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Winter 2023 has been quite a busy season so far at the Sullivan’s Island Bird Banding Station.
We have been banding about 20 to 30 birds per morning, and the vast majority of these birds have been yellow-rumped warblers. This species overwinters in our area and feeds largely on wax myrtle berries. In fact, the subspecies of yellow-rumped warbler found in the Eastern United States is known as the myrtle warbler.
In February, we banded almost 150 myrtle warblers and recaptured about 25. A few of the recaptured myrtle warblers were originally banded in prior winters, showing apparent site fidelity to wintering grounds in this species. Myrtle warblers molt into their alternate – or breeding – plumage before they migrate back to their breeding grounds in Canada and the Northeastern United States. I have enjoyed watching them slowly transform into their gaudier breeding plumage. Males have vibrant bluish-silver hues on their back and wings, while females are more brownish overall. In all plumages, both males and females have the diagnostic yellow rump patch on their lower back.
The yellow rumps we band in October and November are usually thin birds with little or no fat stores. They have already depleted their fat stores from fall migration, and many of them also have a lower muscle mass. In contrast, the yellow rumps we capture in February have really started to pack on the fat grams. The bulkiest of the February yellow rumps have fat bulging out of their furcular hollow – breast bone – and additional fat packed on both sides of their abdomens. A skinny yellow rump, especially a young female, can weigh as little as 11 grams, while an adult male yellow rump pre-spring migration can weigh as much as 16 grams.
Males are slightly bigger than females overall, and their wing length is also slightly longer. Bird banders use wing length measurements as a useful data point that can assist in determining the sex of a yellow-rumped warbler in the hand. Look for them in your backyard now before they all depart in late March into early April. You can easily attract this species to your feeders by offering them a block of suet.
If you are interested in visiting or volunteering at the Sullivan’s Island Bird Banding Station, contact Sarah Harper at sullivansislandbirds@gmail.com.