Hey! Check Out This Donkey...
In a grassy clearing in the western foothills of the Peruvian Andes, a fresh donkey carcass is beginning to attract flies. Warmed by the tropical sun, its fetid smell is rising in the air. A Turkey Vulture far overhead catches a whiff and it, too, is drawn to the dead animal.
Over the next week or so, the carcass will be picked apart and eaten by several species of scavenging birds: Turkey Vultures, Black Vultures, King Vultures, Andean Condors, and Crested Caracaras.
If you were to watch the process of all that tasty, tasty donkey meat being eaten, you would not see all of these bird species sitting around the carcass together, peacefully sharing a meal. No. What you would see are frequent squabbles or outright brawls between members of two different species, with much pecking, clawing, and furious wing-flapping. The losers of these battles wait anxiously on the periphery until the victorious birds are done stuffing their gullets with meat.
A scientific study of this exact scenario found a dominance hierarchy among these five bird species. By documenting thousands of bird fights at carcasses, the researchers found that larger species tend to win fights with smaller species. The bigger you are, the more likely you are to win. Andean Condors are the heavyweights, the big boys, in those Peruvian foothills, so they nearly always win against the other species. The dominance hierarchy then goes King Vulture, Crested Caracara, Turkey Vulture, and Black Vulture. That’s the ‘pecking order’ in those parts.
Dead donkeys, and other animal corpses, are a precious resource for these scavenging birds. But large carcasses aren’t all that common, and they’re distributed haphazardly across the landscape.
When multiple bird species are trying to use the same vital but limited resource, it’s likely they will have to compete to get what they need.
This same thing happens around elephant carcasses in Africa, among multiple species of Old World vultures, which, by the way, are unrelated to those living in Peru. There is a similar dominance hierarchy among those African scavengers.
Anytime multiple species are vying for the same limited and critically important resource—whether its food, nesting habitat, or something else—there’s a good chance those species are competing with each other.
So you don’t have to go to Peru or Africa to see brutal bird battles. The same kind of drama is happening right there at your backyard bird feeder.
This blog post is all about competition between bird species, in other words: interspecific competition.
Before we dive into the topic of competition itself, we need to first talk about the concept of the niche.
Then we’ll talk about the different ways bird species compete. It’s not all just kicking and biting.
We’ll follow that with a discussion of how competition between species seems to have influenced bird populations and species through time. What has resulted from all of that competition?
Last, we will talk a wee bit about the challenges scientists face when researching competition between bird species.
The Niche
You’ve probably heard the word niche before, either in the scientific, biological context, or in its usage as a narrowly defined but profitable market in business.
A bird’s ecological niche is the place where it lives and the resources it uses.
What resources are we talking about? Well, food is the one we most often think of. But we might also be talking about nesting locations, shelter from predators, perches for displaying to potential mates, and so on.
Another way of thinking about a species’ niche is the ‘job’ it performs in an ecosystem. Most broadly, we might think of such jobs as being things like ‘predator,’ ‘scavenger,’ ‘grazer,’ ‘filter-feeder’ and the like.
But if we get more fine-grained in our thinking, we discover that there are literally countless niches, jobs, out there in nature… whether they are currently occupied by existing species, or they are just vacant possibilities.
So, again, a bird’s ecological niche is the place where it lives and the resources it uses.
If a bird species were unconstrained by other species—by competitors, predators, or parasites—we would say it occupies its fundamental niche. The fundamental niche of a species is all the places it can possibly live and all the resources that it can possibly use. In this ideal situation, the species is limited only by its imagination! Just kidding. It would be limited by its anatomy, physiology, behavior, and it’s imagination.
The cold, hard reality is that most bird species don’t get to fly around fully enjoying their fundamental niche; they’re not so lucky. Instead, they operate within a subset at the fundamental niche known as the realized niche. The realized niche is the one we observe species living in. The actual habitats that they use, the space that they occupy, the resources they’re able to use, given the presence of other species that might be competitors or might be predators or might affect them as parasites.
Most species that we know of, perhaps all, are operating within a realized niche, not a fundamental niche.
We can also think of a niche as being broad or narrow.
Some birds are specialists, with narrowly defined niches, while others are generalists, occupying much broader swaths of niche-space.
This is true for people too, regarding our skills in life. Some of us are generalists, Jacks or Jills-of-all-trades, masters of none, that kind of thing.
People who are specialists, on the other hand, are those who have probably put in their “10,000 hours” of practice time to become true masters of one skill.
I know you could come up with a list of pros and cons for a person being a generalist vs a specialist. Similarly, there are pros and cons for bird species occupying broad, generalist niches vs narrow, specialist niches.
For example, specialists can be vulnerable to environmental change. If a bird specializes in eating one type of fruit and that fruit isn’t available during a prolonged drought, the bird may go extinct, at least locally.
No species is a true generalist in an absolute sense.
I mean, you might think we humans are true generalists. As a species, we can live in almost every environment and we eat many types of food.
