Basin Notes - March 2012

Waders awash

On the 24th February there was an unexpected high tide on the Basin. That is to say, the tide was higher than that predicted by the tide tables. This is not unusual, as the prevailing weather conditions have a great effect on tide height; especially barometric pressure.

The birds that frequent the Basin are there mostly to take advantage of the large amount of food that there is available in the mud. However, when the tide comes in, the mud, and hence the food, is covered up and the birds must make a decision whether to go somewhere to roost or move on to another site where food is available.

Most, if not all, the ducks and waders choose to stay on the Basin and find somewhere to roost and wait out the high tide period. This means they need to find somewhere that minimises the effects of the weather and is safe from predators.

As I watched the birds on this particular day, the waders were clearly prepared to stand on a shingle bank near the railway viaduct that was, by now, an island surrounded by the in-coming water from the North Sea. The ducks were quite happy floating around in small groups, but keeping close to the grassy margins.

The waders adopted their usual "head into the wind" position and pulled one leg up into their belly feathers to keep heat loss to a minimum. As the tide got higher they hopped away from the water's edge but eventually the island was inundated and something had to give. The oystercatchers, the largest of the waders on the island, were happy to stay, albeit standing in several inches of water. The smaller waders, like knot, redshank and godwits, flew off to grassland close by, but even then were right on the edge and sometimes just in the water.

Now, all through this re-arrangement there was no animosity between the different species as they were squeezed tighter and tighter together by the rising tide. This lack of aggression suggests that there are benefits to sharing the space and keeping together. The advantages of being in a large flock are that there are always going to be some pairs of eyes watching for predators while others are feeding, sleeping or preening.

When they are just roosting, they want to catch some sleep but birds sleep in short naps and will take a quick peek around at regular intervals. For ducks, this interval is between 2 and 6 seconds, but is dependent on time of year, size of flock and position in flock.

On this day, there was little opportunity for a nap as the tide was rising quickly and looking for somewhere to stand was more important that worrying about peregrines. If one had come along at that point, every bird would have taken to the air in a wild rush that is designed to confuse the attacker and make it difficult to single out a target. So being in a large flock is a definite advantage and if it is made up of different species then everyone benefits.

Some interesting research has shown that flock size affects reaction speed. Large flocks have more pairs of eyes and you might think they would react quicker to danger, but this is not the case. In larger flocks the odds of getting caught are much lower, so the birds can take a moment to assess the danger and not bother to flee so readily and hence save energy. Birds in smaller flocks have a higher chance of being caught if a predator attacks so they must react more quickly and fly at the slightest sign of danger. This means that birds may move around to keep the flock size at an optimum level that detects predators early and has enough birds to reduce the odds of being caught.

Observing bird behaviour is always fascinating, if sometimes unfathomable, and can always tell you something new.