Basin Notes

WHILE helping out at the last Goose Breakfast, admiring the sunrise and waiting for the geese to perform their spectacular mass take-off, I was asked “How do they decide when to take off?” and “How do they decide which direction to fly?”.

My answer was “We don’t know.” Animal behaviour is very difficult to explain and it takes a lot of effort and observation to even understand a small amount about it.

Last week, there was a talk organised by the SWT Angus & Dundee Members’ Centre, given by Dr Will Cresswell from the University of St Andrews, called “Better Red than Deadshank”. As a researcher with over 20 years’ experience of observing bird behaviour, Dr Cresswell was able to shine a light on some of the important factors that affect redshank behaviour in response to attacks by sparrowhawks and peregrines.

One surprising finding is that redshank can distinguish between sparrowhawks and peregrines and adopt a different strategy for each. Sparrowhawks are ambush hunters and take prey close to the ground and, with large prey like a redshank, prefer to force them down very quickly. In consequence, redshank will fly up as quickly as possible when they see a sparrowhawk and if it doesn’t catch them after a few seconds it will give up.

Peregrines, on the other hand, take their prey on the wing at any height off the ground and so redshank will respond by crouching down and even diving under water. However, redshanks are not perfect birdwatchers and sometimes get the identification wrong - with fatal consequences.

However, evading predators when they attack is not the primary strategy for staying alive. The best way to avoid being eaten is to stay out of the predators’ hunting ground. This seems a perfectly obvious ploy, but the redshank is a long-lived bird (up to 25 years) and has a long time to perfect its responses to all of nature’s challenges, so we should not really be surprised by this use of their experiences.

At the estuary site that Dr Cresswell has studied over several decades, he and his co-workers have shown that as long as the redshank stay further than 40 metres away from the trees lining one side of the estuary, then sparrowhawks will not attack them. The main reason is that the redshank will see the sparrowhawk once it breaks cover and have plenty of time to get into the air and fly away before the predator reaches them.

The dilemma for the redshank comes with the fact that there is more food inside the 40 metre zone near to the trees. So the study team found that redshank mortality increased markedly when the temperature dropped below 4 degrees celcius as the need for food increased and the birds had to move into the danger zone to avoid starving. Talk about the devil and the deep blue sea!

Another good strategy among birds trying to avoid predation is to form a flock. This has the advantage of many pairs of eyes available to spot a predator early enough for the flock to escape. When danger is seen they all take flight at once and the predator can then be confused as to the best target to aim for and each of the birds in the flock has a lower probability of being caught.

In the study area it was found that flocks of 40 to 60 birds were the norm and the advantages of being in the flock were evident. However, it was also noted that when feeding as a flock, the rate of feeding dropped as the invertebrate prey of the redshank scattered or submerged themselves deeper in the mud to avoid be eaten.

The situation is complicated by the fact that a flock is easier to see than a single bird, so safety in numbers must be balanced against being attacked more often. It also must be remembered that birds are not reasoning about these different probabilities as a human would and their behaviour has evolved over millions of years of interaction with predators and prey. This makes it all the more fascinating when we try to understand what we can see happening in the wild.

So next time someone asks me a question about bird behaviour I will have to answer “I don’t know, but someone is probably researching the question”. Eventually, all those findings from scientific papers will be available for the general public to read about or see on television programmes. Until then we must just marvel at the spectacle that nature provides for us.

Andy Wakelin