Why are female raptors usually bigger than males?

 A friend of ours foolishly sent me a message yesterday saying he'd got both spotted eagle owls and african wood owls in his garden at the moment. As I'm sure anyone sensible would have realised, that immediately resulted in my inviting the whole family over to his house for lunch today with the join aim of avoiding washing up and seeing some nice owls up close. I'm pleased to report success on both fronts! After the initial surprise of seeing the two species in practically neighbouring trees (eagle owls are well known predators of other owl species, making over 10% of the diet in one study), the thing that struck us most was the extraordinary degree of dimorphism exhibited by the pair of wood owls. In most birds of prey - the owls, hawks, eagles, etc - the female is the larger bird, the male being smaller which is exactly the opposite of what is normally the case in birds. Why this should be is a fairly interesting question which comes in two parts - firstly, when should there be such a large difference in body size in (particularly) birds of prey? And secondly, why should the females specifically be bigger than the males (why is it the opposite of most other species)?

The pair of wood owls that set me thinking. Arusha, Jan 12

For the first question there's a fairly well agreed upon reason for why raptors show such strong dimorphism (i.e. why there are such marked differences between the sexes): it's to avoid competition between members of a pair. The more potential competition there will be between the male and female for food, the stronger should be the dimorphism, so they can concentrate on different prey. Raptors, because they are higher up the food chain, have much lower food abundance than, say, a quelea, eating grass seeds. They also tend to be larger than birds feeding on other food types, and consequently need more food. So it makes sense firstly that they should be rarer than their prey, and secondly that there's going to be much stronger competition for the prey that is available. To reduce the competition between male and female who share a territory, it's very sensible for the two sexes to concentrate on different food types. In fact, it's rather hard to find evidence of this difference: peregrine falcons, for example, show no obvious differences in some circumstances. Nor do all Northern goshawks. But there is enough evidence from a variety of species to suggest perhaps this is fairly normal, though might depend on the specifics of breeding location. Let's accept it for now... One possibility that follows from this, is that the degree of dimorphism between raptors species should also vary with food availability/competition. And there's a degree of evidence that this, too, happens - dimorphism is minimal in vultures, intermediate in hawks and eagles catching terrestrial prey, and highest in those species feeding on birds, where foraging is really tricky. So that makes a certain amount of sense, even if it is a bit of a "Just-so" story, since we can never do real experiments to test the ideas.

Baby Spotted Eagle Owl also in our friend's garden

But why should the females be bigger? This is a harder issues where there is less to go on and even more Just-so type thinking - in fact, no fewer than 20 hypotheses have been proposed! I'm not going to review them all, but will focus on a few of the most popular - and to be honest there really isn't a definitive answer, we just don't know! Still, one of my favourite is that birds of prey need to be more agile than other species if they're going to manage to catch their prey. (It's pretty obvious that a bird that eats another birds really does have to be more agile to stand a chance of success!). How agile birds are depends to a large degree on their mass - heavy birds aren't so agile and can't corner as fast, etc. But when they're about to lay eggs, female birds have to put on mass, which makes them less agile. But a fixed increase in body mass has a lower effect on a larger bird than it does on a smaller bird - so females should be the bigger ones. What do you think? nice idea? Maybe... I'm still not 100% certain it doesn't also apply to other bird species too, where the male is still larger than the female.

So how about this? Raptors need to tear food up to feed young, which is hard work. The females do more tearing to feed the chicks than the males, so they should be bigger. Maybe, but what about raptors (and owls in particular) that mainly eat beetles? And where people have looked at it, there's little evidence of clear sex differences in ripping behaviour anyway.

Adult Spotted Eagle Owl. Male? It was smaller than the chick.

My favourite is to do with prey abundance again - it fits with the original dimorphism argument after all. Smaller prey are usually more abundant than larger prey - there are simple more small animals around thanks to the pyramid of numbers again. Females spend most time incubating and caring for small young, so males are going to spend more time foraging, and should therefore specialise in the species that are more abundant - the smaller prey items. But whilst I like the simplicity of this argument, there's no real evidence it's right, nor is there always evidence that males spend more time foraging!

There are plenty of other ideas out there (reviewed here), but you can try and make your own up too - I think there's plenty of scope for new ideas in this area, especially if they can actually generate hypotheses which might be testable. At least it will give you something to think about next time you see a pair of raptors...

Main references:

Wheeler, P., & Greenwood, P. (1983). The Evolution of Reversed Sexual Dimorphism in Birds of Prey Oikos, 40 (1) DOI: 10.2307/3544210

Helmut C. Mueller (1986). The Evolution of Reversed Sexual Dimorphism in Owls: An Empirical Analysis of Possible Selective Factors The Wilson Bulletin, 387-406
 
ANDERSSON, M., & NORBERG, R. (1981). Evolution of reversed sexual size dimorphism and role partitioning among predatory birds, with a size scaling of flight performance Biological Journal of the Linnean Society, 15 (2), 105-130 DOI: 10.1111/j.1095-8312.1981.tb00752.x