L04 — The Beak

Module: M07 — The Arms Race Periods: 1 NGSS: HS-LS4-2, HS-LS4-4 Lesson Type: Explore / Elaborate

Learning Objective

By the end of this lesson, students will be able to demonstrate how variation in beak structure affects survival by measuring beak “efficiency” with different food sources and connecting the results to natural selection in Darwin’s finches.

Phenomenon / Hook

“In 1835, Charles Darwin visited the Galápagos Islands and noticed something odd: the same type of bird — a finch — had completely different beaks on different islands. Some had thick, crushing beaks. Others had thin, probing beaks. Same ancestor. Same basic bird. Different beaks. Why?”

Show images of 4–5 Galápagos finch species with their beaks labeled: ground finch (crushing), cactus finch (probing), warbler finch (grasping), tree finch (bark-stripping).

Materials

“Beak tools” (one set per group of 4): chopsticks (thin beak), tweezers (probing beak), spoon (scooping beak), pliers/large forceps (crushing beak)
“Food sources” (per group): small seeds (rice grains), large seeds (sunflower seeds), nectar (water droplets in shallow dish), insects (dried pasta pieces hidden in bark/cotton balls)
Measuring cup or scale (to measure “food” collected)
Data recording sheet: [[handouts/L04-beak-lab-data]]
Projector for Galápagos finch images
Timer

Agenda

Opening (7 min)

Show Galápagos finch images — all are “finches” but beaks vary wildly (2 min)
Think-Pair-Share: “Why would different beaks be useful? What determines whether a beak is good or bad?” (3 min)
Key framing: “A beak isn’t good or bad in general. It’s good or bad for a specific food source. That’s the key — adaptation is environment-specific.” (2 min)

Explore (18 min)

Beak Adaptation Lab (groups of 4)
- Each group member takes one “beak tool”:
  - Chopsticks = thin, probing beak
  - Tweezers = pointed, gripping beak
  - Spoon = broad, scooping beak
  - Pliers = thick, crushing beak
- Round 1 — Small seeds (rice): Each student has 30 seconds to pick up as many rice grains as possible using their tool. Count and record.
- Round 2 — Large seeds (sunflower): Same 30 seconds. Count and record.
- Round 3 — Nectar (water drops): Scoop water from a shallow dish. Measure in mL.
- Round 4 — Insects (pasta in cotton): Extract “insects” from hiding spots. Count and record.
- Groups compile data into a class chart on the board
- Guiding questions:
  1. Which “beak” was most efficient for each food source? Why?
  2. If a drought killed all the small seeds, which beak type would survive? Which would die?
  3. How does this relate to Darwin’s finches? What “food source” changed on different islands?

Explain (10 min)

Teacher-led synthesis:
- Show class data — which beak won each round?
- Darwin’s insight: On different islands, different food sources were available. Finches with beaks suited to available food survived and reproduced. Over many generations, beak shapes diversified.
- Peter and Rosemary Grant’s research (40+ years on Daphne Major):
  - 1977 drought: small seeds disappeared. Finches with larger beaks (could crack large seeds) survived. Average beak size increased in one generation.
  - 1983 El Niño: small seeds abundant again. Smaller beaks became advantageous again.
  - Key insight: Natural selection can happen in one generation if selection pressure is strong enough.
- Students annotate their data sheets: “This is evolution happening in real time — the Grants documented it.”

Elaborate (8 min)

Design Your Finch Challenge: Each group receives a new “island card” describing a habitat:
- Island A: Rocky, mostly large hard seeds and snails
- Island B: Wet, mostly soft fruits and nectar
- Island C: Arid, mostly buried insects and tubers
- Groups sketch the “ideal beak” for their island and explain what shape it would be and why
- Share-out: Groups compare their designs — how do they differ? Why?

Closing (5 min)

Exit Ticket: [[exit-tickets/L04-exit-ticket]]
1. Why did Darwin’s finches develop different beaks on different islands? Explain using the four conditions of natural selection.
2. The Grants observed beak size increase after the 1977 drought. What was the selection pressure? Did the finches choose to grow bigger beaks?

Differentiation

Support	Extension
Provide a pre-filled table with food source names and beak tool labels — students only need to record counts	Challenge: If a new species of finch arrived on an island with finches already adapted to its food source, what would happen? Use the term “competition” in your answer.
Assign a specific beak tool to struggling students so they focus on one variable rather than all four	Research: What happened to the medium ground finch beak during the 2004–2005 drought? (The Grants documented oscillating selection — beaks went up then down.)

Assessment

Formative: Observe data collection — are students measuring accurately and recording systematically?
Exit Ticket: Natural selection mechanism applied to finches + “choice” misconception check

Teacher Notes

Prep is everything. Set up food stations before class. Have pre-measured portions ready. The lab falls apart if setup takes too long.
The “did the finches choose?” question in the exit ticket targets the most persistent misconception: that organisms adapt on purpose. If students struggle, return to L01 tokens — “Did the blue tokens choose to survive?”
The Grant research is powerful because it’s 40 years of data from a real place. Use it to show that science is long, patient, and accumulative.
The Design Your Finch activity is creative and fun — let students draw. The sketches don’t need to be “accurate” so long as the reasoning matches the environment.
Bridge to L05: “We’ve seen selection on what you eat. But what if something is eating you? That’s next — the chase.”