Chicks abused by older birds are more likely to grow up to become abusers themselves, scientists have found.
Researchers studying a colony of Nazca boobies, a colonial seabird, found the birds perpetuate a "cycle of violence".
Juvenile birds that are maltreated by older, non-relatives grow up to become more violent towards other chicks.
It is the first evidence from a wild animal that, as in humans, "child abuse" can be socially transmitted down the generations.
Details of the discovery are published in the journal The Auk by Martina Müller, David Anderson and colleagues from Wake Forest University, North Carolina, US.
Nazca boobies are sea-going birds that live in the eastern tropical Pacific, nesting on the Galapagos Islands, and on islands off the coasts of Equador, Peru and Colombia.
Both parents tend to raise a single chick each year, feeding mostly on fish and the occasional squid.
The birds nest within dense colonies, and this proximity to each other encourages bouts of violence to break out.
While parent birds are away feeding at sea, non-breeding adults seek out unguarded nests and attempt to interact with the chicks within.
These can be positive interactions, but frequently they are abusive; the visiting adults try to perform sexual acts on the chicks or act aggressively toward them.
"The maltreatment of nestlings by adults is really obvious," Dr Anderson told BBC Nature. "Essentially all nestlings experience some maltreatment."
To better understand the causes, the researchers studied the interactions between a breeding colony on Espanola Island, Galapagos, over three breeding seasons.
Many of the birds were ringed from birth, allowing the researchers to follow them over the subsequent years, recording their interactions with birds in other nests.
The scientists found a strong correlation between the frequency that Nazca booby chicks were attacked by non-breeding adults, and the frequency that they themselves attacked chicks when they reached adulthood.
The high repeatability of the behaviour revealed it to be a consistent characteristic of each bird's personality, the researchers wrote in the journal.
The team believes the nestlings' experience "conditions" them for life, and may even affect other aspects of the birds' personalities.
The scientists think they can rule out a genetic cause for the abusive behaviour, especially because the abuse is perpetrated between adult boobies and unrelated victims.
"The link we found indicates that nestling experience, and not genetics, influences adult behaviour," said Dr Anderson.
He suspects that being a victim of abuse raises levels of stress hormones, and these hormonal levels later trigger aggressive behaviour, completing the cycle of violence.
"The cycle of violence effect may be a widespread cause of variation in the social behaviour of vertebrates, having been identified in humans in semi-natural conditions, Nazca boobies in natural conditions, and several mammals in artificial captive conditions," Dr Anderson told BBC Nature.
"The Nazca booby model may be very useful for studies of the phenomenon, especially manipulative studies, that cannot be done with humans."
By Matt Walker Editor, BBC Nature
http://www.bbc.co.uk/nature/14418174
