Sunday, 31 May 2015

A costly way of thinking

Flight is a very costly activity indeed. Birds spend a great deal of energy lifting off and staying in the air. Long distance migrants, like our Terns, direct so much energy towards flying that they must reduce the energetic demands elsewhere in their bodies. One way in which birds have reduced energetic expenditure is in the brain. For many regular and day-to-day behaviours, birds operate on a “rule of thumb” basis. They essentially skip the thinking part by following the simple rule “if such-and-such a situation is happening, then… do this”. For example “if there are white speckled things in my nest, then…sit on them” soon followed by “if there are cheeping, gaping things in my nest, then…feed them”. Following this rule of thumb, birds will incubate their young as eggs and then feed them as chicks. By using this method of thinking, the brain does not need to be as big and complex and more energy can be directed to activities like flying.

But alas, although they evolved to work very well and save a great deal of brain power, sometimes a rule of thumb behaviour just ends up wasting your time. For example, see the case of the Black-headed Cardinal feeding goldfish in a garden pond. Following the rule “if I see a red coloured, gaping hole, then…feed it”, the Cardinal's brain is triggered by the shape and colour of the goldfish mouths and, thinking they are actually the gaping mouths of baby chicks, feeds the fish as if they were its own young in the nest.

I saw a similar example of time wasting in the colony today during a rowdy encounter between the Little Terns and Ringed Plover. To explain it, I must first explain two rules that Little Terns and Ringed Plover follow.

Rule #1: “If there is a sick individual attracting attention to the colony, then…chase it away” – Little Terns.

Nesting in groups helps protect Little Tern nests from predators. When a big black crow flies into the colony (or when I go in for colony nest checks), the Terns get together and torment it until it retreats. Similarly, the Terns get together in a gang to chase away any sick individuals near their nests. Sick individuals are loud and flap about, creating a landing beacon for any hungry predators lurking nearby. If there is a sick bird flapping and crying in the colony, the Terns will mob it until it either leaves, or in grim circumstances, gives up and dies.

Rule #2: “If a predator is near my nest, then…attract its attention to me instead” – Ringed Plovers.

A strategy often used by ground nesting birds, when a predator is near the nest, the parent attract its attention and leads them away from the nest. Birds do this by pretending to be sick or injured – an easy catch and an easy meal for a hungry fox. When I approach a Ringed Plover nest to do my colony nest checks, the Plover whistles and cries out to me, then pathetically limps the opposite direction waggling a “broken” wing falling all over the place. It is effective – I do get very distracted! Predators will follow the “injured” Plover away from the nest a sufficient distance before she miraculously heals and flies off into the distance.

Maybe you can already see where I am going here. This morning in the colony, a Plover, sensing some sort of predator danger, began her broken wing exercise to protect her nest. The Terns, on seeing this sick and injured individual near their nests, began to mob and harass her. The Plover felt more threatened and upped her limping and squealing antics. The Terns became more concerned and upped the shouting and dive-bombing. The scene escalated to a tight cluster of 30 Terns furiously screaming over the head of a single Plover rolling around on the ground having an epileptic fit.

Nobody was incubating their eggs. Nobody was fishing for food. Yet there were no predators in sight and no dying birds to attract them. Following their rule of thumb, while usually quite beneficial, just didn't really do the job this morning. The Terns eventually dispersed and returned to a state of peaceful incubation with their Plover colony-mates.

There you have it, you can never be prepared for everything.

Susan and Paddy

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