Great steaming piles of... bark?

Glesni chews some bark.
Image (c) Dyfi Osprey Project 2013

During this early part of their nesting season, the Dyfi ospreys – Monty and Glesni – have been bringing some strange items back to their nest.  As well as conventional materials, they have also been collecting dried cow pats, seaweed, and pieces of tree bark.  A lot of the bark appears to be silver birch, though other species have been sampled as well. Not only have they been collecting this stuff, but on occasion they have been eating it – and on camera, as well

As might be expected, this hasn't gone down very well with the legion of dedicated osprey fans.  They have been disapproving of the selected “nibbles”: ospreys eat fish and only fish, and birch bark and cow poo are not supposed to be on the menu.   So what's really going on...?

This is a kind of detective story.

Almost everything that animals do, they do for a good reason.  That reason might not be obvious to us, ensconced in our little anthropomorphic bubbles as we are, which is why we often misinterpret their actions.  But on other occasions, even a close and objective study of the behaviours cannot provide the correct answer: to get to THAT, we have to think outside the box – and even right outside the nest.  But first, let's define the problem. Fish, we are told, is an almost complete foodstuff in itself and ospreys shouldn't need anything else to sustain them.  That's what it says in all the books.  And here's the thing...

The books are quite right. They don't.

And yet is seems unlikely that Monty and Glesni are eating cow dung and chewing bark just because they like the taste.  I mean, would YOU? That means they must be getting something important from these things – something to do with nutrition or the maintenance of good health. And there, my dear Watson, is our first clue.   A second clue comes from something that cow pats and bark have in common, and that fish don't.   The cow dung consists of partly digested vegetable matter.  Once it has lain around for a while, and the urea and sulphide compounds have degraded or been leached out by the (very occasional) Welsh rain, it doesn't smell too niffy nor taste too acrid.   The birch bark includes an inner layer of woody cells (part of the cambium) which contains and transports many of the nutrients used by the tree during its own development.

Molecular structure of
unbound thiamine pyrophosphate
(c) Warwick University

There are some substances vital for life which animals (with a few rare exceptions) cannot synthesise for themselves.   One of these is vitamin B1, or thiamine.  Thiamine is important stuff: it is needed for the production of ATP, a molecule which supplies energy to the cells of the body, and also plays a role in the correct functioning of the heart, kidneys, and central nervous system.  Chronic B1 deficiency is a serious and potentially fatal condition in all animals, and in humans it has a disease name: Beriberi.

So perhaps the ospreys are getting this and other substances from direct ingestion of plant material, to supplement their typical diet.   If this is so...

Hold on just a cotton-picking minute there, Sherlock. Uncooked fish contains LOADS of B-complex vitamins. Everyone knows that!

And so they do. But some of them might contain other things as well. This is where we need to think outside the box (or nest) because the answer doesn't lie on the green pastures of Derwenlas, but out there in the grey waters of the Irish Sea...

Grey mullet

A favourite prey species of the Dyfi pair is the Atlantic grey mullet Mugil cephalus, which form more than 50% of their total catch.  Mullet are an ideal osprey food: rich in fats, protein and easy to catch because their habit is to feed at the top layer of the water-column.   There they graze on plankton – microscopic plants and animals – and small free-swimming crustaceans. It's been found [1] that mullet also contain quantities of enzymes called “thiaminases”.[2]  The thiaminase enzyme splits B1 molecules in situ, making them biologically inactive.  The amount of enzyme contained in mullet seems to vary seasonally and by location, and may depend on what the fish themselves have been eating. [3]

So this may be the answer to our little mystery: the wonderful mullet which ospreys adore also hides a sinister secret. By eating so much of it, the birds may actually be getting less available vitamin B1, because enzymes in the fish itself are blocking it. To redress their dietary balance, Monty and Glesni need to nibble at an alternative “top-up” source – one that is already part-processed (by tree or cow) but which doesn't contain any thiaminase. This would explain why other fish-eating birds, or other ospreys in different parts of the country, don't have to nibble on cow pats.

No shit, Sherlock?

None at all Watson. Pass me my violin, there's a good fellow....


Notes:
[1] Source: Cornell University Dept of Animal Sciences
[2] NEOPYRITHIAMINE AND THE THIAMINASE OF FISH TISSUES R Sealock & J White; J. Biol. Chem. 1949 181: 393-403.

[3] Or it is possible that this could be an evolving anti-predator adaptation. Other marine organisms use vitamin-busting enzymes as part of their chemical weaponry. Ferns - especially bracken - contain large amounts of thiaminase, which helps them to deter browsing herbivores.