A Novel Context for Evening Grosbeak Flight Calls

A few days ago, I found myself in Blackwood Canyon on the west coast of Lake Tahoe. It is a beautiful spot where Evening Grosbeaks have been reported regularly over the last couple of weeks. The day was lovely, and the birding was terrific with lots of classic mountain birds all around. Warbling Vireos and Hermit Warblers were foraging in the poplars that grow along side the stream that runs down the middle of the canyon. MacGillivray’s and Wilson’s Warblers were leading young birds around through the willows. Spotted Sandpipers were walking and bobbing their tails as they foraged along the rocky stream edges. Female Mallards lead broods of ducklings from pool to pool.

I did find a flock of Evening Grosbeaks, though they were not particularly cooperative in terms of my research plans. However, while I did not get the experimental trials I had hoped for accomplished, I did see something new and cool. I was stopped along the side of a road at around 9:30am beside a small group of spruce tree when I heard an Evening Grosbeak giving really loud flight calls. They were actually kind of spectacularly loud! As I watched, I saw the grosbeak, a male, flying from tree top to tree top and continuing to give these high amplitude calls. After standing below him recording for a few minutes, I figured out why he was behaving this way. There was an adult Red-tailed Hawk perched in the top of one of the spruce trees and the grosbeak was mobbing it. Now, Evening Grosbeaks are known to use their flight calls to coordinate the movement of a flock and they are also used by birds to locate other individuals over long distances. Further, they may possibly play a role in mate choice decisions and in population identification. However, to the best of my knowledge, this is the first time flight call have been observed being used in a predator harassment context! After a few more minutes, the Red-tailed Hawk flew off. The Evening Grosbeak, having succeeded in annoying the predator into leaving, quieted down and then departed on his own business.

This was yet another reminder that calls can have many different functions even when it is basically the same call, and even when that call is has a fairly simple structure. What subtle differences communicate different information to a receiver? Was the volume of these calls an important component of harassing a predator? Are there other differences (speed of delivery, frequency range, something else) between flight calls that are used in different contexts? I have this one recording, and so will certainly examine it to see if there is anything that jumps out at me, but with only the one occurrence, it will be hard to identify smaller differences, even though such small differences may be quite important to the birds involved. This also served as a reminder that exciting things can happen, but you have to spend time out with your study subjects to see them!

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Ways to Maintain Honest Signals

Signals (auditory, visual, chemical, etc.) are the outcomes of co-evolutionary processes between signalers and receivers.  They involve the signaler attempting to influence the receiver in a particular way, and the receiver attempting to make the best decision possible based on the signal it receives.  The goals of the individuals involved in signaling are sometimes parallel, as is times of cooperation, but are quite often in some degree of conflict.  This conflict opens the possibility of cheating being advantageous to one or more of the individuals.  For example, a signal where the signaler and receiver are directly in conflict is between a predator and its prey.  Many prey species have specific signals that they give when they detect a predator.  This signal is given to warn the predator that it has been discovered, and that any chance of that predator ambushing the prey is now gone.  Predators receive these signals and go elsewhere in their search for food.  Given this situation, why doesn’t the prey give their predator-discovery signal all the time?  In other words, what stops the prey from cheating?  If they did, any predator who came close would receive the signal and then leave thinking it had been discovered.  How honesty is preserved has been studied using evolutionary game theory, and three types of signals have been discovered based on how cheating is prevented.

Performance Signals are signals that are inherent in the signaler. They can only be displayed by individuals that possess a specific capability or knowledge.  Body size limits how low a frequency a frog can call at, with larger frogs able to make lower frequency calls.  Therefore, the minimum frequency of a frog call is constrained by the size of the frog, and in this way a receiver can know something about the signaler.  These types of signals are impossible to fake, and so is no way for the signaler to cheat.

Handicap Signals are costly to make or use.  Performing a display that is highly energetic can be costly to a signaler because the energy they use to display is energy they could be using for feeding or other activities that would benefit their survival.  An example of this kind of display could be the flight displays of male Red-winged Blackbirds.  Possessing a display that attracts the attention of predators can carry a high predation cost.  An example could be the calls of Tungara Frogs that are at the same frequency as that which predatory bats use in echolocation.  Forming a display that requires a specific nutrient that is difficult to acquire can carry a high cost because of the time and energy used in location the nutrient cannot be used for other activities.  All these signals can only be used if the signaler can tolerate the handicap of the increased cost.  Signalers that are unable to tolerate the cost are unlikely to survive if they use the signal and so cheating is prevented.

Conventional Signals are signals that have no specific cost, but that have been chosen arbitrarily to convey a given meaning.  An example of this type of signal is the head posture of many sparrow species.  By thrusting the head forward at an opponent, a sparrow indicates that it is willing to fight for a resource.  If that individual’s opponent is also willing to fight both sparrows could be injured.  Cheating is prevented because an individual does not know the condition of their opponent.  If the signaler indicates that it is will to fight, but is cheating and not actually willing or able to fight, they will trigger an unknown response from their opponent.  If their opponent is willing to fight the cheater will incur the costs of the fight with no chance of getting the benefits of the resource.  Cheating with conventional signals is also prevented when individual interact repeatedly.  In this way, a cheater will be discovered over time, and their signals will come to be ignored.

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