This past weekend I taught a class a class on bird language at Earth Native Wilderness School in Austin, Texas. We had a group of about 15 students, and after a brief introduction on bird language basics we headed out on a trail to perform what is known as a bird language "sit" (a term coined by Jon Young, author of What the Robin Knows). Not wanting to overwhelm anyone with too many instructions, I had asked them to observe any and all bird sounds and behaviors, but to make only one distinction: to attempt to distinguish between baseline (songs, calls, feeding, territory defense, etc.) and non-baseline (alarms) behaviors.
The sit is typically divided up into several different periods, with participants recording observations in each period, and then afterward participating in a mapping debrief where the group diagrams everything that occurred through each period. As we got settled in for the sit, a student asked what typically happens to which I replied, "In bird language, there is nothing typical...every sit is different and often unexpected." That certainly turned out to be true in this case! Often times individuals are stunned at what transpires, in particular because the group acts as a multiplier, and when observations are gathered from people sitting in different locations (usually 10 meters or more apart), a comprehensive story emerges, one that is usually invisible to the individual himself.
In this particular sit, song bird activity in the initial period was somewhat repressed, a common occurrence in the first period because people going out on the landscape have created a disturbance to animals and birds alike. As typically happens, the ensuring period sees an increase in activity which happened in this case, with a few songs and several contact calls (a method for bird of the same species to stay in touch with one another) breaking out. During the end of the second period however, a couple of red shouldered hawks called out, creating a low level alarm signature from a nearby woodpecker and blue jay. This mild alarm response from song birds is normal, and unless it's breeding season (when red shoulder hawks can in fact take nestlings) red-shouldered hawks normally do not arouse must interest from other passerines because they are not much of a threat.
A few moments later, I caught a brief glimpse of a hawk flying through the trees, but could not identify it. Naturally, I assumed it to be one of the red-shoulders since forests are their natural habitat, but then immediately afterward the area around me went completely silent! (not at all the type of response expected from a red-shouldered hawk). About 15 minutes passed before baseline behavior (song, calls, and movements from song birds) returned...I had my suspicions about what had occurred, and was very curious to see what nearby students had observed.
During our mapping debrief a couple of students said they had spotted a Coopers hawk flying through the canopy...it was an Aha moment! (no doubt the bird I had seen from afar). As they described it, it matched perfectly with the concept known as Wake Hunting, a phenomenon whereby an accipiter uses a distraction or alarm created by another event (in this case the arrival of a red shouldered hawk) to sneak in behind the initial alarm and catch prey off guard...a kind of surprise attack. To be on the safe side, when corvids or hawks like red-shoulders pass through an area, song birds will often go silent for a short while or seek temporary cover. This allows for the entry of a predator like a Coopers hawk to come in potentially unnoticed, and hence catch a song bird unaware of the incoming danger.
Wake Hunting can take many forms, and in addition to using the passage of other larger hawks as cover, Coopers and sharp shinned hawks will often use urban distractions like moving trucks, the sound of a jet airplane and other disturbances to gain the advantage. Song birds are well aware of these tactics, so when the accipiters are noticed as in the case of our group sit, they then cause their own "secondary" alarm, in this case the near total silence of the forest.
If you are interested in similar topics from the perspective of others who have experienced group bird sits, check out the student blogs from a class I co-taught at the University of Florida recently. Their experiences will make you want to go out and do a bird sit every day!