This one is a cliffhanger, part three

Once I focused on the fact that there were multiple swallow species present, the wide variation in the placement, structure, and stage of the nests began to make sense.

The cliff swallow nests looked the most precarious; they opened outwards and hung where the bridge beams met the supporting columns (see here and here).  These nests were also the furthest behind in development -- while the cliff swallows were still extending their nests and incubating their eggs, the other swallows were busy feeding their chicks.

The cave swallows had built their deep nests in the corners formed by perpendicular beams.  Although I could not see into the nests, the steady stream of feeding visits by the parents was a good indication that there were growing chicks hidden inside.

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Meanwhile, the barn swallow chicks were quickly outgrowing their nests, which were shallow cups balanced on narrow ledges.

Four out of five barn swallow chicks agree: it is time for food!

These chicks eagerly watched each adult swallow that came nearby and opened their beaks wide for food, regardless of whether the adult was one of their parents or not.  Since there were many nests under the bridge, the chicks' gaping, yellow mouths were ignored more often than not.

* To see this video in high definition (1080p), you may need to: 

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Nevertheless, the chicks had soon gown enough to leave the nest and perch on a nearby beam.  There, they continued to watch the all hardworking adults and to wait for food to be delivered.

A few days later, the chicks were out of the nest -- but still waiting around to be fed.