Egret Meets West

Originally restricted to central Africa, cattle egrets (Bubulcus ibis) now occupy every continent on the planet barring Antarctica. Less than 50 years after their arrival to North America, cattle egrets have taken over every US state and now outnumber all other egrets and herons combined. It appears that cattle egrets achieved this feat all on their own*, and have displayed similar population explosions following introductions to Australia and Europe.

As their name suggests, cattle egrets have close ties to livestock grazing, andĀ  agricultural expansion in the 20th century has been attributed to their spread and success. But less clear, and perhaps even more mindbogglingly impressive than flying non-stop across an ocean, is how rapidly cattle egrets have conjured up entirely novel migration routes on reaching new lands.

Some cattle egrets from the original batch in tropical Africa do not migrate at all! I must admit, I was skeptical when I first heard this story. It has become clear that evolution can operate on the same timescale as ecological processes, but for something as complicated and multifaceted as migration?! Apparently so.

It has been posited that the first arrivals to North America learnt by example, following native heron flocks along their long-established migration routes. Seems plausible; at least now we are only trying to explain a phenomenon rather than a miracle. But even still, this requires that the birds have remarkably flexible behavior, and it is thought the unpredictable movements of the Cape buffalo that they originally followed gave them the prior training (pre-adapted in fancy terminology) to make migration decisions on the fly.

Besides, even if we assume all colonizers originate from those birds that did exhibit migratory behavior in their home range, the pathways used to get from Ghana to Algeria will likely be of little use in navigating from Florida to Brazil, or anywhere else for that matter. This is self-evident. It is also evident in the environmental cues that different flocks of birds respond to and the directions they move. For example, African birds move from a cooler summer climate to a warmer winter environment; in contrast Australian birds move from balmy Queensland in the summer to a much cooler wintering range across New Zealand and Tasmania. Similarly, egrets in western Africa and India use rainfall to orient themselves to the seasons and move at the right time, whereas birds in the Americas are though to respond to temperatures alone. With each tidbit of new information, the story becomes more and more incredible, and indeed there are still many more questions than answered.

Cattle egrets are one of the few winners in the rise of man; I advocate we learn from their success, and use that knowledge to improve the chances of the myriad losers.


*with the exception of Hawaii, people brought them there.