But we don’t live in all environments. We’re not living at the bottom of the ocean. We’re not living among the clouds in the atmosphere or 5 miles underground. At least not yet.
And we don’t eat pine needles, wasps, or giraffe dung. At least not yet.
Every species on Earth, including us, has some kind of limited niche.
Niches among bird species can overlap, at least in some dimensions. And the more the niches of two species overlap, whether in one dimension or many, the more likely they are to compete fiercely for resources.
For example, if two bird species living along the banks of a river eat the same type of fish of approximately the same size—well, those two bird species will have a hard time coexisting over the long run, at least in that particular location and habitat. Someone or something will have to give.
This brings us to the competitive exclusion principle. This is the concept that two species with identical niches are going to compete fiercely. The inevitable result, according to this principle, is that the stronger competitor of the two species is going to either drive the other to local extinction, or force it to undergo an evolutionary or behavioral shift. Such changes in the weaker species could end up dividing the single shared niche into two, less-overlapping, niches.
If this principle is true, then no two species on Earth can have the exact same niche. They can’t live in the same place and share the exact same resources. One of the two will always win out in the end, excluding the other, somehow.
The competitive exclusion principle is an important concept in ecology, but there’s a lot of debate about how much of the variation we see in niches results from this phenomenon.
It’s an elegant idea, but its consequences are not so easy to detect in real-world examples.
Interspecific Competition
Okay, so we know competition is going on out there. It’s certainly happening within each species, just like it’s happening within our human species. It’s a dog-eat-dog, cut-throat world, after all.
Remember, though, that in this blog post we’re focused on interspecific competition among birds—that is, competition between species, rather than within species. We’ll save intraspecific competition for another blog post.
Competition should be highest between species with very similar niches, as I talked about earlier.
And competition should be most fierce and have more dire consequences at times and places where the resource being fought over is at its most limited.
For example, bird species that eat insects face the risk of starvation and hypothermia in winter, when their prey is rare. These birds must out-compete similar species in finding food if they are going to survive the cold months.
Conversely, when times are good and everyone can gorge until they pass out… Well, competition for that resource during those seasons probably isn’t so important for determining which species can coexist in a given habitat.
It’s like when the coronavirus pandemic began, everyone ran to the grocery store to buy heaps of toilet paper before it was all gone. But when the pandemic cooled off a bit, we weren’t so desperate to hoard TP. We got back to more important competitions, like who can get more Instagram followers? Or who can eat the most hotdogs?
Forms of Competition
Now let’s talk about how bird species compete.
It isn’t just one bird species pummeling the heck out of another… Feathers flying everywhere and all that. There’s a bit more to it.
There are two primary flavors of inter-species competition: interference competition and exploitation or exploitative competition.
Interference Competition
Interference competition is perhaps the more intuitive of two. This one includes the aforementioned pummeling. It’s any form of direct—often, but not always, aggressive—behavior that interferes with the less competitive species’ ability to get resources.
Our Peruvian vultures and caracaras jostling for dead donkey meat are an example of interference competition. Aggressive behaviors of the larger species limit access to the food resource for the smaller species.
Another classic example that I remember fondly from when I took Ornithology in college, long ago, is that of competition for nesting habitat between Red-winged Blackbirds and Yellow-headed Blackbirds
In marshes of the American Midwest, both species prefer to build their nests in cattail reeds growing in open water. This is safer than building your nest along the margin of the marsh where wily predators have easier access.
Even though Red-winged Blackbirds arrive first to stake out territories in spring, the larger, more aggressive Yellow-heads show up and force the Red-wings to make do with sub-optimal nest habitat at the outer edges.
When Yellow-headed Blackbirds aren’t present, the Red-wings rejoice because they get to expand their niche by building nests in the best wetland real estate.
A really cool study published in 2017 looked at the interactions of 136 bird species at backyard birdfeeders across North America. Aggressive behavior at feeders is a form of interference competition. The researchers were able to construct a dominance hierarchy score for each of these species. This is sort of like our Peruvian scavenger study on steroids.
As in the Peruvian study, body size, i.e. body mass, was the primary factor in deciding who wins, on average.
Interestingly, some bird groups were found to defy expectations based on the overall trend. For example, wood warblers and woodpeckers are more dominant than expected based on their body size. Same goes for hummingbirds. Meanwhile, doves are less dominant than expected. That makes sense to me, since doves seem like such gentle birds. Science can back that up.
Another cool thing about this study is that citizen scientists collected the data via Project FeederWatch, which is operated by the Cornell Lab of Ornithology. Thousands of people contributed observations from their humble backyard feeders to make genuine science happen and for real discoveries to be made about these birds.
So check out Project FeederWatch as well as eBird and iNaturalist if you’re interested in making significant contributions as a citizen scientist.
Exploitation Competition
The other fundamental type of competition between species is exploitation competition. This type is less direct. In this case, one species can out-compete the other just by being better at exploiting a particular resource. If we’re talking about a food resource, if species A gobbles up most of the food, there’s not much left for species B, so species B is the loser, even though there wasn’t any direct interaction or direct competition.
