Saliva!

Today I had the privilege of spending some time in a hide overlooking a Wedge-tailed Eagle nest in the Perth hills. I wasn't sure how successful I would be with capturing footage when I arrived in darkness around 5 am, but I when I left in daylight nearly 6 hours later, I sure was happy with the result! Both adult eagles attended the nest regularly during my presence and I was able to capture lots of interesting behavioural footage and photos. This will be added to my wildlife footage library and used in future documentaries about eagles and wildlife in the Perth hills (stay tuned!).

Two interesting things happened today that I had never observed before, one I knew about and one I had never heard of. When both adults were at the nest, the male eagle spent much time plucking small portions of prey (the red stuff in the above photo, in this case Western Grey Kangaroo), and passing them to the female (as seen above). She delicately accepted an offering, passed it back to the male, and the pair would exchange the portion back and forth several times, before one of them swallowed it. This happened about half a dozen times and was quite a deliberate gesture between the adults, which ignored the two chicks during these exchanges (they had been well fed earlier). The only explanation I can think of is that this is another ritual which strengthens the pair-bond between these long-lived, monogamous (mate for life) eagles.

During each exchange, the eagles' beaks would start to dribble saliva, which dripped of the tip of their bill before one ate the piece of 'roo. This brings me to the next point. While feeding the chicks, saliva drizzling from the adult female's bill was clearly visible as it ran onto the piece of prey and into the eaglets' mouths. This was (as far as I know) first documented by well-known conservationist David Fleay, who was able to observe such intimate behaviours by breeding Wedge-tails in captivity, and he concluded the saliva helps very young chicks to swallow and digest prey (saliva is no longer used when the eaglets reach a certain age).

Another thought which has been suggested is that by passing on saliva to their offspring, adult eagles expose them to various bacteria, helping build their immune system at a young age. Just like letting our children play in the mud and eat soil! If it helps build a healthier, stronger animal, then why not!? Perhaps this could also explain the adults exchange described above - by passing saliva to each other, both adults may be ensuring they both 'have the same germs' to avoid any unexpected bouts of sickness.

So many occurrences in the natural world we can only guess at, making it all the more wonderful I think. As I often say, such incredibly powerful predators can demonstrate the most gentle, caring behaviour when tending their chicks or nurturing each other. Few things are as fascinating to watch!

To finish with, here is a short, unedited clip of the female feeding her chick and using saliva to lubricate the meal.


Saliva! from Simon Cherriman on Vimeo.