Understanding how insect-eating birds contribute to ecosystem balance is crucial for ecology and conservation. One widely used method to measure bird predation on insects involves placing artificial caterpillars made of plasticine in natural environments. These models retain predator bite marks, which researchers can later analyze to estimate predation pressure. However, despite its popularity, the method’s accuracy has been questioned. Do plasticine caterpillars truly reflect real predation rates, or do they introduce bias? A new study by Laura Schillé tested this method using camera traps, and real dead and artificial plasticine caterpillars (Fig. 1).

Figure 1: Photographs of the experimental design in the field. Red arrows point toward the camera traps. Blue and yellow arrows point toward five plasticine models (a) and three corpses (b), respectively.

Testing the plasticine caterpillar method

To assess whether plasticine caterpillars accurately measure bird predation, we attached both plasticine models and real, dead caterpillars (corpses) to tree branches and monitored them using camera traps. The goal was twofold:

1. Compare the predation rates detected by human observers examining plasticine caterpillars with actual bird activity captured on camera.
2. Determine whether birds interact with plasticine models in the same way as real caterpillars.

By pairing direct evidence from camera traps with traditional observational methods, we hoped to uncover the strengths and limitations of the plasticine caterpillar approach.

Key findings: The method has flaws, but remains useful

The study found that the plasticine caterpillar method has limitations but remains one of the best options available for studying bird predation. Here are the main findings:

• Camera traps revealed more bird activity than predation marks on plasticine models. This suggests that plasticine caterpillars may underestimate the predation pressure
• Observers did not always agree on what counted as a predation mark. This highlights the potential for human error in interpreting bite marks.
• Despite these issues, plasticine caterpillars remain the most practical method for measuring bird predation. Using real caterpillars raises ethical concerns, and existing camera technology is still not advanced enough to replace traditional methods entirely.

In the meantime, plasticine caterpillars remain a valuable tool for studying bird predation, provided researchers acknowledge their limitations. This study is an important step toward refining ecological research methods, ensuring we get closer to understanding the true impact of insectivorous birds in our ecosystems.

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Reference: Schillé, L., Plat, N., Barbaro, L., Jactel, H., Raspail, F., Rivoal, J. B., … & Mrazova, A. (2025). Camera traps unable to determine whether plasticine models of caterpillars reliably measure bird predation. PloS one, 20(3), e0308431. DOI: 10.1371/journal.pone.0308431