Bird competition: How warblers share trees without fighting

If you’ve ever watched songbirds squabble at a backyard feeder, you’ve seen a small glimpse of the competition animals face for limited resources.

While some species seem to be constantly at odds, others – even those that appear quite similar – manage to coexist in surprising harmony.

For decades, a classic theory from the field of ecology has suggested that forest-dwelling warblers share trees by using different parts of them.

Now, a new study adds a modern twist to that idea and shows it may be more complicated than it seems.

Revisiting a classic idea

The research, conducted by scientists at Penn State and the American Bird Conservancy, revisits an influential idea that was first proposed by ecologist Robert MacArthur in the 1950s.

MacArthur observed that several species of warblers could live in the same tree without crowding each other out, simply by foraging in different zones – some near the trunk, others at the tips of branches, and at varying heights.

This concept has been a staple in biology textbooks ever since. But thanks to DNA analysis and decades of fieldwork, the new study suggests that the story goes deeper.

A close look at species interactions

Study co-author David Toews is the Louis Martarano Career Development Professor of Biology in the Penn State Eberly College of Science.

“MacArthur suggested that we needed to look more closely at species interactions. We wanted to revisit this idea with modern technology, using molecular methods and an evolutionary perspective that weren’t available in MacArthur’s time,” explained Professor Toews.

“This allowed us to not only directly quantify their diet, but also better understand how warblers are partitioning their habitat and how what we see today may have been shaped by competition over the warbler’s evolutionary history.”

Analyzing bird competition

The team observed 13 warbler species that nest in northeastern New York forests during the summer.

Over 20 years, the researchers tracked where in the trees each bird preferred to forage – how high up they searched, how dense the foliage was, and how close they stayed to the trunk.

The experts also collected fecal samples during five breeding seasons to analyze what the birds were eating.

Using a technique called fecal meta-barcoding, they could extract insect DNA to identify prey species. Alongside this, they studied the physical traits of each bird, including size and beak shape, using public databases.

Songbird foraging behaviors

The findings showed that each bird species did indeed forage in slightly different ways. “The implication from MacArthur’s study is that, if the birds are foraging in different areas of the tree, then they are eating different things,” said Professor Toews.

“MacArthur tried to test this using data from his colleagues, who attempted to analyze fragments of insects from stomach contents, but this was biased toward fragments that weren’t digested, and they were extremely difficult to identify.”

“Now, we can extract DNA from fecal samples and use a technique called fecal meta-barcoding to see which insect species have been eaten by birds.”

Pressures over evolutionary timescales

While the birds’ movements and strategies differed clearly – smaller species tended to hover more, and those with longer legs stayed closer to the ground – their diets were surprisingly similar.

Eliot Miller, BirdsPlus Index Manager at the American Bird Conservancy, noted that a bird’s and shape are products of many pressures operating over evolutionary timescales.

“But numerous studies, including our own, show that chief amongst these pressures is foraging behavior,” said Miller.

Subtle differences in songbird diets

The DNA analysis revealed that although the warblers fed on thousands of insect and spider species, there was a great deal of overlap in their diets.

“The birds we studied collectively consumed thousands of arthropod species, and there was a considerable amount of overlap in their diets,” said Professor Toews.

“The differences in diet were quite subtle, and we found that closely related species had more similar diets. This suggests that MacArthur’s emphasis on diet may have been partially misplaced, though the general idea of portioning their resources by foraging in different parts of the trees is sound.”

Many birds were simply taking advantage of what was available. Professor Toews noted that more than 75% of fecal samples contained the most common insect in the area, a snipefly, and more than half of the samples contained a common, invasive leaf weevil.

This kind of opportunistic feeding may suggest that while competition shaped the songbirds’ physical traits and foraging strategies, diet may not have played as strong a role in pushing species apart.

Bird migration and competition

“Because of the notable differences in foraging behavior, we believe that competition has shaped how these species forage today, as well as how they evolved to forage over evolutionary time,” Miller said.

“But we don’t see many differences in diet, so it’s likely that other forces may have more directly influenced warbler evolution. It is worth noting that these birds migrate to Central and South America over the winter, so competition for resources in those locations as well as the stress of migration may also shape the traits we see today.”

The researchers are planning future studies to look more closely at what warblers eat during their time in the tropics and how the nutritional value of their food might vary.

This could offer deeper insight into how competition and migration shape these songbirds beyond their breeding grounds.

A more nuanced view of coexistence

“MacArthur was right in that these warblers are partitioning their habitat in fine ways, but it’s a little more complex than he originally thought,” said Professor Toews.

“People often observe competitive interactions between species at their bird feeders, and studies like this show how these kinds of interactions that happened hundreds or thousands of years ago can shape the species we see today.”

The takeaway? Even birds that seem to get along may be playing out a long, slow competition – one that stretches across trees, continents, and generations.

The full study was published in the journal Biology Letters.

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