Birds have no capsaicin (8-methyl-N-vanillyl-6-nonenamide) receptors in the GI tract.

I cannot speak about all birds, my specialty is in parrots, birds in the order Psittaciformes, however I know that hot peppers are particularly healthy. The capsicum provides many benefits including antioxidants, immunity support and possibly even pain relief. In addition, parrots have about 400 to 500 taste buds per millimeter of their tongue, while humans have almost 4000 to 5000 per millimeter, meaning it takes significantly more flavor for parrots to taste. Birds have evolved their gustatory system more on an avoidance mechanism more than for preference, meaning they taste things and immediately know if they are toxic or not and choose what to eat based on what it nontoxic. One other thing to keep in mind is birds see in the ultraviolet, those bright red peppers are not "red" to them, they are some fantastic color we cannot even image. However, from my own personal experience, parrots seem to prefer red fruits and vegetables over others (my Congo African Grey will not touch a green apple, but loves red ones, same goes with strawberries vs. blueberries.) Their visual systems are so much more precise and advanced that just about any other living creature, they need to be able to immediately (in mid-flight) tell that that fruit over there is ripe and ready to eat but that one in front of them is not ready, perhaps "red" (or whatever they perceive red to be) in the wild is an indication of ripeness. Think about it; if they go down and start eating something that's not ready, that's just opening them up to be someone else's dinner without anything advantageous coming out of it for them.

Why do mammals have capsaicin receptors but birds don't ? It sounds too convenient to be an evolutionary adaptation.

Incidentally putting hot pepper in hummingbird feeders to deter bats also deters the hummingbirds.

Given that birds in general do not have capsaicin receptors (as Kimberly Chang remarked) while mammals do, I think that the red fruit of peppers evolved to be eaten by birds and not mammals, a fact that is proved by their taste for these fruit and the red colour, which is seen by birds and not mammals. Moreover, the small fruits of wild peppers are eaten whole by birds, while mammals usually chew fruits of the same size, thus destroying the vital and most important part of the fruit, i.e. the seeds. Birds digest the pulp and disperse the seeds with faeces, thus contributing to the spreading of these plants. Nothing in Biology Makes Sense Except in the Light of Evolution ;-)

What about bell peppers, which are too large for a bird to swallow whole, and which lack the hotness? Are they a later branch, evolved for the mammals to eat and spread the seeds? The seeds do spatter out readily so would probably get spread in the pocess of a mammal chewing the pepper up.

I believe bell peppers are a sweet artificial variety selected by man. The wild pepper (Capsicum annuum), from which bell peppers originate, bears small fruits and contains capsaicin as a chemical defense the plant produce in its "effort" to avoid being eaten by "destructive" mammals. Normally, red colour in fruits is correlated with birds acting as seed dispersal agents.

Just a few clarifications on this.

1. Birds DO have the TRPV1 receptor like mammals, which is also commonly called the capsaicin receptor. However the bird TRPV1 is insensitive to capsaicin as beautifully shown by the work in David Julius lab (Jordt S-E and Julius D (2002) Molecular basis for species-specifig sensitivity to "hot" chili peppers. Cell 108: 421-430).

2. Some of the suggestions included by previous comments have already been tested experimentally by Josh Tewksbury (Tewksbury JJ and Nabhan GP (2001) Directed deterrence by capsaicin in chillies. Nature 412: 403-404 and Tewksbury JJ, Reagan KM, Machnicki NJ, Carlo TA, Haak DC, et al. (2008) Evolutionary ecology of pungency in wild chilies. ProcNatAcadSciUSA 105: 11808-11811).

The first paper proves that chilli seed germination is decreased in the GI tract of mammals but not by the passage through the GI tract of birds. In this case birds can benefit from the rich contents of the chilli peppers by not being sensitive to the capsaicin. The plant benefits by getting the seeds dispersed efficiently.

The second paper has little to do with birds but it is a demonstration that an increase in the amount of capsaicinoids in fruits reduces fungal infection and seed mortality.

Obviously these papers do not provide all the answers so there is plenty to be studied in this respect. In the past I tried to collect evidence of which bird species are insensitive to capsaicin but have found very little. The fact that capsaicin does not activate the chicken TRPV1 could mean that this is a general feature for all birds because chickens are rather basal birds. It was interesting to read that hummingbirds avoid chillis but many bird species are neophobic to new tastes and that does not necessarily mean that they do sense the heat in the chillis.

The Phainopepla or Northern Phainopepla (Phainopepla nitens) in the southwest USA eat a lot of hot chili pepper in their diet.

As has been stated by both Giorgio Chiozzi and Jordi Altimiras above, it's not necessarily that birds have adapted to process capsaicin whereas mammals have not, but that chili peppers have specifically developed capsaicin-producing metabolic pathways as an anti-mammal defense mechanism. A lot of the mammals that would otherwise feed on chili peppers either have small home ranges (in the case of most small species) or would destroy the seeds of these plants through mastication, so they are not a very favorable seed-dispersing species compared to birds, which generally cannot crush the seeds and are more mobile. The use of capsaicin in peppers is sort of like how humans use antibiotics that target certain morphological features of the undesired species (i.e., the peptidoglycan cell wall in bacteria) but leave the desired cells (i.e., eukaryotes, which do not have peptidoglycan) alone. Capsaicin in particular has been suggested to have originated as an anti-fungal compound that was later exapted into an anti-predator chemical. See

http://link.springer.com/article/10.1007/s00442-006-0496-y#page-1

http://www.pnas.org/content/105/33/11808

in my phd thesis i was used diets content 1% , 1.5% and 2% hot papper for 42 day in brioler chicken