Corvids - crows, ravens, jays, and magpies - are well known to be among the most intelligent of birds. Along with parrots, corvids have been a frequent subject of avian intelligence studies. Experiments have documented problem-solving and tool use, both in the lab and in the wild. Many of the recent experiments have tested intelligence in New Caledonian Crows (Corvus moneduloides), but tests of Ravens (Corvus corax) and Rooks (Corvus frugilegus) have also found problem-solving capabilities. New Caledonian Crows are a common subect for research because they are known to use a variety of tools in the wild.
A classic test of bird intelligence involves a piece of food dangling at the end of a string. Multiple bird species can figure out through trial and error how to retrieve the food by pulling the string. Corvids and psittacids retrieve the food so quickly that researchers have suggested that these birds may solve the problem at an advanced level. That is, instead of relying on trial and error, these birds might be able to picture a solution in their minds. Experiments with Ravens and Keas (Nestor notabilis) have provided some hints of advanced problem-solving, but the birds have difficulty retrieving food at the end of a string if the string is crossed by another of the same color.
A new PLoS ONE article takes the intelligence tests a step further with New Caledonian Crows. Once again, a piece of meat is suspended at the end of a string. This time, however, the reward is screened from view by a piece of plywood with a small hole in it - just wide enough to pull the meat through. If a crow goes under the board, it can see the food and string. From above the board, it can still see the reward through the hole, but not well enough to judge its distance or whether pulling the string is moving the meat closer. The image shows the setup for this study; this image shows examples of prior experiments. Successful retrieval would require pulling the string part way, stepping on the portion pulled, and then pulling the rest of the string. {
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