She allowed these eggs to hatch, then took samples from the nestlings to see who was the father.

ECONOMIST: Male birds cannot afford to relax their standards

Her work revealed that males who did not get enough food as nestlings have a smaller range of songs.

ECONOMIST: Parasitology: The song does not remain the same | The

Each juvenile bird had an average of more than 23 parasites on it, and more than a quarter of these nestlings were dying - apparently as a result of infection.

BBC: By Jonathan Amos

The team found that it adjusted its cheeping-rate as it grew, so that the combined signals of cheep and gape, when plugged into the formula, exactly matched the value that would have come from four warbler nestlings.

ECONOMIST: Bird-brained | The

Humans wearing a condor-shaped glove puppet (developed by Mr Wallace) have fed the nestlings, and they have been kept in close proximity to a group of older condors so that they can observe and learn from their elders' behaviour.

ECONOMIST: Coping with troublesome young birds