After a cursory glance over my old posts, I don’t think I’ve done a great job outlining my thesis research (Perceived predation risk and the responses of adult and nestling Tree Swallows). I also don’t think I ever expressed my love of crudely done MS Paint drawings here either. And with those powers combined, I give you this blog post!
A major part of my thesis pertains to bird calls, so to begin, let’s get some background on the noises birds make.
There are songs:
And then there are calls:
Birds song varies from species to species, some species don’t even use songs at all. Fun fact; while song is typically associated with male birds, females of some species are able to sing as well. Songs tend to sound more musical compared to a call, and generally are only used in the spring time for two purposes.
Calls, on the other hand, are used year round by males and females alike. Even baby birds use calls to communicate to parents and siblings. Calls sound less musical than songs do, but have several jobs to do.
The type of calls I focus on in my thesis are alarm calls. These are the calls that birds use to tell other birds about danger. There are two types of alarm calls:
1) Non-specific alarm calls
2) Specific (also called referential alarm calls)
Referential alarm calls are unique in the sense that birds that use them are capable of encoding important information about the predator in them. They might call faster or slower, higher or lower, potentially add more syllables onto the call- it all depends on the type of predator and how much danger it poses to them.
Now enter the Tree Swallow!
Uh…I mean the Tree Swallow; Tachycineta bicolor!
Eventually, Tree Swallows will get their own post without my…artistic renditions. Tree Swallows are a small bird that naturally nests in tree cavities, but will gladly kill a man to live in a nest box. Since they nest in cavities, the nestlings may respond differently to predatory threats compared to birds that nest in an open cup or other cavity nesting birds. My thesis looks at how adult and nestling Tree Swallows respond to different nest predators. I broke down predatory threats into three groups: Box Penetraters (black rat snake and chipmunk), Grabbers (raccoon and kestrel), and Wait-That’s-Not-A-Predator (an American Robin).
My thesis has two main objectives: to determine if parent tree swallows use different alarm calls for different predators and to see how nestlings respond to their parents’ alarm calls depending on what the predatory threat is. To collect the data I needed, I placed a camcorder in the back of a plastic container attached to the nest box so I could record the nestlings. After the nestlings acclimated to the camcorder, I placed a predator or the robin in front of or on top of the nest box. I used a microphone and a recorder to record whatever noises the adults were making during the trial. Or visually…
I’ve already watched all of my videos and I’m processing the audio still. Please note, I haven’t done any statistical analyses and this is all speculative. My ideal expectations were that they would crouch in response to the grabbing predators (to avoid being grabbed out of the nest box), crouch in response to the robin and then resume normal behavior (because robins don’t eat baby birds or would have an interest in taking over a nest box), and they would crouch in response to the box penetrating predators unless they were close to leaving the nest anyway, in which case they would attempt to fledge (as a way of upping the odds of survival, twelve day old tree swallow+snake=bye bye baby, but eighteen day old tree swallow+snake=bye bye baby to the outside of the nest box, potentially).
My personal observations from watching 68 videos is that the trend in behavior is for the nestlings to crouch down in the nest and stop calling when they here a noise outside the nest box or the parents start calling. However, this isn’t entirely consistent with all nest boxes. I had one box with the snake on top and the parent came to feed so the offspring all freaked out and started to beg. I’ve had other boxes were the nestlings stop crouching and go back to hanging out in the line of sight, calling occasionally. There were a couple of boxes where the nestlings did jump around, but I think it was because they heard the parents calling and thought they were about to be fed, not that they were trying to fledge.
The lack of the behaviors I expected for each predator type doesn’t mean the parents’ aren’t using changing their alarm call for the different predators. Based on one study that documented Tree Swallow alarm calls, it appears that adults call more or less when different predators are present. They’re also able to change the frequency of the shriek call , which can make it sound higher or lower. Going through the spectrograms (visual representation of sound), I am seeing variation in the number of calls given at a time, how long a bird waits between calling, and some possible changes in frequency. But like I said, it’s still a work in progress and I haven’t run any formal analyses on anything, so everything is speculative so far.
I’m planning to have everything analyzed, written, and defended so I can finally be a Master of Science by August. Once everything is over and done, I’ll write a detailed blog post about the results I found (and my experience defending my thesis) and what they mean in a biological perspective. Tree Swallow vocal repertoires haven’t been well categorized beyond two papers from the 90s and a study like mine hasn’t been done with Tree Swallows yet. I think there’s potential for finding out some cool information about how the shriek alarm call is used in different situations and learning if other calls are incorporated (such as the anxiety call). If you’d like a detailed post about Tree Swallows or bird noises sooner rather than later, let me know in the comments below and I’ll get to work on it. And as always, we can also chat in the comments about any questions/ideas you have. Catch y’all on the flippity flop!