One of the better known studies providing evidence of exploitation competition was published in 1987 and took place in the frozen forests of Sweden in winter. The birds studied were the Goldcrest and three species of tit. When these birds forage for insects and seeds in the winter, they often flock together. The Coal Tit and Goldcrest are smaller and less socially dominant than the Willow Tit and the Crested Tit. The researchers did an experiment where they removed the smaller, subordinate species from several mixed flocks. What resulted was that the larger two species expanded their foraging habitat to include parts of the forest canopy—the outer branches of conifer trees—that the smaller species usually focus on.
The best explanation is that the diminutive Goldcrest and Coal Tit normally out-compete the larger species in collecting food from the outer branches. The little guys win in this classic example of exploitation competition. Good for them.
The Outcomes of Competition
So what are the possible long-term outcomes of all this competition?
What are the patterns we would expect to see out there in natural ecosystems if bird populations have been shaped by interspecies competition?
Well, at least theoretically, there are several outcomes we would expect.
Where competition is actively occurring, where two species have strongly overlapping niches, we would expect that the dominant species would have a population that is increasing and the subordinate species would have a decreasing population.
But this effect of competition has been a little hard to measure and therefore demonstrate in nature. Just because there is apparent competition between two species, it doesn’t mean that there are any ongoing population or species-level consequences of that competition.
It’s likely that population fluctuations are influenced more strongly by other forces, such as predators, extreme weather events, and disease.
Nevertheless, there are examples where in the absence of its apparent competitor, a species increases in local population density. This has been seen on isolated islands where there are subsets of species that co-occur on the nearest mainland.
What we see more often are cases where bird species seem to have worked out ways to avoid competing. Such that they can live together in perfect harmony, side-by-side. Well, maybe not perfect harmony, but with relatively little outright strife.
Now consider, if you will, a big mess of shorebirds on a tidal mudflat.
On this small patch of coastline, you see curlews, yellowlegs, dowitchers, plovers, willets, whimbrels, sanderlings, sandpipers, godwits, and more.
All of them seem at first glance to be foraging practically shoulder-to-shoulder, for what looks like the same type of food.
But paying closer attention, you see that actually no, they have a wide variety of bill shapes and lengths, which they are using to catch different sorts of invertebrate prey. And their legs are of wildly different lengths. The long-legged species are out wading around in deeper water, while the little, stubby guys are zipping back and forth across the drier flats.
These shorebird species are not actually competing for the same food.
What we’re seeing in this example is habitat segregation. Over millions of years, competition has driven similar species to evolve different lifestyles, such that they can now minimize competition by exploiting different microhabitats in the same general habitat.
A concept similar to habitat segregation is resource partitioning. This is where similar, closely related species have evolved to divide up a resource by size or some other dimension.
For example, research on kingfishers in South America shows that five species living in the same habitat differ dramatically by body size and bill length, such that each species eats fish of a different size. These birds are partitioning the resource of fish prey by size.
There are many, many examples of bird species divvying up their habitats or resources. Darwin’s Finches in the Galapagos Islands are another classic case. Real-time evolution in the beak sizes of these species has been documented and strongly linked to competition.
The overarching pattern seems to be that the more species can exist in non-overlapping niches, the more they can occupy the same habitat.
Studying Competition Between Bird Species
The idea that competition between species is very important in determining what species can coexist in a habitat comes from classical ecology, going back many decades. But this is still a topic of heated debate among biologists. Research is ongoing.
As I mentioned, it turns out that it’s difficult to document present day competition.
Sure, you can stake out a rotting donkey carcass to record which scavenging bird species are beating up their rivals, but are those interactions influencing the populations of those birds such that evolutionary change results? Hard to say.
All the lovely examples we have of habitat segregation and resource partitioning are often referred to by scientists as “ghosts of competition past.” These patterns we see today are, we assume, the end-result of competition among species that was occurring a long time ago.
Most of the research on this topic is observational. Scientists use observations to develop hypotheses about bird competition. Predictions are made from these hypotheses, and then further observations in the field either support or refute the hypotheses.
Using this approach, it’s often difficult to measure competition between species that has clear evolutionary or ecological consequences.
An experimental approach is better, where variables can be controlled. One of the few examples of this approach is the study in Sweden I mentioned, where two insect-eating bird species were removed to see what happens to the remaining two species. But experiments like that are challenging, logistically and sometimes ethically, which is why we don’t see many of them.
Whether we humans can measure it, competition is most certainly going on out there among bird species. Through evolution, extinction, or behavioral changes, birds have in countless cases reduced competition such that they can coexist in remarkable, beautiful ways, all across the planet.
If you have a birdfeeder filled with seeds or a hummingbird feeder, maybe take a little time to watch the inter-species interactions you see out there. Can you figure out the dominance hierarchy in your backyard?