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Science Journaling Club

INTERACTIVE MODELFIELD NOTEANIMAL BEHAVIOUR

The Reconstructed Hunt

A live model accompanying “Twenty-Three Minutes of Chewing, One Kilometre Up”

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13 sectionsabout 16 min
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Two things closed this case: a recording of the hunt itself, and, eighteen years earlier, an isotope signature that could prove what greater noctules were eating but never how the bird died. Both are built here from the numbers the papers actually report — scrub the hunt below, then try to break the isotope case yourself.

Model 1. Scrub the recorded hunt

Pick a chase, then drag the time slider or press play. Position, descent and the spacing of the robin's distress calls are this model's own staging of the reported durations and counts — illustrative, not measured telemetry. The durations themselves (30 s, ≈180 s, 23 min) are exactly what the papers report.

Playback
phase: searchaltitude band: >1 km, total darknessdistress calls heard: 0 / 21chewing sounds: 0:00 / 23:00

Chase A never leaves the high band and never triggers a capture. Chase B closes to a capture at 3:00, then the bat descends to feed for the remaining 23 minutes without landing — the 21 distress calls and the chewing duration are real counts from the recording; exactly when each of the 21 calls fell within the pursuit is not reported, so they are spread evenly here for legibility.

Model 2. Try to break the isotope case

bird share (%) × passerine signature + (100 − bird share) × invertebrate signature

Slide the assumed bird share of the diet from 0 to 100%. The marker moves along a straight mixing line between the paper's two measured endpoints — invertebrates and passerines — using a plain linear isotope mixing model, the simplest version of the same logic the 2007 paper used. Then flip the switch below it. Nothing on the plot moves, because nothing about the chemistry changes: a kilogram of freshly killed robin and a kilogram of already-dead robin leave identical isotope signatures. That is the whole reason this needed a tag rather than a syringe.

implied δ13C: −23.55‰implied δ15N: 8.50‰autumn field value (2007): near-pure-bird zone [3]
Consistent with both a live catch and a scavenged carcass. The isotope signature does not move when you change how the bird died, because it cannot: this is the exact eighteen-year gap the 2025 recording had to close with sound and motion, not blood chemistry.

Endpoints are the paper's own reported means: invertebrates −25.64‰ / 6.45‰, passerines −22.43‰ / 9.60‰ [Popa-Lisseanu et al. 2007]. The mixing line between them is the simplest possible model; the real diet is not necessarily a clean two-source mix, which the 2007 paper itself notes.

What the two models are for

Neither model reproduces the published analysis. The hunt model stages real durations and counts on an illustrative flight path; the isotope model runs a two-point linear mix on real endpoints. What both are built to show is the shape of the argument: one kind of evidence can place a diet, and a completely different kind of evidence was needed to place a death.