On Sundays they would pile the family into their secondhand station wagon and gape at the prosperous neighborhoods of Bellevue, Laurelhurst and Washington Park.
At the Wagah border checkpoint in Punjab, picnicking families gather to gape and wave to Pakistanis waving back from the other side of the barbed wire.
The use of gape-area to help assess how much food to bring means that the quantity automatically increases as the chicks grow, without them having to cheep more in order to get extra food.
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.