Birds Nip Youth Mating Game in the Bud : Wildlife: Some species disrupt adolescents’ advances on females. Scientists say elders are recruiting young males to care for siblings.

Like teen-agers who have to pass up a hot date on Saturday night because their parents make them stay home and baby-sit, some birds who want to mate must overcome the opposition of their elders.

Adult male white-fronted bee-eaters, common birds on the savannas of eastern and central Africa, actively disrupt the mating attempts of their male offspring and other males in order to recruit the adolescents into helping with the care of their younger siblings, Cornell University researchers reported Thursday in the British journal Nature.

In a clear demonstration of instincts honed by evolution, the older birds chase their sons away from potential mates, prevent them from giving food gifts and block access to nesting sites.

The tactic works. Young males were twice as likely to abandon their mating quest and return to the family nest when harassed as when they were left alone.

Why don’t the sons put up more of a fight? Because by helping to ensure the survival of its siblings, say the scientists, the son increases the likelihood of its own genes being passed on to subsequent generations, even if it does not pass them on itself.

Such helping behavior is common among birds and many other species, such as coyotes and hyenas, but the report marks the first clear demonstration of the role of harassment by parents, said animal behaviorist Stephen T. Emlen of Cornell University in Ithaca, N.Y. The results are not directly applicable to human behavior, he said, “but we are on to some important generalizations about the underpinnings of social behavior.”

White-fronted bee-eaters, colorful birds about the size of a Western bluebird, live in colonies of as many as 200 birds. The young remain at their parental nest until they mate at one or two years of age. When they mate, males build new nests in the same community as their parents, but females leave to join the community of their new mate. About half of all nesting groups include young non-breeders who help provide food and protection for their younger siblings.

Emlen and Peter H. Wrege of Cornell studied two populations of individually marked, white-fronted bee-eaters for five breeding seasons at Lake Nakuru National Park in Kenya and documented the generational conflict.

Virtually all the harassers--91%--were male, about 65% were the fathers or grandfathers of the harassed birds and three-quarters were older than the individuals they harassed. In addition to chasing the younger males and interfering with food gifts, the older birds would make repeated visits to the pre-nesting site of the younger birds to disrupt the mating. About one-third of the time, the harassed bird would become frustrated and return to the home nest.

The risk of being harassed falls off rapidly as an individual ages and older close relatives die. By the time an individual is three years old, “it is virtually immune from harassment and typically has become a harasser itself,” the authors wrote. The birds typically live an average of 3.7 years after the age of first breeding.

If the researchers are correct in their theory that transmission of family genes is the driving force behind the behavior, then the harassment should be less effective against other birds. That is what they found.

The more distant the relationship between harasser and harassed, the less likely the younger bird was to help in the care of young. Harassed birds who were unrelated to the harasser were least likely of all to become helpers.