When you hear the phrase “wildlife spectacle,” what comes to mind? Many people would think of wildebeest on the Serengeti, Sandhill Cranes in Nebraska, or raptor migration in Mexico. The spectacle of Tree Swallow migration has been known to science for many years, but this event is not familiar to most wildlife enthusiasts.
On the lower Colorado River (LCR) where GBBO runs its Riparian Birds Survey project, our crews have been enjoying the sight of Tree Swallow migration for years. Even the sight of thousands of birds streaming over a survey site on their way north can be amazing to see, but the real spectacle happens when the birds go to their roost for the night. On their chosen lakes, sites with suitable extensive marsh to roost in, swallows stream over the water, swirl over the marsh, and gather in flocks high over the lake. Eventually, all the swallows gain elevation and stream into masses high in the sky. The ball of swallows moves through the sky with the liquidity of smoke, merging and splitting, climbing and shifting as the light dims. Eventually, just before dark, a few hundred birds break off from the flock, diving almost straight down toward the marsh before breaking at the last moment and settling into the cattails, preparing to roost for the night. As more and more birds join in the plummet to the marsh, they appear as whirling vortices to the naked eye, like twisting pillars of smoke or small tornadoes.
LCR Crew members checking out an earlier swallow swarm in Lake Havasu City
Swallows swarmed over the river, continued to stream in over the trees, and darkened the sky over the nearby marshes and mountains. The swarm just across the river from us was nearly as impressive as any we had seen, but we were in awe when we realized that another swarm, several times larger, swirled across the horizon to the west. The birds gathered into tighter groups and flew higher, until that critical moment when thousands suddenly plunged together in a continuous stream into the marsh. The whooshing of tens of thousands of wings was audible from across the river, though the birds didn’t utter a peep. After about six minutes of funneling, followed by the last stragglers circling again and again before following the rest into the marsh, the skies were once again calm.
In all, we estimated 1.2 million Tree Swallows in the swarms visible from our vantage point. This ties the high count for the lower Colorado River!
The following evening, we didn’t expect another big show, as there surely couldn’t be many birds left to swarm up. The world population of Tree Swallow is estimated at 17 million (Partners in Flight 2012. Species Assessment Database), and huge numbers of this species had already been reported migrating north for at least a month. The birds we had seen the night before were clearly not sticking around, either: we had seen them streaming in from the south, and in the morning watched them head north, with very few around during the day. However, to our amazement, when evening rolled around the birds streamed in and formed huge congregations as they had before. Though the numbers were about half what they had been the previous evening, it was still a spectacle!
Both mornings before dawn, we saw the last few thousand birds leaving their roosts and heading north. The dawn flyout is very different, with the birds forming dense murmurations of smoke cruising relatively low and flying steadily upstream. They also call during this time, which causes quite a cacophony of twitters with a thousand birds overhead! The dawn flyouts we witnessed were nowhere near as spectacular as this one in Florida, or as the evening concentrations, but still were amazing to see.
The LCR Crew looking for swallows on Lake Havasu