Petrodchelidon pyrrhonota--the rock swallow with fiery ears.
We're so used to seeing cliff swallows nesting on buildings, under eaves, and under bridges that it's a minor revelation to see them nesting on their traditional substrate. In Great Falls, Montana, there's a lovely park on the banks of the Missouri River called Giant Springs State Park. Here, a colony of cliff swallows has made its home.
When we visited, the nests were largely under construction, leading to a number of pretty adorable photo-ops.
The swallows gather mud on streambanks and in puddles, bringing little wads of it in their gullets, making thousands of trips for each nest. These seem happy to have made a place to perch. But there's much more to do. The little mud slings are rounded out into bowls.
Pairs work together, trying to get the nests finished before the eggs come. I find it amazing that they can construct these elegantly cantilevered jugs without the whole thing just falling off.
Cliff swallows have little headlamps of white that show up well in the dark nest interior. It's a clear signal that the nest is occupied.
There's a lot of conversation between neighbors, and there's egg dumping and mate switching and even egg switching. It gets kind of crazy. Charles Brown at the University of Tulsa has done fascinating studies of cliff swallow coloniality--its benefits and costs. He discovered a new form of reproductive parasitism when he witnessed cliff swallows entering other nests with eggs in their bills. 
Not only do the swallows lay eggs in others' nests, but they may also carry their eggs in their bills and deposit them in others' nests. They'll throw out an egg in other nests, as well. It's puzzling why they do this. Perhaps they're hedging their bets, increasing the chance that their young will fledge by farming some out.Years ago, I painted that scene, of a cliff swallow with an egg in its bill, for Dr. Brown's species account in the Birds of North America, but I don't have the original any more. He might have bought it. It's all lost in the mists of memory.
These nests are almost finished. A rainproof neck completes the construction, pointing downward. Eggs are usually laid before the entrance tunnel is complete.
There are benefits and costs to living in a colony. Brown discovered that, like honeybees, cliff swallows are able to communicate about food sources, letting other colony members in on an insect hatch, for instance. They seem to have specific calls that function as words, that are used only in the context of communicating about food.
But tremendous ectoparasite loads build up in these nests, with lice, mites and bloodsucking bugs sometimes multiplying to crippling loads.
All of which leads one to wonder why a robin would want to build her nest in the midst of a swallow colony.
She had young chicks already, when the cliff swallows were just starting to lay eggs.
I hope her young fledge before the swallow bugs build up. Looking at it another way, she's probably well-protected from hawks and other avian predators.
You can just see the yellow billtips poking up above the nest rim. Good luck, Mrs. Robin.
