By John Keeling
Our garden in Ajijic is currently over-run by small birds calling “skwit, skwit, skwit”. These are Yellow-rumped Warblers, so-called because the lower back is yellow, though it is often hidden by the folded wings. They also have distinctive small yellow patches on each side of the breast.
These are the commonest of the warbler species. You will see them on your lawn, in the bushes and in the trees constantly on the move. Because they are so small, you will probably need binoculars to see the tell-tale yellow markings.
They have proven themselves to be a very adaptable species. In summer they can catch insects in the air, and also have the ability to pick insects off the leaves and branches of bushes and trees. In the winter when insects become scarce, they switch to berries, particularly the myrtle berry, but also poison ivy berries and privet berries. The Yellow-rumped Warbler has a digestive system specially adapted to digesting the high levels of wax found in myrtle berries.
In April and May these birds fly north to Canada and Alaska where they nest in pine and spruce trees about 20 feet off the ground. It takes 28 days from laying the eggs to the time the babies fly. Sometimes they have two broods in a summer.
In August they plump up and then fly south to points from the Southern States to Panama, which is the southernmost range of the myrtle berry. In winter they travel in flocks, and will quickly leave a location when both the insects and the berries run out.
There are two common sub-species or races: the Myrtle Race, predominant in the east, has a white throat; while the Audubon Race, predominant west of the Rockies, has a yellow throat. The body coloring varies from black and white, through blue-grey to nondescript buffy brown.
The Myrtle Race and the Audubon Race used to be considered separate species, but when it was discovered that they interbreed where their ranges overlap in the Rockies, they were made one species. It is speculated that they were originally one uniform species prior to the last ice age perhaps 70,000 years ago; and that the eastern and western parts of the population became separated from each other by the “Wisconsin Glacier” which covered much of Canada and the Upper Mid-western States until 10,000 years ago. During that time the two populations were able to evolve different color patterns.
You will find that these birds are not shy around people. When you watch them remember that they have powerful eyes for catching insects on the fly, and any sudden movement on your part may scare them. On the other hand, their ears are geared to hear the high-pitched calls of their own species. They hear frequencies from the middle of the human listening range up to frequencies above our listening range. As is the case with other small birds, they cannot hear human conversation because the frequency is too low. Be on the look-out for groups of these birds in your lakeshore garden this winter.
(John Keeling and his wife lead ‘Los Audubonistas del Lago’ which is a loose-knit group of people interested in birds. To receive notices of events please leave your e-mail address at www.avesajijic.com.)