Lecture 16 - C lassic Example Ravens Bernd Heinrich Ravens...

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Classic Example: Ravens – Bernd Heinrich - Ravens are highly intelligent birds o Heinrich studies foraging behavior of ravens in the north woods of Maine during winter o Huge vocabulary of quarks, quorks, queeks, yodels, screams, drums, etc Feed on carcasses in winter Carcasses are scarce, hard to find and hard to break open because they freeze Ravens are also very common - One day Heinrich saw 15 ravens eating a dead moose o Ravens yelled to attract other ravens (he thought) o But why would they do this? Why not just keep the food for themselves to eat all winter long? o How was yelling at the carcass adaptive? o Heinrich tested several different hypotheses to discover the answer HYP 1: Ravens are one big family Prediction: birds yelling at carcass should then be related Test/Results: DNA fingerprinting revealed that birds were not related HYP 2: Ravens yell to signal a bear or some other animal to come open up the carcass which would allow to eat it Prediction: Ravens should only yell when they first find the carcass Test: hauled out dead cows and watched what happened Result: First raven to arrive didn’t always yell or sometimes yelled when the carcass was already open Conclusion: Ravens are not yelling to signal another animal HYP 3: Ravens yell to dilute the risk to the individual Prediction: Birds should stop yelling once there are a lot of birds around Test: Hauled out dead cows and watched what happened Result: Ravens yelled even when lots of birds were around Conclusion: Ravens are not yelling to dilute the risk to the individual o Breakthrough Observation Heinrich observed that a single bird or a pair of birds never yelled. Yelling only started when 3 or more birds had arrived at a carcass He also knew that pairs defend territories but solo birds did not These observations implied several predictions which led Heinrich to a solution to the mystery o Prediction: resident territory holders should never yell
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