Tree Swallow parents begin feeding their four to seven nestlings as soon as they hatch, and they continue doing so until their young depart the nest and sometimes for several days afterwards. The adult carries food in its bill and places it directly into the open mouth of a begging nestling. The small insects gathered by the parent may be formed into a rounded ball, or bolus, which they hold in their mouth or throat (often not visible to an observer). Both parents feed the nestlings, together averaging about ten to twenty deliveries per hour. During periods of peak nestling demand, parents may feed as many as 6,000 to 7,000 insects in a single day. (Thanks to Jeannie Killam and Terry Ross for photo op.)
You may have heard of “lining” bees – following one honeybee after another, tracking them to their honey-laden hive. Recently I lined Cliff Swallows. My initial observation was of several swallows on a mud flat in the middle of a river, loading their beaks with mud and taking flight, all in the same direction. Knowing that their nests are made of mud pellets (900-1,200 of them), I knew that they must be nesting somewhere in the vicinity. The length of time between their departure from and return to the mud flat was quite short, so I deduced that the distance they were carrying the mud and depositing it couldn’t be too great. After heading off in the direction that the swallows were flying, I eventually discovered the very beginning of a colony of Cliff Swallow nests under the eaves of a nearby barn.
The building of a nest requires not only finding a source of mud, but also ferrying lumps of it (in their beaks) back to the nest site many, many times. Once a source has been found, its location is made known to all members of the colony, and they all make use of it. Cliff Swallows belonging to the same colony not only use the same source of mud, but gather it together as a group, and return to work on their nests all at the same time. They work in roughly half-hour shifts, after which they all take a break and forage for insects for ten minutes or so before resuming work.