Thoughts on food-processing by birds
(writing in progress)
Everyone knows that no bird chews its food.
Instead, as everyone knows, birds swallow their food unchewed, and any comminution of the food is performed in the gut, particularly the gizzard, which is analogous to dentition in birds.
However, this generalisation, while valid, hides some interesting patterns.
Birds vary enormously (more than mammals) in the size of items swallowed.
For example snake eagles swallow large snakes whole – even if the swallowing takes a long time.
The lammergeier swallows large bones with awkward shapes.
By contrast, various other birds never swallow anything but small items, as if their throats cannot stretch much.
The latter birds are either specialised for a diet consisting of small items such as seeds, or they rely on pecking to break up large items into small pieces before swallowing them.
With this conceptual framework in mind, I have noticed two interesting points just among by local suburban birds.
The first point is that I have noticed that it seems to be a feature of crows (Corvus spp.) that they are unable to swallow anything but small fragments of food.
The second is that the beaks of various psittaciforms are so large and powerful that it is surprising that they do not chew their food.
Looking at crows and ravens first: have you ever noticed that you’ll never see a member of the genus Corvus ‘bolting’ any large item by swallowing it whole.
These birds, despite being omnivores and opportunists, need to break up food into small pieces, a process which they accomplish by means of the beak (sometimes assisted with a foot).
This processing of food makes crows and ravens vulnerable because it is time-consuming; but these birds are intelligent enough to minimise this vulnerability in various ways, e.g. carrying the food up into a tree where the bird is relatively safe, or remaining suspicious and vigilant while eating.
With my local species of Corvus in Perth, I can’t recall ever seeing it swallowing any item larger than a pea. How about your experience with Corvus spp. in Africa?
In fact, I would go as far as to say that it is hard to observe crows or ravens swallowing anything at all, so carefully do they process food by pecking at it and so vigilant are they while doing so.
One sees Corvus foraging and food-processing but not actually ‘eating’, in general.
Looking at psittaciforms: birds such as cockatoos have remarkably large and strong beaks, which would surely be capable of mastication were these beaks designed differently but with no extra mass or volume.
So, I wonder why psitacciforms have not gone to the obvious next step, and designed their beaks to comminute food by grinding?
The remarkably large and powerful beaks seem to be specialised for extraction of edible items from their containers and packaging, rather than for comminution of the edible items.
As a result of the above considerations, both Corvus and psittaciforms wind up swallowing only small items.
But this is for different reasons. In the case of Corvus it is because the bird insists on breaking up the food with the beak despite the fact that the design of the beak seems not much different from that of birds that swallow food in large pieces.
In the case of psittaciforms it is because the birds specialise on diets which involve extraction of small items from large and hard packagings.
It is so easy to envisage crows just swallowing large chunks of food, and the more omnivorous psittaciforms just chewing various foods.
But instead some sort of constraint seems to operate and I have yet to understand this fully in the context of the central adaptive strategies of birds.
Two additonal thoughts occurred to me just after sending this.
The first is that you don’t see Corvus ‘throwing its head’ to ingest items even after it has laboriously and carefully broken the food in order to swallow only small pieces.
There is something peculiar about the eating behaviour of Corvus, to the effect that it seems to get the food item to the throat without jerking the head in the normal avian way.
I think there is some sort of specialisation in the whole mouth anatomy and function of Corvus that may have been overlooked in the literature.
I did not mean to imply, below, that I’ve actually seen any crow or raven swallowing an item the size of a pea or smaller. In fact, I have never seen these birds swallowing anything, as such.
The second is that this peculiarity of Corvus might help to explain another well-known quirk if this genus, at least here in Perth.
The local species of crow is well-known, even among lay persons who are otherwise fairly ignorant about natural history, often to place food (e.g. bread or crusts) in water, leaving the food in the water and presumably coming back later to eat it.
This is ‘dunking’ behaviour and as far as I know Corvus is the only genus of birds that does this here in Perth.
My point is that this use of water to soften food would be consistent with the peculiar inability or unwillingness of crows and ravens to swallow large items.
Because Corvus is also perhaps the most proficient of all birds in the making, modification, storage and use of tools, it occurs to me that the way Corvus goes about using its beak is more along the lines of a ‘built-in tool’ than a part of the mouth.
Most birds do use the beak to break up food but then trade this off by accepting large pieces with an elastic throat and a head-jerking mechanism of throwing the food back.
What Corvus does instead is to use its beak as a tool, so that swallowing becomes so inconspicuous that I wonder if you or anyone you know has ever seen Corvus swallowing anything as such.
Postscript:
I have for the first time spotted a crow swallowing a food item. The item was about pea-size and the bird did ‘throw’ its head slightly.
So slightly that I got the impression that the food is drawn from the beak to the throat as much by the tongue as by the motion of the whole head.
So, I have caught a crow in the act; but I stand by the gist of my remarks, above.
(writing in progress)
Comentarios
Agregar un comentario