the rare tropical sparrow of walmart woods
humans can be good for an animal.... until they're not
for my 30th birthday, brittany and i spent a week in pristine costa rican forest observing birds unlike any i’d ever seen before. but the morning before our flight home, i had to visit one more spot, home to one of the world’s most range-restricted birds. i wanted to visit the woods behind the walmart across the highway from the airport.
my target was the cabanis’s ground sparrow, a bird species totally endemic to costa rica. i sometimes associate endemism—when a species only lives in one place—with the creature having some restricted habitat preference leading to its small range. but this sparrow’s preferences allow it to survive between a walmart and an airport in the country’s capital city.
i’d been warned that the walmart birds were shy, and friends of mine had struck out on their own attempts. but the hardest part for me was parking the car—“soy un observador de pájaros” was not enough to convince security to allow me entry into the empty walmart parking lot before the store opened. once i found a spot for the car and started walking, and once the habitat changed from “the road” to “the side of the road,” i immediately encountered my target on the sidewalk.
the triangle of woods measured maybe three acres, and was bordered by the walmart, a highway, and a residential area. it went downhill from the street into a depression formed by a stream, littered with an overturned shopping cart, takeout contianers, and clothes. i’m not great at tropical plants, but i did recognize some invasive bamboo.
the sparrow wasn’t the only life in the lot—in my 20 minutes on the sidewalk, i encountered 17 bird species. that’s because, while lots of species prefer towering ancient forests with trees covered in lichens and bromeliads, others prefer the habitats that develop after disturbances cause forests to lose their trees.
those habitats form as part of ecological succession. disturbances to the canopy from forces like fire and wind flood forest floors with more light than was previously available. seeds that sat dormant in the soil or those blown and pooped in from elsewhere sprout into aggressive plants fighting to take advantage of the resource. the result is a denser habitat with lots of shrubs and vines, since those are the kinds of plants that thrive in these new conditions. a different set of animals can take advantage of disturbed habitats—here in the eastern united states, chestnut-sided warblers and yellow-billed cuckoos are fond of them, for example.
eventually, certain plants will win and shade out the competition, and the habitat will look like it did prior. left to their own devices, forests will continue this cycle of succession in response to disturbances, making them dynamic mosaics of different-aged habitats with unique characteristics. forest edges, with their abundance of sunlight, will retain this shrubby looking habitat, too.
humans are the ultimate forest disturbers, but we do things differently than natural forces like wind or fire. we flatten the forest with asphalt, mow whatever tries to grow, and generally try to prevent forest succession. we still allow some habitats to regenerate in parks, unmaintained lots, fallow farm fields, and along roadsides. we also create habitats that look like disturbed habitats, such as farms and coffee plantations. certain species can adapt to these second-growth forests, our name for forests that regenerate after human-caused disturbance to primary forest, like the cabanis’s ground sparrow.
but here was the head-scratcher for me. if the cabanis’s ground sparrow can tolerate human-altered habitats, then why is its range still so restricted?
the answer to that question, first, lies in the geography and ecology of central america. though smaller than west virginia, costa rica hosts over 900 bird species, many only found in costa rica and neighboring panama. we attribute that diversity to the varied topography in the already-biodverse tropics at the juncture between two continents. the cordillera central and the cordillera de talamanca, the mountain ranges forming costa rica’s spine, interact with the climate to form lowland tropical forests, dry forests, foothill forests, cloud forests, highlands, and the central valley between the ranges. bird families from both north and south america have colonized this habitat and filled the many available niches.
in cases where birds populations became isolated, many evolved into new species. a 2017 paper led by university of costa rica professor luis sandoval used dna to hypothesize that the cabanis’s ground sparrow’s ancestors originated in southern mexico in the late miocene, the era that spans 5 to 12 million years ago. some of those birds dispersed northward, evolving into birds like the familiar california towhee. others dispersed southeast across the isthmus of tehuantepec into guatemala and beyond, becoming the white-eared ground sparrow and a bird formerly called the prevost’s ground sparrow.
the prevost’s ground sparrow further diverged, and a 2014 paper led by sandoval found that it had evolved into two species: the white-faced ground sparrow ranging from southern mexico to honduras and the cabanis’s ground sparrow isolated to costa rica’s central valley. the american ornithological society recognized that split in 2017, and the sparrow became an endemic species of costa rica. while there are dozens of birds restricted to panama and costa rica, there are only four (or five or six, depending on who you ask) restricted solely to mainland costa rica.
so, it’s a happily-ever-after story for the cabanis’s ground sparrow, right? after all, its limited range overlaps with the costa rican central valley, the most populated part of the country, it likes disturbed forest, and humans are a forest disturbance factory.
unfortunately, that’s not the case. forest clearing isn’t just part of the succession process, it’s also part of the urbanization process. so, while some areas have regenerated into appropriate habitat, others have been permanently altered into business districts, suburbs, plazas, and shopping malls—these areas are not appropriate habitat for the cabanis’s ground sparrow. and those second-growth forests that do regenerate might be lower-quality because of invasive species or further disturbance.
as a result, cabanis’s ground sparrows aren’t actually doing well. one 2021 paper led by ornithologist roselvy juárez, now at zamorano university in honduras, found that the birds had bigger territories in urban and suburban areas than in rural areas—they were more densely packed in rural areas, making those rural areas more valuable from a conservation standpoint. another 2021 paper led by university of costa rica researcher pablo muñoz modeled the habitat in the bird’s range and found that 74% of it was unsuitable for the sparrow. that’s because, while the sparrow was able to adapt to the second-growth forests, those habitats aren’t protected. today, humans are clearing second-growth habitats for development and further fragmenting them, to the detriment of the sparrow.
muñoz’s paper gives some ideas, and hope, for the continued existence of the sparrow. the bird takes readily to coffee plantations, which represent the largest patches of habitat in its range. the researchers stressed the importance of maintaining these plantations as habitat, and not further fragmenting them. they also suggested that the costa rican government implement an incentive program to pay landowners not to destroy appropriate sparrow habitat. these measure would benefit species beyond the sparrow—other threatened birds, such as the threatened golden-winged warbler which migrates to the eastern united states and canada to breed, uses these habitats, too.
cities serve as wildlife habitat in surprising ways, but i hadn’t considered that in some cases—like the cabanis’s ground sparrow—the city is the only place for them to go. as always, i think we have a responsibility not to displace the creatures that live on the land we occupy, and it’s crucial for us to maintain our cities in conversation with the surrounding habitat. which feels like it shouldn’t be that hard when the animal is okay living in the shrubs behind walmart.
postscript
they offer good boy points for doing well at my full-time job, and i had accrued enough to earn plane tickets—so i’m taking myself to costa rica again for my birthday at the end of the month. that inspired me to dig up this story, one of my favorite city-wildlife interactions. also, it was too much nyc in a row.
but guess what: it’s seagull season!!!!!!! once the temperature drops below freezing, a switch flips in my head such that the only birds that bring me joy are gulls. rare ones, common ones, i don’t care. if you want to look at gulls, too, meet me at the public restrooms at the end of stillwell avenue in coney island on sunday 1/14 from 10-11:30am. please register here if you plan on coming.
this past weekend, i drove up to connecticut to try my hand at finding a rare gull that’s been visiting the stamford coast for five years now, and successfully located it despite having only a general sense of where it might be. the species is called the common gull, but it’s not common in the united states—it’s a eurasian species. specifically, this individual is a member of the kamtschatschensis subspecies from kamchatka, identifiable by its larger size, longer beak, and darker back. look how handsome!!
