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A recent encounter among birds at backyard feeders sheds light on the fierce competition for survival that animals encounter in nature. However, some species that share ecological characteristics appear to coexist harmoniously. A foundational study conducted by Robert MacArthur, a pivotal figure in the development of modern ecology, indicated that similar wood warblers—vibrant, insectivorous songbirds inhabiting forests—can share trees due to their tendency to occupy different parts of those trees and presumably consume distinct insects. A contemporary study is revisiting MacArthur’s findings using advanced techniques, affirming their relevance in current biological research.
Conducted by teams from Penn State and the American Bird Conservancy, this new investigation offers a more intricate view of how these songbirds coexist than what MacArthur initially described. The findings, published in the journal Biology Letters on April 16, encompass a range of factors including foraging behavior, physical traits, diet, and evolutionary background of warblers.
A fundamental tenet of ecology states that two species vying for the same resources cannot persist indefinitely in the same environment. Typically, one will outcompete the other. In his 1958 study, MacArthur proposed that five species of wood warblers, believed to be “ecologically equivalent,” could coexist by foraging in different areas of the same tree. For instance, some species may forage near the trunk while others utilize outer branches or upper parts of the tree.
“MacArthur highlighted the need for a closer examination of species interactions,” remarked David Toews, Louis Martarano Career Development Professor of Biology at Penn State Eberly College of Science and co-author of the study. “We aimed to delve deeper into this notion utilizing modern technologies that weren’t available to MacArthur, allowing for a more accurate quantification of their diets and a clearer understanding of how these species have adapted over time in response to competitive pressures.”
The research team focused on 13 wood warbler species found in the forests of northeastern New York during their breeding season. Over a span of 20 years, they meticulously observed the birds’ foraging behaviors, documenting their height in trees, foliage density, and foraging distance from the trunk. They gathered fecal samples during five summers to analyze the dietary components, which primarily included insects and spiders. Furthermore, data regarding each species’ physical attributes and beak sizes were sourced from public databases.
“The premise from MacArthur’s findings indicates that if birds forage in different areas of the tree, they are likely consuming different food sources,” Toews explained. “MacArthur’s attempts were hampered by biases in the identification of stomach content fragments, complicating the analysis. Today, we can extract DNA from fecal matter using a technique known as fecal meta-barcoding, allowing us to identify what insects the birds have ingested.”
Consistently, the researchers corroborated MacArthur’s findings that foraging behaviors significantly vary among species.
“A bird’s morphology reflects many evolutionary pressures, with foraging behavior being paramount,” noted Eliot Miller, BirdsPlus Index Manager at the American Bird Conservancy and co-author of the study.
The analysis revealed that variations in foraging behavior were associated with the birds’ physical traits. Smaller species were observed hovering more during foraging, while those with elongated leg bones typically foraged nearer to the ground. Interestingly, dietary differences were found to be minimal.
“The species we analyzed consumed thousands of arthropod species, with notable overlap in dietary habits,” Toews added. “While dietary differences exist, they are subtle, and closely related species often share similar diets. This indicates that MacArthur’s focus on diet may not fully capture the complexity of their coexistence, though the overarching concept of resource partitioning through distinct foraging locations remains valid.”
The birds’ foraging was found to be opportunistic, with over 75% of fecal samples containing the dominant local insect, a snipefly, and more than half containing a prevalent invasive leaf weevil.
“Given the significant discrepancies in foraging behavior, it’s evident that competition has influenced how these warblers forage today, as well as their evolutionary adaptations,” Miller explained. “However, the lack of significant dietary variation suggests that other factors might have played a more direct role in shaping warbler evolution. It’s also important to consider that these birds migrate to Central and South America during winter, where they may face additional competition and challenges that could impact their traits.”
The research team aims to further explore the diet and foraging behavior of warblers during their wintering periods, as well as evaluate the nutritional quality of their food sources to determine the effects of competition on their diets.
“While MacArthur correctly pointed out that these warblers partition their habitats in intricate ways, the dynamics are more complex than he initially envisioned,” Toews concluded. “The competitive interactions observed today at bird feeders echo the historical dynamics that have shaped the species we recognize currently.”
The research team included Andrew Wood, a former research technologist in Toews’s lab now at the University of Minnesota, Duluth, and Marcella Baiz, a postdoctoral researcher formerly in the lab now an assistant professor at the University at Buffalo. The collaboration also involved Andreanna Welch from Durham University, Robert Fleischer from the Smithsonian’s National Zoo and Conservation Biology Institute, and Adrienne Dale from Texas Tech University.
This research was supported by funding from the U.S. National Science Foundation, Schmidt Sciences, the Cornell Lab of Ornithology, and Penn State.
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