Survival of the fittest?

A week ago we visited a sea eagle nest on Mull with two chicks in it. This was to be the last nest check on Mull this season. We hoped we'd end the week on a high - and with the nest some 50 feet up, it didn't get much higher. These two were almost a month behind our twins at Loch Frisa and it showed.

While Frisa and Skye's chicks are now well feathered, these two were still very downy - a grey down with only the very first hint of feathers just appearing. One chick was big and strong and quite feisty. But there beside it was a smaller bundle of grey down. One was clearly way ahead of its sibling. 

Our expert climber and ringer Justin ringed the bigger of the two but the little one was just not quite ready. The feet of eagles develop rapidly and so they can usually be ringed at about four weeks of age. But poor little 'Half Pint' had clearly not been getting his fair share of the wealth of food being brought in by the parent birds: fulmar, gull, goose and deer were all on the menu. In situations like this it is always entirely possible that the smallest chick may not survive.

Sometimes, they just do not receive enough sustenance and slowly lose condition and lose their fight for survival. Sometimes too the bigger chick bullies the sibling so much that it just doesn't have the strength to fight back. This is the infamous 'Kane and Abel Syndrome' and is more commonly seen in golden eagles than sea eagles.It is more unusual for golden eagles to rear two chicks to fledging but it does happen.

For sea eagles, it seems much more common. Perhaps there is just more harmony in the nest. There is nearly always an off duty parent sitting nearby keeping a watchful eye on the nest. Perhaps the chicks just get less 'bored' when mum or dad is nearby and so they don't resort to sibling rivalry.

Whatever the reason, we knew we had to leave the smaller chick to the mercy of the elements and his nest mate and just hoped he would make it through. With such a big disparity in size, it's always tough to know what to do. Do you mess with Mother Nature and intervene? Rarely a good move. Do you try and foster the weaker chick into another nest with a similar aged chick? Or do you let nature take its course?

You take a long look at the wee one. Take a deep breath and turn away. Justin got some brief footage of the nestlings for us on the ground to view. In this case, there was no other similar aged nest. This pair had hatched late. All the others were several days if not weeks ahead. This one was going to have to take its chances. Surely it was in the best possible care of its parents?

As we gathered up the ropes and kit and prepared to get clear of the nest area, a nagging doubt remained. This pair has never successfully fledged two chicks before. And yet they had got these two this far, to four weeks old. This was already a record for them. As usual we'd left them a small gift of fish in the nest to make up for the brief spell of disturbance during the nest check and ringing. The parents would soon settle again and return to their normal pattern. When we were to return in a week - what would we find?  Now it was up to them.

Dave Sexton RSPB Scotland Mull Officer

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