Attractive red mark
Spanish biologists also studied the red mark, but this time with Yellow-legged Gulls (Larus Michahellis). These are relatives of the Herring Gulls that live around the Mediterranean. The marks on the gulls were made bigger or smaller with nail polish. For the chicks this made no difference, there was an equal amount of pecking at bigger and smaller marks. For the partner, however, it did make a difference. When the test gull had a bigger mark, the partner vomited food for the young twice as often. It had been observed previously that healthy gulls have redder marks. It was concluded from this experiment that a gull can see from a big mark that a partner and its young are worthwhile.
A smaller mark led to a different reaction. The number of regurgitations depended on the amount of pecking by the young. The conclusion was less clear here. The researchers suggested that the gull judges that the young is weak and therefore pays extra attention to it getting enough to eat.