In males of some bird species, the hormone testosterone that controls male aggression also depresses the immune system. Males have balance between the reproductive benefits of aggressively defending territories and females and the costs to their overall health. We don't know yet whether testosterone controls female house wren aggression or whether it negatively impacts health.
To get a measure of health, we are making blood smears to look for avian parasites. To do this we take a small blood sample from the female's wing (which is very similar to when you get your blood drawn at the hospital). Then we smear a few drops across a microscope slide. When the stained slides are viewed under a microscope we should be able to see a variety of parasites in the blood, such as avian malaria.
Preparing a house wren wing for a blood sample
There are many different ways to measure immune function in wild birds, some more complicated than others. We chose blood slides since they're relatively fast and easy when we're already taking blood sample. It should also be fun to see the parasites under the microscope!
Slides box with house wren blood smears taken a few weeks ago. It's more full now!
Stained slides are in the background and unstained slides are in the foreground