By Christine Parisek & Jon Walter
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Food webs are the backbones of ecosystems: they chart the flow of energy through ecosystems in terms of who eats whom, and their structure helps determine whether an ecosystem will be stable over time. While food web studies often focus on relationships within a particular habitat type – a lake or a forest, for example – broader predator-prey relationships can couple a lake to the forest that surrounds it. We conducted such a study, examining predator-prey relationships at the aquatic-terrestrial interface, and we report an unexpected new finding: fish are important predators of birds in food webs spanning water bodies and shorelines (i.e., riparian zones) across North America.
Using a synthesis of riparian-zone bird datasets from 41 streams and lakes across North America, we found that bird biomass tended to decline over time (Figure 1), prompting us to ask why these declines were occurring. Across North America and globally, bird abundance has declined markedly over the last few decades (e.g., Johnston et al. 2025). There have been several proposed explanations – including habitat loss, climate change, changes in prey or resource availability – and the ecological drivers of these changes have been debated.
Food Web Study
To assess whether trophic mechanisms contribute to bird biomass declines over time, we conducted a food web study. We used stable isotopes δ15N and δ13C to map food webs at each study location. We then estimated each organism’s trophic position (TP), a measure that represents the organism’s functional role within the food web based on where the organism derives its energy sources. Birds in riparian areas often prey on aquatic insects emerging from waterbodies, so we sampled not only land-based organisms but also aquatic organisms like zooplankton, insects, and fish. The results at first flew over our heads. Much to our surprise, predatory fishes were consistently at a higher trophic level than birds, and this was true not only for small songbirds but also for larger waterbirds, like ducks, geese, and swans (Figure 2).
Fish Gut Content Analysis
Intrigued, we followed up by examining the fish gut contents. Stable isotopes provide a longer-term record of where an organism fits in a food web, but gut content studies directly identify, enumerate, and weigh the food an organism ate by examining the digestive tract. This was crucial to validate our food web findings. Corroborating the surprising implication of our stable isotope study – fish are eating birds – a follow-up gut content analysis revealed numerous instances where birds and bird body parts, including feathers, beaks, and feet, were found in the guts of fishes. In fact, 26% of all fish and more than 80% of fishes longer than 12 inches (total length) had birds or bird parts in their bellies!
What’s more, our data suggests this is a pervasive phenomenon with far-reaching implications for bird population trends. When we compared long-term trends in bird biomass to the biomass of predatory fishes at each site, we found a negative relationship such that the sites with low predatory fish biomass tended to have stable or even increasing bird biomass, and that bird declines were more severe at sites with higher predatory fish biomass (Figure 3).
Conclusions and Implications
Birds of prey have long been known to consume fish, but our research shows how the reverse is also true; fish consume birds at surprisingly high rates, leading to fish being important predators in avian foodwebs. While against prevailing expectations, our findings are not without precedent. Fish attacks on low-flying birds have been reported (Figure 4). Additionally, naturalists have often noted disappearances of young waterfowl and wading birds, and smaller birds have been observed avoiding certain shoreline habitats. Though these observations have previously been attributed to terrestrial predators like snakes and weasels, the role of fish as predators in such interactions must be rethought. Synthesizing our findings with earlier observations, we propose a new model for lake food webs (Figure 5).
We acknowledge this study might ruffle a few feathers. However, moving forward, integrating bird-fish management frameworks will be essential to avian conservation. Indeed, this work demonstrates how regimented fish stocking programs can produce unintended consequences for bird populations by unintentionally increasing predation pressure on birds. In an effort to promote bird-safe fisheries management, we suggest limiting fish stocking in areas known to have sensitive bird populations – such as migratory flyways – or during times when they are more susceptible to risk – such as the juvenile and hatchling life stages.
We further suggest that even minimal additional management of shoreline vegetation – which can aid in obscuring bird prey from their fish predators – could be disproportionately valuable towards helping birds remain more protected while they forage.
Additionally, stocking birds into riparian habitats could be a valuable supplement to fish diets, including for some of California’s endangered fishes. For example, given declines in zooplankton density in the San Francisco Estuary (e.g., Bashevkin et al. 2023), delta smelt may benefit from looking above the water for new prey items. Future research could investigate the palatability of tiny hummingbirds, such as Mellisuga helenae, to delta smelt.
Ecology, as a discipline, would benefit from improved understanding of these fin-and-feathered entanglements more closely. Further monitoring efforts and additional aquatic-terrestrial food-web studies will be needed to understand the implications of the findings presented here.
In summary, fish are better than birds. The authors also note that the timing of this post (April 1, 2026) may be relevant to the reader’s interpretation.
About the Authors
* Both authors contributed equally. Author order was established via a carefully adjudicated coin toss of an Icelandic 100-króna – specifically, the one with the lumpfish.
Christine A. Parisek is a Postdoctoral Research Scholar at the Center for Watershed Sciences at the University of California, Davis. Her research integrates global-scale patterns in landscape limnology, local-scale community and trophic dynamics, and freshwater ecosystem science. Christine is also Managing Editor of the California Water Blog and a Science Communications Fellow at the Center for Watershed Sciences.
Jonathan A. Walter is a Senior Researcher at the Center for Watershed Sciences at the University of California, Davis. A (jokingly) self-described “indoor ecologist,” Jon specializes in developing and applying statistics and mathematical models to study the dynamics of aquatic organisms and ecosystems.
Data Availability
Data underpinning these analyses are available at https://github.com/jwalter/fisheatbirds
Further Reading
Hamilton, A., May, R. & Waters, E. 2015. Here be dragons. Nature 520, 42–43. https://doi.org/10.1038/520042a
Frobish, N. J. 2015. Exotic animals deployed as Delta ‘weed whackers’. California Water Blog. https://californiawaterblog.com/2015/04/01/exotic-herbivores-deployed-to-mow-down-waterweeds-clogging-delta/
Johnston, A., et al. 2025. North American bird declines are greatest where species are most abundant. Science. https://www.science.org/doi/pdf/10.1126/science.adn4381
Bashevkin, S.M., et al. (2023) Long-term trends in seasonality and abundance of three key zooplankters in the upper San Francisco Estuary. San Francisco Estuary & Watershed Science 21:3:1. https://escholarship.org/uc/item/2b87w198
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