You will have to excuse me with this blog. It is spuriously linked to trees and forestry in as much as Goshawks live in the woods…….but that’s about as far as it goes. So, if you don’t fancy a few minutes of ecology geekiness then look away now.
We are lucky enough to manage woodlands with Goshawks nesting in them. These amazing raptors were once driven to extinction in Britain by egg thefts, shooting and poisoning but thankfully are now making a recovery. The Goshawk is a relatively shy and illusive hawk with an uncanny knack of nesting in the next block of timber to be worked and thereby causing the whole job to be replanned, moved or cancelled.
That being said, they are awesome birds able to fly at over 40 mph through the tightest of woodlands to hunt other birds, small mammals and squirrels. With fiery red eyes and a muscular build they are the ultimate areal woodland predator. But as with most other hawks they have something unusual about them.
In virtually the whole of the rest of the animal kingdom the sexual dimorphism (differences in appearance between male and female) leads to the males being larger than the females or more colourful or both. (Think Lions, Elephants, Peacocks etc.) However in Goshawks this is reversed, with females being on average twice the weight of males. In fact, in some areas populations the females can be as much as three or four times the weight of the males. So, the question is why?
Well, there are two possible hypotheses for this. Firstly, there is the food related one. As a top predator on a woodland, male and female Goshawks are in direct competition with one another for food. Given that prey is hard to catch, and that prey availability is a major constrain on the success of Goshawk populations it is not ideal to compete with yourself. Reverse sexual dimorphism is a cunning way around this. With males and females being vastly different is size, they are able to hunt different size prey. Males specialising in small birds and other prey of a similar size, whilst females hunt far bigger prey such as rabbits, squirrels and pigeons. Not only this, but during the summer months the males provide much of the food for the females on the nest and for the young. In summer months the woodlands are in full leaf and even harder to hunt in and manoeuvre through. By being smaller the males are able more easily to hunt in these cramped conditions and provide for both their mate and their young. (Very clever if you ask me).
The second theory relates to babies! The Nest Defence Hypothesis states that females are larger to allow them to better protect their nest and chicks. A study in North America found that a quarter of all failed raptor nests were predated. As such, it makes perfect sense for the bird who is on the nest most, to be larger and better able to defend the nest. Bigger females have more success and thus females get bigger.
Personally, I think this is not an either/or situation and that BOTH hypotheses are true. If bigger females can better protect the nest and smaller males can better provide for these females and young, then to me that is the perfect answer. Ecology is never straight forward, but I do love a bit of geeky ecology stuff in my spare time, and this is a particular favourite of mine.
Goshawk (Accipiter gentilis) - British Birds - Woodland Trust
Latest news
