Sunday, 1 July 2018

Defence strategies of tundra birds

A male Spotted Redshank comes close in defence of his chicks
I spent the past two weeks in northern Norway studying birds, mostly waders and even though I am familiar with them, I continue to be impressed by the efficiency of their stategies in defending their eggs or chicks.

A Spotted Redshank can hide or run when a predator approaches
Most Spotted Redshank pairs split soon after the eggs are laid and the male incubates them, then rears the chicks alone. The chicks stay in the nest for the first day then they can walk more than a kilometre to reach a feeding marsh.

An male adult Spotted Redshank in breeding plumage - no other bird like it 
When danger approaches, the adult flies close to the intruder and pipes loudly, very loudly, causing confusion in a potential predator's mind - mine anyway. Meanwhile, the chicks run away or hide until the danger passes.

Incessant piping
Other waders, such as Whimbrel fly up and around any potential predator, calling in alarm, or guidance to their chicks. We as humans, cannot tell which, but the chicks can either run or hide. What vocal signals do the adults give to do either?

A Whimbrel up a tree - from where it is safe from ground predators and it can see its chicks hiding in the ground vegetation
The main terrestrial predators are Red Fox and Stoat, but there are also Ravens and Hooded Crows which can take the chicks. Perhaps that is one reason why the chicks can either hide or run.

Wood Sandpipers habitually sit in trees when protecting their chicks
The waders usually have four chicks, so scattering to feed, hide or run gives the others a chance if a predator finds one of the brood.

When on eggs =- Wood Sandpipers sit tight on approach
If they have eggs, these birds usually adopt a different strategy, one of sitting tight on their nest, relying on camouflage and concealment to avoid detection by any predator, until the last moment when they burst fom their nest to escape capture.

very tight
Once they leave their nest, these waders will call in alarm, then either fly away and try to distract the predator, or adopt a rodent-run display, where they either impersonate a rodent or a bird with a broken wing to lure the predator from their nest.

A Broad-billed Sandpiper gives a distraction display, pretenting to be a rodent or have a broken wing
- to lure me from its nest and eggs 
If anyone has been with a dog when it has disturbed a nesting bird they will know how efective this display can be. Most dogs, especially town dogs fall for this trick and run after the bird well away from the nest.

Jack Snipe sit extremely tight on their nests 
Jack Snipe are one of the tightest sitting waders, I am sure I must have stepped over birds on their nest. And when one sees how well they are camouflaged in the mire vegetation, I know why.

Jack Snipe nest just above the water level below the marsh vegetation
- they see the habitat differently from we humans 
It is not only waders which behave like this, other ground nesting birds act similarly, such as Willow Grouse. They have larger clutches and broods, perhaps 7 - 11 eggs or chicks, so the scattering of the brood if they run or flutter away is especially successful at saving at least part of the brood.

You all saw the male Willow Grouse, on the right first, maybe then seeing the female afterwards
Their chicks are most vulnerable in their first week, but after that they can fly in a fluttering flight for several metres, then more as they age.

A two-day old Willlow Grouse chick sits quietly and motionless - to avoid detection while its parents try to distract me
The male grouse is more brightly coloured than the female, and this one was more demonstrative to me. The female was running around low and farther away from me. That way, if any predator managed to kill any bird it would likely be the male. He is more expendable, if he died the female could rear the chicks by herself. So, a test of his fitness as a mate is not only his bright plumage, but his ability to escape predation which that plumage might attract.

The male Willow Grouse came within metres in attempting to lure me away from his chicks

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