Cliff Swallows on the Reserve, by Cami Vega (Reserve Intern)


Spring brought life and color onto the Reserve. Dry vernal pools were filled with pockets of beautiful wildflowers. Burrowing bees escaped the depths of their burrows to pollinate the blooms of Goldfields, Downingia, and Meadowfoam. The warm breezes and blooming flora set the breeding season into full throttle. After a short winter, Cliff Swallows returned from the tropics to their optimal breeding site in the Reserve.

Cliff Swallows are small, compact birds with pointed, broad wings and square, pumpkin-colored tails. The metallic colored birds can be easy to miss, as they are often zooming around and foraging near water. Their flightiness makes them difficult to catch for predators but makes them proficient at catching flying insects by their wings. These diurnal foragers are normally seen around the summer time in large groups, often chasing swarming insects or hiding from the heat and predators in their gourd-shaped mud nests on cliff faces. Cliff Swallows are very social creatures; though they are able to nest alone they usually form colonies of over 100 nests. Some of the largest sites recorded have had colonies with about 3,000 active nests.  The onset of urban construction has heavily influenced their breeding range, as bridges and buildings now serve as a habitat for this species.

Each colony works in synchrony to build their nests, avoid prey and find food.  All birds are eager to land the optimal nest; fights may break out if two birds seek the same nest. If they do not have mates the males may start to build a nest to attract a mate.  The intricate architecture of their mud nests take a lot of work so pairs and colonies usually take over existing nests from previous sites. The males and females work together to build the nests by gathering mud in their beaks from nearby water sources. The mud pellets are transported to the nesting site and are carefully molded into place and shaped to their liking.  Thousands of mud pellets and their delicate little beaks model the nest design which consists of an elongated entrance shaped like a cylindrical tube with the inside is cushioned with dried grass. Once the nest is built and claimed the inhabitant will often sit at the entrance and lunge at intruders. Neighboring nests are preferred to reduce construction timing, the spacing between the nests create a honeycomb structure.

Cliff Swallows have one mate with whom they raise young with but may have several mates. Females may carry its eggs to another nest or lay their eggs in the nest of another. Clutch sizes range from 1-6, incubation occurs before the last egg is laid and lasts for about 12-15 days with both parents incubating. Young are reported to fledge in about 23 days, though the colony at Black Rascal Creek fledged a couple of days early.  The immensity of their colonies has led to the evolution of their sophisticated vocal systems that aid them in distinguishing their offspring. Parental recognition of the chicks’ calls are developed by the time the chicks fledge; siblings tend to have similar calls structures.  After fledging, the young assemble together in crèches to feed.

The cliff face by Black Rascal Creek provides the perfect habitat for the Cliff Swallow colony (see photo of this colony, taken by Clayton Anderson).  The site gives the birds protection from predators (primarily birds and snakes), is close to a water source, and is on an open area speckled with insects for foraging. The limited space and nests left from previous years has spread out the colony into three separate locations. There are around 300 nests from previous years that were teeming with Cliff Swallows in full breeding season. The largest colony inhabits the nests on the cliff side at Black Rascal Creek. In flight the colony creates unusual shapes that continually warp as they glide through the air. The structure of the cluster is difficult to describe as the colony is inextricably linked to one another. It is both an informal self-organizing group of pairs and acquaintances and a hierarchical organization with no true leader. Their emergent behavior is still a mystery. The clusters approximate small-world networks. The colony inadvertently relies on each other for survival. The nests serve as information hubs, birds will observe their neighbors and follow them to a food source.  This swarm behavior makes feeding and foraging more efficient and effective. By flying in a swarm they protect each other from predators, eager for a snack. 

While observing the colony on the cliffs of Black Rascal Creek, I witnessed the unfortunate demise of one swallow by a small falcon, probably an American Kestrel.  The colony had congregated outside of the nests, all flying in synchrony, unbeknownst that the falcon had carefully mimicked their flight pattern when suddenly it shifted directions and snapped a swallow right out of the air.  The entire colony was instantly flustered by the action and all immediately dispersed with a chorus of loud chirps and purrs. Last week, we witnessed an American Kestrel capture a Cliff Swallow from the nesting colony on the side of the power plant near the North Bowl parking lot. The swallow struggled in the air and in the end it was victorious and was able to escape the Kestrel’s tough talons.

The short inhabitance of the Cliff Swallows on the Reserve opened my eyes to the intricacy of this ecosystem. I learned so much from observing their complex nest structures to their synchronized behavioral patterns.  The colonies’ interconnected web of interactions and behavior work together for the survival of the colony, I think this is something we can all learn from. The once boisterous cliff side that was teeming with the flighty, purring Cliff Swallows continuously brought me back to the site, always with more questions than before.  

June 3, 2014