I hope your question is more from the lack of knowing or curiosity than not seeing that many of these species are only extinct or on the brink of extinction because humanity could not be bothered to be a little considerate while modernizing. I'm all for progress, but when we trample on other humans or other species just to save a little time or money, it's kind of saddening. I always thought that because we have the ability to care about the survival of species other than our own that it contributes to what makes us "human." Some animals may care in way because certain species are an important food source, but we've come to care about animals for reasons beyond that.

The extinction of the passenger pigeon was no accident or even carelessness. People literally hunted them at an industrial scale with large nets at their nesting sites (many ended up as canned food for the military or railroad workers). The passenger pigeon is not even closely related to the sometimes reviled rock pigeon that inhabits Europe and as a non-native invasive species in North America. Instead, it is a close relative of the North American mourning dove. I assume that is why this commenter[0] has a "knee-jerk" dislike of the passenger pigeon with no explanation given.

Passenger pigeons were perhaps the most numerous bird to ever exist and yes, while they did primarily eat seeds, so do many other living species including the European house sparrow brought to North America around 150 years ago. House sparrows and other species have mostly filled the niche that the native passenger pigeon once inhabited. I mean house sparrows are so numerous in North America that most people do not even realize there are a couple dozen native species of sparrows here that are only distantly related[1][2]. Unlike their distant European relative, these native sparrows tend to be prettier and less numerous due to being territorial in breeding pairs all year round, though some sparrow species[1] will flock together during migration.

This might sound like something made up, but the house sparrow, along with the European Starling were brought over to North America by some guy that wanted all the birds of Shakespeare's plays to inhabit the New World. Ironically, both species are now in decline throughout the Old World while they dominate and displace species that rely on nest boxes and nest cavities in the New World. The movement in the past 50 years to put up nest boxes for blue birds, purple martins and other native species in North America was at least partially motivated by trying to shore up their populations as many were threatened in the past.

Another species that met a tragic demise that could have been saved was the ivory billed woodpecker[3] in the early 1900s. The ivory billed was the largest woodpecker in North America (about the size of a crow), with the still living pilated woodpecker a close second (both resemble Woody the Woodpecker). Efforts were made to have its last refuge made a protected forest, but ended up going nowhere as elected officials considered the wood in the forests to be more worth while. There's been unverified sightings by people that claim the ivory-billed still exists in remote locations in the Southern United States. It's more likely those that claim to have seen it really saw a pilated woodpecker, as no images or recordings have been made by those that claimed to have seen it.

There was also a lesser known parrot[4] that lived in the temperate Eastern/Southern United States that died out around the same time as the two mentioned above. The Carolina Parakeet lived as far north as Ohio and was the only native parrot in the continental United States. Reasons for its extinction are a bit more complicated, though it has been partially attributed to deforestation and the spread of the European honey bee in North America as they competed for nesting cavities in trees (random fun fact that honey bees are not native to the New World).

Many birds are considered a prime indicator of the health of the environment around them as well, due to wide variety of foods they eat and the elaborate nesting/mating rituals some go through that can be disturbed when things are thrown out of balance.

I'm only in my late 20s, but I've always had an interest in birds and studying them is sort of a hobby of mine. They're one of the few animals in the wild you can see on a daily basis and many of them are as intelligent as primates (such as crows/ravens/rooks, certain parrots and jays). I can pull up other references for anyone interested on where I found the info above, but it takes a bit of time. A good starting point is the Cornell Ornithology site linked below though[1].

[0] https://news.ycombinator.com/item?id=8255389

[1] http://www.allaboutbirds.org/guide/white-crowned_sparrow/id

[2] http://www.allaboutbirds.org/guide/White-throated_Sparrow/id

[3] http://www.allaboutbirds.org/guide/Ivory-billed_Woodpecker/i...

[4] https://en.wikipedia.org/wiki/Carolina_parakeet