From New Scientist:
Robins outdo other animals in judging meal size
11 August 2012
ROBINS appear to have an eye for numbers, at least when it comes to choosing the biggest meal.
“Discriminating between two large groups of objects that are close in number would be pretty exceptional for any animal or human, but that’s exactly what the robins did,” says Alexis Garland at Victoria University of Wellington in New Zealand.
Garland let 36 wild North Island robins choose one of two wells after seeing different numbers of mealworms dropped en masse into each. Most picked the fuller well as long as the ratio was below 0.75 – correctly selecting, say, 64 over 32 worms. The mechanism at work here is called ratio-based representation and involves guessing which large group of items has the bigger bulk.
The robins did even better when the worms were dropped into the wells one by one and covered so that the masses could not be compared: they managed a ratio of 0.88, albeit with a smaller number of worms. For the largest trial at this ratio – 14 versus 16 worms – most robins chose correctly (Animal Cognition, DOI: 10.1007/s10071-012-0537-3).
Other animals tested like this have only managed to track about four items. Robins hide multiple food items in several places so it may be advantageous to distinguish more from less quickly, says Garland.