I don’t know if “ubiquitous” is the word for it, but I just can’t imagine going somewhere and not seeing pigeons.
Is there a country that doesn’t have pigeons? Is there a climate they can’t support?
I assume you are referring to the rock dove, aka the common pigeon that is found in towns and cities the world over, and not all pigeons as a group.
The rock dove is ancestral to southern Europe and northern Africa. Outside of that range it is usually only found in cities or nearby some other human habitation. So you won’t find any pigeons in, say, the Amazon rainforest, or deep in the Canadian wilderness.
Some sources indicate the Red-billed Quelea (Quelea quelea ) of Africa is the most numerous bird in the world with an estimated population of 10 billion.
The Quelea may be the most abundant wild bird, but someone was telling me the other day that the most populous bird on this planet is the domestic chicken; which I find completely believable.
Yup, it’s the chicken. Because most people (and bird flu strains) find them cheap and tasty.
Candidates for the most widely distributed bird species include the Osprey and the Barn Owl.
Since the world is 2/3 ocean, I cast a vote for the lowly Sea Gull.
I’da guessed the crow.
While Corvids do indeed seem to be widely distributed, I’d guess that no single species has a world-wide range.
We need Colibri to come along and give the definitive word.
Same problem there - too many gull species, none with world-wide distribution.
My guess for a single species with the widest distribution would be the House Sparrow, Passer domesticus. This guy. Their distribution is nearly worldwide (an african and eurasian native) plus they’re easily found where humans are: in urban, suburban and agricultural habitats.
On the other hand, since I’ve caught more than a 1000 of them this field season, it may just seem like I can’t get away from them. But I’d bet dollar to donuts that if someone traveled to each continent and recorded the birds they saw, the House Sparrow would be on each list, and close to the top.
Well . . . that’s why I didn’t say “Corvids,” but crows, specifically. Corvus brachyrhynchos/caurinus covers every danged inch of N. America, IIRC, which comprises almost every climate you’d find globally. What my thinkin was based on.
Yeah, I thought of that nasty little rodent of a bird too, but then I don’t see them, sometimes AWAY from cities; deep dense wilderness kinda places. Whereas there’s ALWAYS a crow somewhere between here an the horizon. As I’d assume you’d agree with, from your username (which I’ve always admired 
).
My National Geographic Birds of North America book says its range extends over much of the 48 states and Canada, but covers less than 5% of Alaska and hardly touches Mexico at all. So it looks as if this is not a worldwide species.
With regard to birds whose distribution has been influenced by humans, the domestic chicken undoubtedly has the widest range, but has only become feral in some areas. The Rock Dove or Rock Pigeon (Columba livia) has been introduced on every continent except Antarctica, and I believe has the widest range of any feral species, and perhaps of any land bird species. But it is generally limited to the vicinity of human habitation except in its original range. House Sparrow comes close, and is world-wide, but isn’t quite as ubiquitous as pigeons, being generally more resticted to the area of cities in much of the world.
For land bird species, the most widely distributed species is probably the Common Barn-Owl Tyto alba, which has a nearly world-wide distribution except for polar regions and central and northern Asia.
Of birds associated with fresh water and coastal area, the most widely distributed are the Osprey Pandion haliaetus and the Great Egret Egretta (or Casmerodius) alba. Both are essentially worldwide in temperate and tropical regions. The Egret has a wider distribution as a breeding species.
For seabirds, the species with the widest distribution is the Parasitic Jaeger (aka Arctic Skua) Stercorarius parasiticus, a gull-like bird that nests in the Arctic but is found at sea almost world-wide except in the Antarctic. Other species with very wide ranges at sea include the Pomarine Jaeger, Red-necked Phalarope, and Arctic Tern.
As has been said, gulls comprise many species and none of them is as widely ranging as the species mentioned above. Crows aren’t even in the running; the widest ranging corvid is probably the Northern Raven, which extends across northern Asia and North America only.
Kind of off-subject, but brachyrhynchos, your name leads me to believe you know a lot about crows. I am totally intrigued by crows! I absolutely love them. When I recently moved from Virginia to California, I noticed that the crows here are larger. Is that true, or was I just seeing things? What else can you tell me about your knowledge/experience with crows? Do you know of any particular way to attract them to your yard?
Is this with C. brachyrhynchos and C. caurinus combined, as conspecifics? Not defending; I don’t know myself, it was just an educated guess.
I’m not surprised to see Ospreys in the running; I’ve seen them in the dangedest places. But the Barn Owl surprises me: in my lifetime of bird watching, east coast to west, I’ve never managed to see one. Seen an Arctic Owl well south of its normal range, but never seen a barn owl.
C. caurinus has a pretty small range, so even if merged with C. brachyrhynchos (which I haven’t heard any proposals to do) it wouldn’t increase the total range significantly. Northern Raven, on the other hand, has a total range, in North America alone, that is similar in size to that of C. brachyrhynchos (ranging both farther north and farther south, but not present in much of the east), and also extends all the way across northern Eurasia.
This is because they are almost exclusively nocturnal. If you know where to look for them, they can usually be seen at their day roosts pretty easily.
So… where do I look? I would love to see one of these beautiful creatures in person!
Barns are usually a pretty good place to start. 
Especially ones with lots of mice. Barn owls like to roost in human structures, including belfries, abandoned buildings, etc, especially ones near open fields where there might be a lot of rodents.
In New York City, I have seen them roosting under a highway bridge across the Hutichinson River, and a couple used to roost in crevice at the pavilion at Orchard Beach. Here in Panama, in farmland they tend to roost in brush along watercourses.
The best bet is probably to get in touch with your local Audubon chapter if there is one. If an Audubon Christmas Count is held in your area, someone will have the good owl roosts staked out.
I don’t know why, but “feral chicken” is the funniest thing I’ve read all week.