WHERE AND WHEN TO go?

Questions for nomadic birds

- JOHN MENDELSOHN AND SUE MILTON

A great many birds in arid and semi-arid parts of the world spend much of their lives seeking solutions to those two simple questions. Richard Dean too spent much of his life puzzling, talking and writing about potential answers, so it seems fitting to present this article as a tribute to him and to extend the conversation to a broader group of people. Many of the ideas and observations described here came from our discussions with Richard and from his unpublished notes.

Biological production across the world largely follows time scales that are perennial, seasonal or ephemeral. Perennial production happens steadily and slowly during the year, with relatively few changes between one area or time and another. This is typical of tropical environments. Seasonal changes in production are usually substantial and predictable, as in temperate climates where blooms and booms of life in spring and summer follow months of autumn and winter dormancy.

Challenges of where and when to go seldom arise in perennial and seasonal environments where most forms of life simply wait or move regularly between known areas. These questions prevail, however, where ephemeral production is largely driven by unpredictable changes in arid and semi-arid areas, especially after significant amounts of rainfall. Life then explodes, as countless seeds, spores and eggs germinate or hatch, aestivating animals wake, termites emerge, plants and animals grow, and most forms of life reproduce as rapidly as possible. What was a quiet, sleepy environment becomes a hustle and bustle of productive life.