & Marquiss.M., (1979) Sex Ratio Among Nestlings of the European Sparrowhawk. The American Naturalist, 113, 309-315.
- Newton.I., Marquiss.M., Weir.D.N., & Moss.D., (1977) Spacing of Sparrowhawk Nesting Territories. Journal of Animal Ecology, 46, 425-441.