The Accipiter - Vets Corner December 2005

Vet's Corner - December 2005

John Chitty, the park’s vet, gives us some insight into the healthcare and maintenance of raptors as well as the injuries and illnesses that they fall victim to, and how he treats them...

This month, John looks at the means available to determine the gender of raptors.

Sexing Birds of Prey

It is not always easy to tell which are male and which are female! As breeding these species obviously first requires finding a male and a female we therefore have to use techniques to help us.
Several can be used:

Plumage differences. Some species, although not many, have obvious plumage differences. The classic example is the Snowy Owl (Fig 1) where the male is white (camouflage while hunting over the snow) and the female has bars to blend into tundra while sitting on the nest.

Fig 1. Male on right and female on left - photo John Chitty

Size. The falcons and many hawks show a marked difference between genders with the females being much larger. Hence the male falcon being referred to as a “tiercel” derived from “third less”. While this is great when there is a large difference there is often some overlap between large males and small females and it can be especially tough with hybrid falcons where there is a huge size range!

Fig 2. The male is much smaller than the female - photo John Chitty

DNA. Like mammals male and female have differences in chromosomes. However, there is a different arrangement – in people males are XY and females XX. In birds the male is ZZ and the female ZW. The basis of DNA sexing is finding a substance derived only from the W chromosome. If this is detected then the bird must be female. If not then it is non-female, ie male! DNA can be collected from freshly plucked feathers or (more accurately as there is less risk of contamination) a blood sample. In general this method is very accurate but problems do occasionally occur where labs have trouble with a novel species.

Endoscopy. The birds’ gonads are held inside the body so can’t be easily viewed. However, insertion of an endoscope via the body wall into an airsac permits visualisation of the gonad, hence sexing. The disadvantage of this method is that the bird must be anaesthetised and undergo a surgical procedure (ie there is a small risk). However, it is useful on two counts. Firstly, DNA sexing may take a couple of weeks for the lab to report on the result. Secondly, DNA only tells us what gender the bird is. Sometimes we have birds of known gender that appear unable to breed. Endoscopy allows us not only to tell the gender, but also the state of the gonads – ie are they fit to breed.

Figure 3. Goshawk ovary viewed through the endoscope. Note the immature egg follicles on its surface - photo John Chitty

Figure 4. The normal testis is smooth and oval. In this macaw it is round, shrivelled and most likely infertile - photo John Chitty

Finally, how do birds tell their genders? We find it tough but they don’t! Firstly, with the exception of the New World (and possibly Old World) vultures these birds have a poor sense of smell so it is unlikely that pheromones play any significant part.
However, they do have vision in the ultra-violet spectrum which we don’t. When viewed in ultra-violet there are often massive differences in plumage between genders even when they appear identical in the visible light spectrum. This opens up many possibilities in non-invasive sex determination of birds. Sadly, at present, the equipment required is extremely expensive!

John Chitty BVetMed CertZooMed MRCVS

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