Northern Mockingbirds (Mimus polyglottos) are year-round residents in North Carolina and defend territories throughout the year. In the winter, a female sometimes defends a separate territory adjacent to that of her mate from the previous summer, or a pair sometimes defends a territory together throughout the year. Males and females can only be distinguished by behavioral differences. In particular, only males sing.
A mockingbird spends nearly all of its time within its territory. In behavioral studies, a territory is a defended area used exclusively (or at least mostly) by the resident and its mate. A great variety of different species, vertebrates and invertebrates, defend territories. In some species a territory provides only some of an individual's requirements, such as a nest site or food, but not both. In these cases, territorial individuals must of course leave their territories to obtain their other necessities.
In contrast, a mockingbird's territory, like those of many other small songbirds, includes all of its requirements: places to feed, rest, sleep, and seek shelter from predators. During the breeding season, nesting also occurs within the territory. Consequently, each mockingbird spends nearly all of its time within its own territory.
Mockingbirds feed mostly on insects during spring and summer and fruit during fall and winter. The red berries of the holly trees around Coker and Wilson Halls are favorite sources of food during winter. A mockingbird defends its territory primarily against other mockingbirds, but sometimes it also attacks other birds that feed on fruit inside its territory. Why is fruit guarded so jealously? If food becomes too scarce during winter, a mockingbird must occasionally leave its own territory and fend for itself in other birds' territories, where it is subject to attack whenever discovered.
Territorial animals defend their territories by means of outright attacks and fights when necessary. Much more frequent, however, are displays which serve to advertise that an individual has claimed and is prepared to defend an area.
These advertisement displays take diverse forms in animals: many territorial mammals use scents (secretions from special glands or simply urine or feces) to mark boundaries or conspicuous points in their territories. Many territorial birds use long-range sounds to advertise territories. Most birds' songs, including the mockingbird's songs, fall into this category. A male mockingbird uses conspicuous perches as song posts from which to proclaim its presence on a territory.
Males sing primarily from January (warm days only) through July and then again in October. From November through February, they use another distinctive sound, a sudden harsh chat or chat-chat-chat, to proclaim their territories especially early in the morning. A tape-recording of a mockingbird's songs or a burst of chats, when played from a speaker inside a mockingbird's territory, often elicits a prompt aggressive response from the resident. What might explain this behavior?
Work in groups of 4-5. Use a paper and pencil for recording observations, a watch to time behavior, and binoculars. Each group will record the behavior of one mockingbird (or a pair) for one hour.
Two students will serve as observers with responsibility for keeping the bird in sight (or knowing its location, if it is in a bush out of sight) at all times. A third person will record the locations of the bird on a map. The fourth person will keep a written account of the bird's behavior, and the fifth person will serve as timer. Everybody should take a turn looking at your subject with the binoculars -- does it have colored plastic bands on its legs?
In recording the behavior of an animal, we need to take into account two kinds of activites. Behavioral events are actions that take place essentially instantaneously (or are too brief to allow us to time their duration), such as eating an item of food or attacking an opponent. Behavioral states are activities that last long enough for us to time their duration, such as resting on a perch, singing or preening continuously, or displaying repeatedly to a neighbor at a boundary. When you make a record of your mockingbird's activities, you will want to count all events but time the duration of all states.
To estimate the time an individual spends in different states, you should use scan sampling, the procedure used in the exercise on fish schooling. Instead of recording the time at the start and stop of each behavioral state, record the bird's behavioral state every 10 seconds (the person serving as timer can help here). In one hour you will then have 6 observations/minute X 60 minutes = 360 scan observations at 10-second intervals. How can you use this information to estimate the proportion of time spent in each behavioral state?
You also will want to pay attention to a bird's locations and conspicuousness. Perching on a high bare branch (conspicuous both to other mockingbirds and possible predators) is not likely to be equivalent, from the point of view of a mockingbird, to perching out of sight in the middle of a dense bush. Think like your mockingbird when you decide what to record about its behavior!
Especially important are the locations at which your subject attacks or challenges opponents (if any). Also include the locations of other mockingbirds that you notice while watching your subject. Then determine the area defended by your subject, the area used exclusively by your subject, and the total area used by your subject. Do these three areas coincide?
After recording your observations for one hour, prepare the above three summaries. You should have made a start in understanding your subject's territorial behavior. Give your summaries to your Teaching Assistant.
This week you will conduct an experiment with playbacks of tape-recordings. During the week or two before this exercise, you will have a chance to check all the equipment we use.
Be sure that all of the equipment works and that you understand how it works.
During this exercise, you will again observe your bird's behavior for one hour and make a map of its territory, in order to obtain some additional information about your subject's behavior and the limits of the territory. Afterwards, you will use playbacks of tape-recordings to create the impression of different kinds of intruders in the territory.
The loudness of the playback is a important variable in these experiments. Too quiet and the playback might not be heard. Too loud and the mockingbird might conclude that its territory has just been invaded by the mockingbird equivalent of Godzilla! Encourage students to use their own ears during the playbacks to adjust the volume. It is no use adjusting volume in the lab (sound reverberates in the room and makes any estimates of loudness useless outdoors).
As before, use scan sampling to estimate durations of behavioral states and counts to determine the frequencies of behavioral events. For these experiments, you should also record the latencies of conspicuous responses (the time from the start of playback until the subject's first reaction).
Use the results from your entire section (or all sections) to compare behavior before, during, and after the playback. Use simple statistics (see the supplement to this manual).
Also compare responses to the three different kinds of playback. How would you use simple statistics in this case?
Do mockingbirds respond to conspecific (same species) calls as much as to songs?
Do they respond to other species' songs as much as to conspecific songs?
How do responses to playbacks resemble responses to natural intruders? How do they differ?
Why do certain stimuli evoke territorial aggression?
Why do we record data before and after the playback as well as during it?
Why do we choose positions for the speaker and then flip a coin to determine which to use first?
Why do we move the speaker between playbacks?
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