I'm very happy to welcome Aaron, my Undergraduate Research Assistant (URA), to the wren project this summer! Aaron is a Fisheries and Wildlife major from Michigan State University. Aaron will be helping me complete my experiments this summer, as well as conduct a small project of his own. He's been here for almost two weeks now and is doing a great job! This summer he'll be looking at egg coloration in house wrens. Stay tuned for more news on his project!
If aggression helps female house wrens, why aren't all females highly aggressive? This summer I'm looking for a cost to female aggression that might explain the variation I see in my population. Last summer I thought that more aggressive females may have less time or energy to raise their offspring. However, it turns out that aggressive moms actually have bigger, faster growing babies. This summer I'm investigating whether aggression has an immune cost.
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
This summer is off to an interesting start. The birds arrived a month ag,o but they spent quite a long time at the beginning of nesting before the first female was ready to lay her eggs. This is possibly because the temperatures have been slightly lower than normal.
Possibly because of this late start, most sections of the field site are in a state of territorial unrest. Both males and females are involved in these territory fights. Two days ago we mist netted the male at B5. After we caught the male, we played female playback to try and get the female. We caught an intruding male instead! It will be very interesting to get the genetic results back from the offspring to see whether the territorial male or the intruder fathered the egg she laid the next day.
At C5 two females have been having a week long fight. The female initially banded there lost her territory for several days to an unbanded female that laid several eggs. However, on the 4th egg laying day the old female had tossed all the eggs on the ground and reclaimed the territory. How do we know a wren got the eggs? Notice the tell-tale beak-sized punctures in the egg below.
Egg found on the ground in front of box C5 with several puncture wounds from a female wren beak.
For one of my experiments this summer, I'm kicking birds off territories to increase the number of floating birds that can challenge territory holders. I'm hoping to determine whether females that are more aggressive in my tests are able to keep their territories longer. Although I have only evicted 3 pairs so far, the birds seems to be doing a good job of this themselves. I might also be getting a helping hand from the resident mammals. Destruction of the box at B3 by a flying squirrel appears to have set off a chain reaction of territory switches in the B's and C's over the past week.
Several females have now made it back. There are currently at least 3, but there could be more waiting in the wings. Only one of these females has previously bred at the field site. In general, fewer known females return to the field site each year. In comparison, a larger chunk of males are previously banded and the first males to make it back from migration are general ones that have previously bred at Lux, possibly because they know where they're going. Of the 11 males that have been identified so far, 9 have bred in previous years. Offspring that return are also more likely to be males.
In mammals, females are more likely to stay in their natal groups while males are usually the sex that disperses. Birds show the reverse pattern, with males being more likely to stay in their natal areas and females disperse. These patterns, known as sex-biased dispersal, serve a useful biological purpose. If one sex disperses farther than the other, it's less likely that siblings or other close relatives will meet as breeding adults and try to mate. This reduces inbreeding and it's genetic consequences. The patterns I've been seeing with the house wrens might be a reflection of this wider behavioral tendency.
First female caught this season at territory A7. She was actually born at Lux Arbor in 2012 in a different portion of the study site. She appears to have paired with the male at A7 and sings in response to simulated intruders, despite the fact they haven't started a nest yet.
This field season is off to a very slow start. As of yesterday there are now at least 14 males back at the field site, however, many of these males aren't building nests yet despite being around for several days. The other day I even observed a male courting a female with an empty box! This doesn't appear to have turned her away- she was still there the next day following her male around and reacting to my presence with agitation.
Usually nest building begins immediately after males arrive. The slow start may be due to the weather. Temperatures have barely made it past 60 this week and morning temperatures have still been in the 30s and 40s. The next couple days are supposed to be much warmer, so I expect things to take off quickly.
Nest started at A6 by the first male bk/wh, gn/al who moved across the field. This nest is now 3 inches tall.