BATHTUB BLUES (And reds)

"In birds, the emergency life-history stage is promoted by a release of the stress hormone corticosterone. However, how corticosterone reduces the expression of parental cares remains to be clarified. One hypothesis is that the release of corticosterone may also affect prolactin levels, a pituitary hormone widely involved in regulating parental behaviours” [1]

Ospreys are a long-lived species and, as such, their reproductive strategy is spread over multiple breeding seasons. In this context, the survival of the adult female is paramount. When something goes “wrong” during the early stages of nesting, female birds have a physiological response to stress hormones that can switch away from normal nesting behaviour, and back into a mode that more resembles everyday life for a non-breeding individual. The “wrongness” could be one single event – nest disturbance, sudden reduction in food supply, prolonged bad weather, or desertion by (or absence of) the paired male – or a number of events that, taken together, add up to a “wrong” situation.

This season (2015), we have seen several high-profile examples of disrupted nesting behaviour – notably at Glaslyn (non-return of the incumbent male) and at Loch Garten (incumbent male apparently evicted by intruders.)  Such events seem to be an integral part of the osprey ecology, and have probably been occurring for many millions of years. They are a necessary thing if the species is to continue in its traditional breeding range

From ref. [1]

Scientists have discovered some of the mechanisms that lead to this “emergency life-history stage”. One of them is the effect of stress-related hormones such as corticosterones on serum prolactin – the “parental behaviour” chemical that normally takes precedence during nesting.

When prolactin levels crash in an emergency life-history stage, we see the female apparently “abandoning” or “ignoring” eggs, or even newly-hatched chicks. For human observers, this action seems to be ethically and/or morally culpable, but we must not look at it this way. The female bird has no choices in the matter: without the correct circulating hormones to trigger her behaviour, she no longer recognizes the eggs, does not know what to do with them, has no interest in them at all. Nor has she really “decided” to adopt a new strategy for her own survival or breeding success at some later time – evolution has decided these things for her.

This leaves the question of how the female's behaviour can alter so quickly (or slowly) to take account of the changing situation. To find out, we need to understand the way that hormones are created in the body, how they are disposed of, and how long these processes take – a branch of science called pharmokinetics.

From ref. [1]

In birds, egg production seems to be associated with a substance called luteinizing hormone (LH) – a glycoprotein that is released from the pituitary gland. Parental behaviour (incubation and caring for young), as we have already seen, is promoted by prolactin. The relationship between LH and prolactin levels in birds is very complicated, and both circulating factors are affected by other substances. However, we do know that they can be secreted in very large quantities, when conditions are appropriate..

And it's this basal secretion rate that provides the answer to rapid behavioural changes in birds, because NEITHER of these hormones hang around in the bloodstream for very long. The “serum half-life” of prolactin (the time taken for 50% of a given quantity to be cleared) varies according to the size of the bird, but in larger raptors seems to be around 40 minutes.

Think of trying to fill a bathtub when you have forgotten to put the plug in the plughole. If the amount of water coming in through the taps is greater than the outflow, then the bath will fill eventually – but as soon as the filling rate slackens off, the water level will start to fall again. Turn off the taps completely and the bath will empty very rapidly.

This is the way that seasonal reproductive hormones act in birds: Only if the nesting situation is favourable in most respects – and remains so, will hormones be maintained at the correct level.  The existence of an emergency life-history stage is a remarkable adaptation. For birds in general, it is not an incident of failure – it is one of the long-term secrets of their success.

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SOURCES:

“How does corticosterone affect parental behaviour and reproductive success? A study of prolactin in black-legged kittiwakes” F Angelier, C Clément-Chastel, J Welcker, G W Gabrielsen, O Chastel, 2009 

“Neural Circuits Underlying Parental Behavior” C Dulac, L A O’Connell, and Zheng Wu. 2014.
Neural Control of Maternal and Paternal Behaviors. Science 345,no. 6198: 765–770.