Here is Phil’s novel approach to measuring the length of a Bee-eaters burrow:
"Periodically Rainbow Bee-eaters burrow in the street nature strip in front of my mother's house here in Townsville. A precarious place to nest with parked cars, wheelie bins and pedestrians. But the birds do it repeatedly so it must be safe enough. This October a pair of Bee-eaters decided to excavate new burrows closer to the house, inside the front fence beside the garden footpath, and safer than in the nature strip. Being close to the smooth concrete path the birds diligently flicked the excavated sand across the path. This presented an opportunity to non-invasively estimate the depth of the burrow from the volume of sand and confirm references citing burrow lengths being an average 82.5 cms and up to one meter as a probable maximum.
About seven litres of sand was collected in a 10 litre bucket and the burrow diameter appeared to be about six cms. References state the burrow width is typically really close to the width of the bird, whose entering and leaving acts as a sort of ‘plunger' keeping the air breathable. I assumed that the burrow is the same width the whole way (ignoring any increase for the brooding chamber) and the bulk density of the sand in the bucket is the same as in the un-excavated form.The volume of the cylindrical tunnel is Pi times the radius squared times the length, which for the seven litres of sand (7000 cubic cm) gives a length of 2.47 metres. If the error was 30% to account for changes in bulk density of the soil after excavation and the unknown nesting chamber volume, then we still have a length of 1.73 metres, well above the quoted values!”