The Hawk and Owl Trust fitted a nest box in St John the Evangelist Catholic Church in Bath, and peregrine falcons have been nesting there since 2006. From March this year we've been following the intimate lives of this family. Genetic tests and close observation has revealed some extraordinary, rare behavior, which for us humans makes uncomfortable reading.
In addition to the main breeding pair, we've observed a juvenile who is not only still begging for food from the father, but in return has appeared to pay his way by spending time incubating the pair's eggs, and more recently bringing food to the newly hatched chicks. Why has this young male not left to start a family of his own? And why should he be investing so much time caring for his parents' eggs?
What's more intriguing, is that genetic tests strongly suggest *(99.9% probability) that the mating pair are not just closely related - they are in fact mother and son. This son took charge after the disappearance of his father back in 2008, helping his Mum raise that year's chicks and since then they have mated each year. Is this normal behaviour? Is it viable? And is it a one-off or are there other cases of this in the avian world?
It's usually assumed that inbreeding increases the likelihood of recessive, deleterious genes being expressed. But can it ever have desirable effects? Or can it be accepted and/or justified?
Peregrine falcons have suffered a number of population crashes, for example as caused by use of DDT in the 1960s. These dramatically reduced the variety within the peregrine gene pool, making some inbreeding inevitable.
A study in 1999 on peregrine falcons in the midwestern United States investigated several cases of close inbreeding. They identified 4% of the population in which the adults were closely related (half siblings, full siblings or mother and son). The researchers also observed a very similar family situation as we're seeing in Bath. Interestingly, they did not record any indication of genetic problems in the offspring. All offspring were normal and breeding success seemed unaffected.
So is something happening here that doesn't result in the usual negative effects of inbreeding, not at least at the same rates as we see in other complex animals?
So returning to the juvenile helping his parents on St John the Evangelist Catholic Church in Bath, perhaps the reason that he hasn't left the parental nest is due to the lack of suitable nesting sites in the city - has the population recovered so much that it is hard for territorial peregrines to battle over prime property?
Moreover, could it actually be beneficial for our juvenile to invest his time in these 'half-siblings' as they actually share 75% of his genes. If he were to risk leaving this home-nest and starting a life on his, his offspring would only share 50% of his genes.
Maybe he's worked out what is best for him!