Flocking

Sociobiology is defined as ‘the subset of behavioral ecology that specifically deals with social behavior.’

Types of groups that you can imagine include the following subsets:
1. Unrelated individuals (A school of fish for example).
2. Uniformly related individuals (not siblings). {For example, a group of cousins}
3. Variously close and distant relatives (perhaps with siblings) (ie. Mother, daughter, son, cousins, etc). This is probably the most common type
4. Siblings (with or without one or both parents)
5. Groups of genetically identical individuals.

Living in a group has several risks. Some of these include:
1. Increased intensity of competition for resources, including mates.
2. Increased likelihood of disease and parasite transmission.
3. Increased conspicuousness of predators or prey.
• Of these, you’ll always have 1 and 2 occuring, while 3 is not always the case.

Consider then the problems of the organisms in a group.
1. Subordinate male is rendered effectively strile by an aggressively dominant who keeps him away from ovulating females.
2. Dominate male who is cuckolded by a sneaky subordinate that he has not ostracized completely or killed.
3. Females unable to secure all the parental attention of the father of her children because of other females nearby
4. Colonial birds who must nest close to each other with maximum risk of having someone elses eggs deposited in their nests.

Selective background of group living.
1. Susceptibility to predation may be lowed because of aggressive group defense or because of oportuity for individuals to use the group as cover.
2. Nature of food resources may make splintering off unprofitable (large prey or patchy prey.).

Performance of a bird in a group: Shows that pidgeon’s reaction distance is increased with larger flocks. In other words, the larger the flock, the sooner they see the hawk. Thus, there is an advantage at least to wood pigeons when an attacking goshawk is nearby.

Predatory success while birds are in a group: (Kenward 1978) – The more birds there are in a group, the less successfull a hawk is. Some birds that form a ball when a predator is around. This in turn, may increase the risk of the predator because of the increased chance of getting hurt.

The idea that flocking reduces vigilance is a big deal: How important is predatation in the evolution of flocking? This can be done by comparing islands with mainland areas. Ed Willis came to Hawaii and compared them wrong. As it turns out, the birds do flock AND there are predators.

BIRD VIGILANCE:
Part of the reason birds flock is because of vigilance behavior. A bird looking up can be considered more vigilant than a bird looking down. Thus, you can determine the amount of vigilance in a bird. You can also look at differences between males and females of dichromatic species. If you look at the vigilance rate, compared to the number of near-neighbors, you find that the vigilance rate goes down with increasing flock size. But the vigilance rates of single males are much higher than female birds.

Flocking: One aspect of flocking is interspecific flocks of birds. There are some flocks in Peru that have over a hundred species of birds. As it turns out there is lots of communication in the flock … including dis-communication.

Study in Hakalau: There were two different sites: Pedro and Pua Akala. In the less dense area, they would have to go way out of their way to find a flock. As it turns out, these flocks are full of insect-feeding birds. In general there are no differences in the flock sizes in the areas where the density is large, compared to areas where the density is lower. Pedro is the low density site. But, it shows that at the low density site of Pedro, that flocking is so important that the birds go out of there way especially to find these flocks.

Alternatives to the Benefits of Flocking:
Social-Facilitation: the feeding rate, for example, may be increased when others are around. In other words, when others are around, there is an increase in fitness somehow.
Local Enhancement: This concept deals with one individual that is successful at a particular location, attracting others to that location.
Information Center: When individuals can learn from each other. For example, bank swallows hunt for insects. The idea is that the swallows are using each other to find the insects. If they nest too early, there are not many others to help find food. Another example is Ospreys feeding for fish. Schools of fish are also very patchy. Ospreys that nest near each other can look at other ospreys and see what type of fish and where their neighbor Osprey comes from. (For fish, such as flounder that are not schooling fish, a find does not attract other Ospreys to head out in the same direction.

Foraging in a Group:
• The probability of starvation plotted over the food requirement allows us to see that at different 'food requirement conditions' there are better times to be a solitary, compared to a flocking bird.
• In a flock, not all organisms are equal. Is it better to be a dominant or a subordinant individual in a flock?
• The larger the flock size, the more aggressive confrontations there are in a flock.
• There are less scannings in larger groups, so the feeding curve also has an optimal level. In essence you can predict the optimal flock size.
• Flocking in birds has risk averse benefits. They reduce the varience in patches. In a risk prone environment they make take a chance and not forage with a group and end up dying.

By Rob Nelson


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