The following is from:
Dr. T. Luke George
Ornithology Lecture Notes


Why do birds breed cooperatively?

Cooperative breeding poses an enigma for evolutionary biologists because helpers are apparently behaving altruistically. Closer
examination of cooperative breeding systems, however, has shown that helpers are actually acting in their own self interest.

Analysis of Fitness Trade-offs of Cooperative Breeding

First we must weigh the costs and benefits of helping to the breeders and the helpers to determine if helpers are suffering lower
fitness as a consequence of helping.

For example:

The Florida Scrub Jay (Aephelacoma caerulescens) is a subspecies of the Scrub Jay. It inhabits scrub oak woodlands in central Florida. A population has been under intensive study for nearly two decades by Glen Woolfenden and John Fitzpatrick.

This population is sedentary; individuals are long-lived; and long-term monogamy is the rule as a mating system. Groups are permanent residents on large, all-purpose territories.

Approximately 1/2 of the breeding pairs are accompanied by helpers; pairs have as many as 6 helpers that remain with them for 1-5 years; most helpers are offspring of one or both of the breeders.

Helpers assist breeders in the following ways. Helpers:


The evaluation of fitness benefits to helpers and breeders is crucial. Woolfenden and Fitzpatrick evaluated the question of whether or not helpers really help (increase reproductive success of breeders) in the following manner.

So, we conclude that pairs are benefitted by helping. There are costs, however. Therefore, helper success = 1.94 - 1.62 = 0.32. But, this must be devalued by 0.5 because individuals only share 1/2 their genes with the siblings that they are helping to raise. Thus, 0.32(0.5) = 0.16

In a few cases, young birds are successful in establishing themselves as breeders. These novice breeders produce 1.03 young/nest. Again, we must devalue this because an individual shares only 1/2 of its offsprings genes. Thus, 1.03(0.5) = 0.51

Conclusion: Florida Scrub Jays would do better (fitness) breeding as novices than helping to raise kin. Other studies generally reveal similar results, i.e., helpers do help but they would do better to breed on their own.

So, why don't they breed on their own?  There are other benefits to staying home and helping including:

Thus, in terms of lifetime fitness, young Florida Scrub Jays do better by staying and helping. The most important factor in this 'decision' appears to be 'ecological constraints' (see below; the probability of a young scrub jay establishing a territory is low).
 

Evolution of Cooperative Breeding

The ultimate explanations for cooperative breeding systems (why has it evolved?) fall into three general arguments:


Back to BIO 554/754 - Avian Mating Systems