Day 5: Who’s feeding the babies??

You may be wondering whether the mother or father is feeding the babies. Catbirds can’t be sexed just by appearance, so we can’t actually tell. In the breeding season we can catch birds and inspect them for a brood patch on a female—a smooth, featherless portion on the breast to facilitate heat transfer to eggs—or a cloacal protuberance on a male—which is basically what you think it is.

Males and females feed the nestlings at similar rates at this point, so telling them apart is impossible without catching them first.

For birds outside of the breeding season we often take a small blood sample and use molecular techniques to figure out the sex based on separation of DNA fragments on a gel (called gel electrophoresis). If you look at the image of a completed gel below, you can imagine 5 columns: the first column is a standard of DNA fragments so we know what sizes to look for, then in the next two columns you can see that two bands separated while the other two columns only show one band in each. Just like how human males have XY chromosomes but females have XX, we are just looking for bands of different sizes. But it’s actually swapped in birds: males have ZZ chromosomes and females have ZW. So the columns here with two bands are actually females!

Gel electrophoresis showing a DNA size ladder followed by female (2 bands) and male (one band) catbirds

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