Bsrgvty

This is rather speculative, but draws on a few birding/wildlife books I've read and it's too much for a comment. It also has a little UK bias, but I've visited 4 continents on wildlife trips, and much of this seems universal.

First, to get them out of the way: invertebrates may be interesting but in the general population there's a bit of a "yuck factor". Only butterflies (and maybe bees) really have much general appeal. I do know a couple of people who are as keen on their lepidoptera as birders are on their aves.

Mammals are broadly interesting. They're what people with only a passing interest in wildlife go on safari, or to the zoo, to see (along with the largest reptiles). But for many people, especially city dwellers, sightings of wild mammals are rare. The creatures themselves are, and they're often nocturnal. I'm in a village and cycle at dawn and dusk, so see more than most, but still only see (introduced) grey squirrels and rabbits regularly, with deer and foxes occasionally, and badgers/weasels almost never. Rats have the yuck factor again for many people, and are to be avoided, while most people will only see smaller rodents if they've got a cat to bring them in. So to see interesting mammals takes effort. Much of this applies to reptiles/amphibians too.

Birds on the other hand are visible, audible, and common. Having a few different attractive species visible from your own home is normal. Some (e.g. European Robins in the UK) approach close enough that you can really appreciate them, while others draw attention to themselves at a distance by singing - and birdsong is generally pleasant. If you have the beginnings of an interest in wildlife, it's most likely to be revealed by garden or local birds. Then we have a self fulfilling effect - people are interested in birds, so they go to bird reserves, which means there are facilities for people to go to.

It's also a case of accessibility, birding really doesn't take any special equipment beyond say a pair of binoculars and birds will visit peoples backyards and bird feeders. While mammals can be shy or limited in area requiring trips to see them. Fish take special equipment to catch and mammals especially the biggers ones can often be dangerous to humans.

The next thing is there are tons of diversity among the birds you can see while there is a much smaller number of mammals compared to birds, compare this list of birds to this list of mammals.

This also may depend on where you are, it is faily common in national parks in the U.S. to have massive elk/moose/buffalo/bear traffic jams and I doubt that you would ever see that for a bird.

I'm not a bird-watcher myself, but I have bird-watchers in my family, so I had rich opportunity to observe their conversations on their hobby. Here's a collection of points that I think contribute to the appeal of bird-watching. Note that for reasons of homogenuity, I'll use websites and quotes mostly from the UK as an example in the following, but the arguments should hold for most other temperate climates just as well.

(1) Changing availability encourages emotional investment

One aspect that sets birds clearly apart from other types of animals is their strongly seasonal pattern. True, in temperate climates, there are hardly any insects during winter, but the mammals that you can observe in the UK are observable throughout the whole year. With the migratory behavior of birds, you have immense fluctuations across the seasons: for many species, you'll be able to observe them only at specific times of the year. This seems to lead to emotional investment: You can look forward to the time when species A should be observable for the first time, and cherish if it eventually has returned from its winter quarters. Also, bird-watchers seem to develop forms of FOMO if a rare species B has migrated to an area where you'd not expect it normally.

(2) There are so many to collect

The Royal Society for the Protection of Birds claims that there are 574 bird species that can be observed in the UK. In comparison, the Mammal Society lists only 101 mammal species in the British Isles, and there are apparently only 6 reptile species in Britain. This means that while you may quickly have observed all reptile species that exist in your area, you will never run out of targets if you are a bird watcher.

(3) It's very accessible

Some birds are so common that you will recognize them without much training. The entries in the list of 19 common British birds are distinct enough that anybody should be able to identify at least a few entries already. This means that you can start bird-watching right away just by going to the local park, even with young children.

(4) There is room for personal development

You may start watching birds straight away, and even children should be able to reliably recognize Great tits and Blue tits. Yet, you need to invest some time, and you will require some experience, in order to be able to recognize a Marsh tit if you see one. I've been told that keeping waders apart, e.g. a knot from a redshank is not trivial, and that it can be a satisfactory experience to realize how many different species you can already spot.

(5) It's a social activity

As an outsider, I was surprised to learn how much of a social activity bird-watching actually is. If you're at a observation post, it's not rare that you'll be addressed by complete strangers who want to show you that rare species that they've just discovered. Bird-watchers are also well-organized, be it in traditional societies (such as the RSPB) or on social media, including dedicated networks like BirdTrack where you can record and share your bird observations.

Granted, some of these points might in principle apply to the observation of other vertebrates as well, especially the last one: I'm sure that there are societies dedicated to the observation of reptiles or of mammals – but there are just not as many. Other points are fairly unique to observing birds, I think.

This might have to do with migrating behaviour. Different seasons will see different birds, hence more variety, and their arrival has more of an event character.

It's nothing more complicated than this: The hobby is accessible. Birds come to us.

The suburbs are a very, very, very hostile place for land animals - vast and sterile by their standards, with pavement, lawns (no cover), structures, fences to impede mobility, and thick with predators (dogs and cats). So prairie dogs, salamanders, wolves, emus, giant turtles, rattlesnakes just can't get to us to be seen.

Birds, however, glide over all. The predators are no threat to them. The urban environment has enough trees, power lines, or other roosts; there's food and water to be had from squirrel feeders and bird baths.

So all you need is an easy chair, and you can see a huge selection.