This could apply to all animals who have no way of warning others about dangerous foods.
Lets stick with birds and food. If it is edible they all eat it and survive. But if it is poisonous they usually
So how do they go from 1 to 3 as a population. How does this info get passed down generations.
Many bird species do learn about food location and obtaining from their parents.
Observations of both wild and my pet birds show that when encountering a new food they tend to explore/taste very hesitantly at first. Poisons that are bitter or sour or otherwise unpleasant may be sufficient to warn them and prevent them from eating inedible or dangerous things.
I’ve also noticed with my pet birds that with a new food they prefer to see me eat it first before trying it. If that carries over into the wild then birds to some extent rely on “well, you ate it and lived, this food item is safe”.
And a certain number of birds probably screw up or make mistakes and die. We don’t see their little corpses, though, because in the wild freshly dead bird quickly becomes someone else’s lunch.
Natural selection?
What fruits/seeds are poisonous to birds?
chocolate, avocado, common fruit pits like apple, cherries, plums, apricots and peaches, comfrey, onions and garlic (in large amounts). That’s the common ones found in human household bird owners have to watch out for. There are probably a bunch of ones in the wild the average person wouldn’t know about.
None of these are things that wild birds would likely encounter or be able to consume if they wanted to.
I doubt this very much.
Most of them are, actually.
You are wrong.
So why aren’t birds all going extinct from eating all of these supposed poisonous fruits/seeds?
Because they don’t eat them. Duh.
Broomstick has described some of the reasons.
Let’s take one example: cherries Birds are the primary vector for the distribution of cherry seeds.
Cherries are adapted to appeal to birds (they turn red when ripe) - in regions where cherries grow, birds are adapted to be able to eat cherries - so on those cases, cherries are harmless to birds.
Birds have a generally fairly rapid digestive transit; so in the case of birds that are big enough to eat cherries whole, including the seed, the cherry fruit flesh is digested and the cherry stone passes through more or less unscathed - The bird deposits the undigested seed some number of hours later in a different location.
(the partial digestion of the seed coat is in some cases necessary for germination of the seed, and the excreta of the bird may also benefit the seedling in provision of essential plant nutrients)
Some smaller birds may ‘cheat’ by eating the fruit flesh and discarding the seed - which is less optimal for the tree, as it means the seeds fall directly to the ground where they may not grow so well, or at all, in competition with the parent tree.
Problems are most likely to arise when a bird is exposed to fruits or seeds that are in some way toxic, and are not native to the same geographic range as the bird - they may lack the adaptation to be able to safely digest the food, or they may be ‘too good’ at eating it (i.e. if they are able to crack the hard seed coat and be exposed to poisons in the seed kernel)
In such a case, the bird will teach its offspring to avoid such food in the future. Nestlings learn which foods are good to eat based on what parents bring them in the nest. After leaving the nest, fledglings follow the parents around for several weeks and they show them what to eat.
Birds that eat such food will be selected against because they won’t leave offspring. Birds that avoid such food will leave offspring who will also avoid such food. In the long run, the species will evolve an innate avoidance to such food through natural selection. This process, however, is much slower than in case 1.
In both cases, the population will end up avoiding it.
Very good post. Thanks.
Such as Nandina (Heavenly bamboo), for example.
Many years ago I read an account of a conservation group that had brought a troop of monkeys to the U.S. They were concerned that the site contained a plant that would be toxic, although slow acting, to the monkeys. They found that for reasons they didn’t understand that somehow the monkeys learned to avoid the plant.
Primates have quite a well-developed sense of taste - so it might just be that the plant didn’t taste good (there is a loose correlation between toxicity and unpleasant taste in plants - toxic components such as alkaloids, tannins and glycosides tend to be unpalatably bitter)
Nobody told birds around my house not to eat our cherries. Why would prunus fruit be harmful to birds?
The pits contain cyanide. Most birds pass them through the digestive tract, but if you give them to parrots or other large seed-eating birds they can crush the seeds and release the poison.
This will work for some types of poisonous foods, but not for others. Natural selection works only for heritable traits. Recognizing and avoiding bitterness is heritable, for instance. But what if the food looks and tastes fine but can kill you, anyway? I don’t know about birds, but there are things that can kill a human that taste just fine (the death cap mushroom, e.g.).
It will work for any kind of poisonous food in the long run. If foods that “taste just fine” kill enough individuals, eventually the species will evolve so that food no longer tastes just fine and individuals will reject it.
If death cap mushrooms provided strong enough selection by killing individuals who ate them, either 1) the species would evolve to be able to distinguish them in some manner from other mushrooms; or 2) they would simply reject all mushrooms if there were no way to distinguish them.
Death cap mushrooms taste fine to humans because they have been a negligible source of selection on us.
Or 3) become extinct.
(at least, I think that still counts as selection)