Sunday, 4 September 2016

Parent-offspring recognition in European ospreys

4th September 2016

'How sharper than a serpent's tooth it is to have a thankless child' 
                                                                             [King Lear, I, iv]

Over the last couple of weeks I've been inundated by literally two e-mails, both asking much the same question:- 

Do parent ospreys recognise their returning offspring from previous seasons?”

There is no simple answer to this seemingly-straightforward issue. It seems clear that many species of birds can recognize each other as individuals, once they become acquainted. Of course, the term “kin-recognition” encompasses a whole raft of responses and behaviours, but in this article I am going to focus on the question asked, which concerns parent-offspring recognition.

This has been proven in birds that nest communally, and the selection pressures for this ability to evolve in communal nesting are fairly obvious. Mechanisms seem to include both vocal and visual recognition. Penguin adults and chicks can locate each other – apparently by voice – even in the noise and visual confusion of a busy colony. Beecher (1988)[1] showed that bank swallows are able to do much the same thing. Birds of prey, in contrast, tend to nest by territory – but Alcaide et al (2007)[2] demonstrated that these species (ospreys included) also carry the MHC gene sequence, which had previously been associated with kin-recognition ability (and lots of other things) in mammals.

Ospreys, at best, nest semi-communally and their migratory lifestyle complicates the issue still further. Their young leave the nest at the end of summer, and undergo a complete feather-replacement moult into adult plumage before returning north to their natal regions. This means that a returning youngster will be completely different in appearance, calling into question the whole principle of visual identification by the parent.

All this stuff isn't really getting us anywhere...

UV on the nest
(detail from original
watercolour by
K Davies)
To find out more, I turned to one of the most closely-observed young ospreys on the planet. Yes, you've guessed it – it's UV again.

UV is the trifecta for this job: he has a high-visibility leg ring for ease of spotting, he also has a satellite tracker for detailed following, and his parents' nest at Kielder Forest Park is monitored by video recording cameras, (courtesy of Forestry Commission England.) On 1st July 2016, he landed on that nest while his mother was looking after this year's brood of his siblings. Such an event – a year-two returnee alighting on the natal nest and interacting directly with a parent – is sufficiently rare that filmed recordings of it are almost non-existent.

But we got one.
Video (c) 2016 Forestry Commission England
Used by permission
Short though it is, this sequence repays careful study. We know from tracking data that UV approached the general nest area from the south-west, and that he altered course directly towards it when about 1000m up-range. The nest female sees him approach and goes onto the nest to defend her brood. The chicks - of which there are four - cannot yet fly at this date, so they crouch down in response to the female's evident alarm-calls.

UV makes several passes over and round the nest, before finally landing on it. He alights right on the edge, prepared to take the air again at a moment's notice. His mother mantles and calls in protest at his temerity, and within seconds she has chased him away.

Is there any evidence of “recognition” here? The answer has to be “no”... ALL the birds involved behave and react in exactly the same way as if the intruder had been an unrelated individual.

Expert opinion is still divided on the initial question. Some hold strictly to the view that there is no parent-offspring recognition in European ospreys. Others prefer the view that such recognition “ought” to have evolved, and that there is no evidence to confirm its absence. A few (this author included) suspect that the parent birds cannot recognise a returning youngster, but that the youngster has some instinctive awareness of the kinship.

 But all are agreed on ONE thing: it doesn't make a blind bit of difference who is right! The returnee is no more welcome at a nest site that any other osprey would be, and the breeding adults would always react in exactly the same way, offspring or not.

So is that the end of the story? Not quite.

UV nabs a free meal at Nest 2
(Forestry Commission / J Dailey
For the remainder of summer 2016, we continued to follow UV's activities. He remained interested in Kielder Forest, and intruded at all the osprey nests there on several different occasions. All the nests, that is, except N1a... He only made one more return visit to his parent's nest and did not land on it again. For most of the time, he maintained a discreet distance from it while moving around the Park.

 Does this mean that UV is somehow aware that the parental nest is not a proper place for him, whereas all the others are fair game for investigation? I am not sure. Only the birds know these things for certain.

And they're still not telling.

[1] “Kin recognition in birds”: Beecher, M.D. Behav Genet (1988) 18: 465. doi:10.1007/BF01065515 

[2] “Characterization, Polymorphism, and Evolution of MHC Class II B Genes in Birds of Prey”: Alcaide, M., Edwards, S.V. & Negro, J.J. J Mol Evol (2007) 65: 541. doi:10.1007/s00239-007-9033-9