One thing I've learned here: A catchy title is crucial. So technically, it should probably be "Invasion of the Birdie Displacers" or "Invasion of the Breeding Cavity Snatchers", but... those just don't have the right ring. If this was Fox Noise, I could whip up a frenzy describing how our fine, native birds are under siege from alien hordes... but, nah. Although I might keep with their anti-elitist tone and curse Shakespeare (just this once).
Few things illustrate the law of unintended consequences better than invasive species.
A hundred-odd years ago, some genius got the idea to create a Shakespeare Garden in Central Park. It was planted with species mentioned in his works and populated with birds he'd included in his writings. Starlings were there because of these lines in Henry IV, which played off their abilities for mimicry:
“When he lies asleep,
And in his ear I’ll holler ‘Mortimer!’
Nay, I’ll have a starling shall be taught to speak
Nothing but ‘Mortimer,’ and give it him,
To keep his anger still in motion.”
[1st Henry IV – I, 3]
Well it seemed like a good idea at the time. Apparently, it seemed like such a good idea that they brought over a second batch after the first group died out. And a third group after the second died off... and that third group must have included a few very clever birds. They survived, and they bred. And... they spread. And they spread and they spread and they spread...
Now they're found across the continent, where they've adapted to a wide range of conditions. Good for them to be so clever and find a way to make it in a brand new place... but bad news for the native birds they've displaced. Starlings are cavity breeders, and good nesting cavities are a limiting factor in many places. There's a bit of labor involved in creating cavities (mostly done by woodpeckers) and many species compete for the cavities when woodpeckers give them up. The bigger, aggressive starlings win out over bluebirds and other natives.
Female House Sparrow (bigger version here)
The introduction of the House Sparrow is a little murkier - some accounts include them as another Shakespeare Garden bird, but it seems as though they'd been introduced before that time. It may be that they'd been brought over to remind earlier European immigrants of their birds "back home". Some accounts say that they were brought here to control insects. In any event, they made themselves at home and spread across the nation; like starlings, they're cavity nesters so they've been displacing natives all along the way.
The male House Sparrow in breeding plumage is actually a rather handsome bird.
Ironically, while they've been wildly successful in the New World (they're also widespread in South America), their populations have been in steep decline in their native turf. It's also interesting to compare the success of House Sparrows with their close cousins, the Eurasian Tree Sparrow. The tree sparrows were introduced in St. Louis in the late 1800s and established a self-sustaining population, but their range has not expanded beyond St. Louis.
Pigeon dude struts his stuff...
The consumate urban bird, the Rock Pigeon (commonly known as just plain Pigeon, derisively known as flying rats) came over with earliest immigrants. Many colonial houses had dovecotes to give them a home. They do not seem to have caused the same problems for native bird populations as the cavity nesters, but it may be that we're not aware of what was here before they arrived. To many people, the biggest problem with pigeons is their sheer numbers in urban areas; no one likes to find themselves under a pigeon roost. Someone is happy about them though - the success of many urban peregrines is due in part to the presence of their fellow "cliff dwellers" in the city canyons... pigeon is a major food source.
Rock Pigeons (bigger)
The pigeons came with the pilgrims, and starling and house sparrow population explosions took place in the late 1800s, when few (if any) understood the problems with invasive species. But the spread of Eurasian Collared Dove is going on now. After appearing in the 1970s and 1980s in Florida, they have spread across the lower tier of the country and are now spreading rapidly in California.
(BTW, this is another area where Citizen Science has been very helpful. The spread of these invasive species has been well documented due to studies like Christmas Bird Counts, Project Feeder Watch, eBird and other ongoing monitoring programs. Taking part in these citizen science activities really does help us understand what's happening to our bird species.)
Non-natives species are not destined to become invasive - indeed, many or most die out without breeding successfully. Many others do manage to establish small, self-sustaining local populations, but they do not spread widely. (Even these small local populations can be problematic if they displace natives.) Examples of these local populations include the monk parakeet colonies in Chicago and New York, San Francisco's movie star parrots, and St. Louis' Eurasian Tree Sparrows.
Some species have established themselves across a wide area, but at low enough population levels that they don't generally cause problems. This is often the case for game birds - ring-necked pheasant, chukar, Himalayan Snowcock. Wild Turkey, which is native to North America, has been introduced into areas that were outside of its historical range. Here in California, their numbers have grown so much that there is concern about damage they're doing... oh well, we'll learn someday.
House Sparrow, surveying the terrain. (bigger)
Despite all the problems they create for our native birds, I have to admit a certain affection for some of these invaders, especially the starlings. As with another big introduced problem, the housecat, my beef is with the people who created the situation, not the critters themselves. There are many non-lethal actions we can take to help keep their populations in check.
The most important step is to discourage them from breeding. House sparrows and starlings breed pretty much unchecked in urban areas, and each year's new crop of fledglings spills out into wilder areas. Both species like to nest in eaves, in roof tiles, crevices and open vents. To the extent that you can close off those openings, do it. If there's a torn screen on a roof vent, repair it.
Starling displaying near nest (under eaves)
A few weeks ago, NestMaster wrote a diary on nest boxes, and one thing he mentioned was the size of entry holes - an important factor in excluding invasives. A typical gift-shop bird house is built without regard to the size of the opening, and the holes are usually large enough to allow house sparrows to use the box. Before you buy or build a birdhouse, check the specs!
Food is another factor - if you're tossing breadcrumbs for birds in the park, you're probably feeding pigeons and house sparrows. It's fun, but... don't. Cheap birdseed mixes include a lot of millet, which house sparrows love, but many native birds don't. It's more expensive to feed things like black oil sunflower seed, but the house sparrows don't like it nearly as much as the natives do. (That's worked for me, but it doesn't help everyone - you can find more suggestions at this bluebird site
Starlings love suet, but so do woodpeckers and chickadees, so I'm not gonna stop offering it. The good news is that they make starling-excluding suet feeders, which require birds to cling to the bottom of the feeder to eat.
You can't put the toothpaste back into the tube - they're here to stay, so we might as well admire their tenacity (but test it just a little bit when we can).