Hornbills Adjust Response Speed According to Solvability of Patterned-Strings Problems

Cognitive adaptations for processing causal information are fundamental to flexible problem-solving across species. Patterned-string tasks offer a classic means-end problem, requiring subjects to identify the functional connection between an action and its distal outcome. This study investigated patterned-string pulling in East Asian hornbills, a cognitively underexplored avian taxon, to assess their problem-solving abilities and underlying cognitive mechanisms. Nine hornbills were presented with three string configurations of increasing complexity: parallel (baseline), broken (contact/no-contact), and crossed strings. Subjects reliably selected the correct string in the baseline and contact conditions, demonstrating sensitivity to physical continuity cues. In contrast, accuracy declined significantly in the crossed condition, which required recognition of spatial displacement between the baited and accessible string ends. Reaction time (RT) analyses revealed prolonged latencies in the crossed condition, likely reflecting increased cognitive demands or limited habituation. Individuals who failed often adopted suboptimal strategies, such as proximity or side biases. We also observed lateralisation effects, with a left-side RT advantage, suggesting hemispheric specialisation. These findings support the hypothesis that task complexity modulates both decision accuracy and response speed, highlight RT as a useful proxy for interim cognitive processing, and establish hornbills as a valuable comparative model for studying causal cognition in birds.