In nature, the growth of nestlings can be affected by; environmental conditions out of parental control, the age and physical condition of the parents and the factors which are, to a varied degree, under parental control, such as choice of breeding habitat, hatching date, clutch size and hatching synchrony. Studies have shown that the fledgling weight of Great Tits (Parus major) correlates with post-fledging survival and thus demonstrates that variation in fledgling weight correlates with the variation in the fitness of the adults. Carrion Crow (Corvus corone corone) chicks in the urban habitat gain weight slower, take longer to reach fledgling weight and are significantly lighter at fledging than the chicks in the agricultural habitat. Additionally, successful parents in agricultural landscapes fledge, an average 2-7 chicks per year, whereas parents in the urban habitats fledge 1-5 chicks only. Since a much higher percentage of chicks reach the critical body size in the agricultural habitat, the parent crows reach a Darwinian fitness five times higher than the parents in the urban habitats (Richner 1989a).
Three types of ecological conditions are important in the evolution of cooperative breeding. The first type of constraint is habitat saturation which entails a shortage of breeding sites or territories relative to the number of birds competing for them. The second type of constraint is the shortage of mates which, in monogamous birds, is the result of a skewed population sex ratio. The third type of constraint is the environmental harshness represented by erratic changes in the carrying capacity of variable and unpredictable environments. However, a fourth condition, the constraint imposed by body size, could be more important than these three conditions in the evolution of the helping behaviour in the Carrion Crow (Corvus corone corone).The best thing for birds which fall below the size limit in order to acquire a suitable territory, is to help their parents in raising chicks, therefore increasing the chance that some of their kin may reach the critical body size necessary for becoming a breeder (Richner 1989b).
This species breeds in western and central Europe, with an allied form Corvus corone orientalis in eastern Asia. The separation of these two populations is now believed to have taken place during the last ice age, with the closely allied Hooded Crow filling the gap between. Fertile hybrids occur along the boundary between these two forms indicating their close genetic relationship (Parkin 2003).
- Richner.H., (1989a) Habitat-specific growth and fitness in Carrion Crows (Corvus corone corone). Journal of Animal Ecology, 58, 427-440.
- Richner.H., (1989b) Helpers at the nest in Carrion Crows (Corvus corone corone). IBIS, 132, 105-108.
- Parkin.T.D., (2003). “Birding and DNA: species for the new millennium”. Bird Study, 50, 223–242.