Case of Altricial Eye Appeal

Derived with thanks and acknowledgements to Clyde Freeman Herreid,
whose coot case study demonstration titled, "Mom liked you Best," was in turn based on an article published in Nature, 1994, 371: 240-243.

 

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Case Studies

Can being a beautiful baby increase one's chance for survival?

Red can be claimed as the favorite color of birds. At least in many species it is important in provoking certain critical behaviors (aggressive displays and courtship displays being most common). Where bright coloration contributes to reproductive success by affecting mate selection it is said to result from what Darwin called "sexual selection." Does color have other social value?

In the American Coot--a water bird of the family Rallidae--the adults have a white bill and uniformly greyish-black feathers covering most of their bodies.

The newly hatched coots are not drably colored like most nestlings, but are quite striking in their coloration. They have long orange-tipped slender feathers, brilliant red papillae around the eyes, a bright red bill and bald red head. They lose their colorful appeal at about three weeks of age. But in the

meantime, does it help them survive?

One team of Canadian biologists thought that the selective advantage arises in the eye of the (parental) beholder. They speculated that the plumage may make some chicks more attractive to their parents. That is, the bright coloration might in some way affect the quality of care given (mostly in the form of food provided) by the parents. The more attractive chicks might be more successful in begging for food, thus having a higher rate of survival and a greater likelihood of passing on their genes (including those for bright coloration). As many as half of all chicks do die of starvation, so this might actually confer a significant advantage.

  • How might that notion be tested?
  • What is the specific question being asked?
  • What is the hypothesis being suggested?
  • What predictions (deductions) can you make if the hypothesis is correct?
  • How can your predictions be tested? (i.e. What exactly might we do if we were the field biologists who had studied coots for several years.

Remove the color advantage?

Here are some additional pieces of information:

  • Large numbers of floating reed-basket nests were accessible to the biologists.
  • The long orange-tipped feathers could be clipped from selected hatchlings.
  • Coots lay an egg a day for 8 to 12 days.
  • Eggs hatch in the order laid.

Designing the experiment

  • How should clipping be done to test the hypothesis?
  • Clip all the hatchlings in a given nest or just some of them?
  • If just some, then which ones?…and in what order?

Predicting the results

  • If observations and measurements of normal, unclipped "Orange Chicks" produce the results indicated in the graphs a, c, and e, what would you expect in a brood of all clipped "Black Chicks?" (In the graph the boxes show the results of most of the chicks, while the lines extending above and below indicate some extreme cases.) Make your own prediction in the form of boxes positioned level with those already drawn if you think there would be no change. Draw boxes either somewhat higher or lower to show that you think the clipped Black Chicks would fare better or worse.

This graph show how frequently chicks in a nest of all Orange Chicks were fed during feeding sessions.

Where do you think a horizontal line should be drawn to show how frequently chicks would be fed in a nest of all Black Chicks?

After taking a guess click on the graph to see the actual results.

This graph show the relative rate of growth of the chicks in a nest of all Orange Chicks.

Where do you think a line or box should be drawn to show the relative growth in chick in a nest of all Black Chicks.

After taking a guess click on the graph to see the actual results.

This graph show the percent of of chicks surviving in a nest of all Orange Chicks.

Where do you think a line or box should be drawn to show the percent of of chicks surviving in a nest of all Black Chicks.

After taking a guess click on the graph to see the actual results.

  • If you would like to see the result of all three data sets, click here for graphs of the actual field results .

The "Control Broods" were clutches in which all chicks were either clipped and made "black" or simply handled as long as it would take to clip them but returned to the nest still orange.

  • Why do you suppose the biologists handled the orange chicks when they had no intention of clipping them?

The biologists reasoned that if parent birds would show a preference, it would only be demonstrated in broods in which some chicks were clipped (Black) and some unclipped (Orange).

  • How should they go about deciding which birds in a given nest should be clipped and which merely handled briefly?

More predictions

  • What would you predict would be the results in the experimental broods, if the hypothesis of an "orange advantage" is correct?

Make a prediction or guess about where a box or a horizontal line should be made in the graph to indicate how often orange chicks and black chick would be fed when they are members of the same brood.

After taking a guess click on the graph to see the actual results.

Make a prediction or guess about where a box or a horizontal line should be made in the graph to indicate how fast orange chicks and black chick would grow when they are members of the same brood.

After taking a guess click on the graph to see the actual results.

Make a prediction or guess about where a box or a horizontal line should be made in the graph to indicate the percent of orange chicks and black chick that would survive when they are members of the same brood.

After taking a guess click on the graph to see the actual results.

  • If you would like to learn how the experimenters decided which chicks to clip and look at all three data sets together, click here to see the actual results from the field study.
 

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