Cooperative Breeding
Many birds have helpers. Often it depends on environmental conditions (ie. rain ecology). For instance, bee-eaters depend on bees. If there are fewer bees they have more helpers to help raise the young.
Helpers in the pied kingfish (Reyer 1980, 1984). At Lake Naivasha nests have no helpers or one helper, feeding conditionas are good and one helper is therefore enough to ensure that all the chicks get enough to eat. At Lake Victoria there are sometimes second (unrelated) helpers, feeding conditions are poor and the second helper makes a big difference to chick survival.
Example: Acorn woodpeckers are more likely to stay at home if there is no place to go. There are especially likely to stay at home if there is no place to go and it is a high-quality habitat.
Seychelle warbler (Komodeur 1992). These birds went for years without helpers and then suddenly they began having helpers.
Table 1: Factors influencing dispersal from natal groups:
Risk of dispersal Probability of obtaining a vacant territory Probability
of obtaining a mate Prob. Of successfully reproducing once established
Resulting behavior
Low High High High Disperse and breed independently
High Low Low Low Remain in natal group as an auxiliary
Often the breeders work harder when they are in a single pair, than when they have a bunch of helpers.
One of the most interesting things involved in cooperative breeding in birds is to study them in different environments. One example is in acorn woodpeckers in different sites. These sites were studied by Stacey and Bock in 1978. They included Central Californa, Arizona/New Mexico, and Southeastern Arizona. While there may not be an increased success per group, there is an increase per capita.
Florida Scrub Jays serve as a model species to study in cooperative breeding because they have been studied the longest. We know that the parents benefit, but what benefit do the helpers get? It seems that if the young stay around and ‘help’, they may be able to inherit an enlarged territory. This would be a direct benefit to the helpers.
An alternative to the direct benefits would be kin selection. This theory is based on the fact that offspring are related to the parents and thus their fellow siblings. If they help the parents raise a sibling, they ARE actually helping their genes to be passed on to future generations. Kids are related to parents by 0.5. Grandperents by 0.25. Siblings by 0.5. Half-siblings 0.5.
The best study on cooperative breeding has been done by stripped-back wrens in Venezuala: Campylorhychus n. He found that the number of individuals produced depends on the group size. On average a pair of birds will not produce many offspring. But, if there are two parents and a helper, its still very, very low. But when you get groups with two or more helpers, reproductive success increases dramatically.
White-fronted bee-eaters exist in clans. This is possibly the most complicated of the cooperative breeding systems that have been studied. Individuals can be fluid, meaning that they can go from breeders to helpers and back to breeders then back to helpers.
Who is helped by who? This is a good question. Its been shown that whether they are helping as a breeder, or as a failed breeder, they are helping relatives. Thus, natal birds are the most likely to help. 45 percent of the time helpers, helped their own mother and father, 10 percent they helped a father step-mother, 9 percent of the time they helped a mother – stepfather, 10 percent of the time they helped their sons.
The individuals can recognize the amount of relatedness to each other.
In green wood hoohoos, an individual may help someone that fed them earlier in life. Its an interesting situation.
What degree of incest is there in birds? Sons do not try to court the mother. But, if there is a replacement female in the family (stepmother), the son will start trying to court the female.
An Ani will engage in group breeding areas. Some females in these communal breeding systems will toss subordinate females eggs out of the nest.
So why do some birds engage in cooperative breeding and others stay only in pairs? This question is still very interesting, and it has not been resolved. There are a number of species where there are more than a few birds that take care of the nest. It has been documented in Hawaii in the Maui Creepers, and Hawaii Amakiis.
By Rob Nelson
