Wednesday, 16 July 2014

Altruism and Tits

Long-tailed Tit (Aegithalos caudatus)
Browsing through my Royal Society for the Protection of Birds (RSPB) magazine, Nature's Home, just now I came across a reader's letter asking about something he had seen with nesting long-tailed tits recently. He had noticed that several adults were helping to feed the young in addition to the parents and wondered if this was normal.

It is. It is a lovely example of altruism in one of my favourite birds.

The so-called long-tailed tit - it's actually closer to the warbler family than the tits - is one of the UK's smallest bird. Were it not for it's long tail it would actually equal the size of our smallest two, the goldcrest and the firecrest - also, as it happens, members of the warbler family. Both male and female cooperate in building one of the most remarkable of all bird nests made almost entirely out of cobwebs and lichen, lined with about 1,500 small feathers, and suspended in the small branches of, usually, thorny bushes. The nest is domed and has a small entrance hole in one side. It is elastic and remarkably strong. It needs to be because it will expand as the brood of 8-12, sometime more, grows.

Long-tailed tit's nest. Cobwebs and lichen.
I still remember finding my first long-tailed tits' nest and taking one of the eggs for my collection when this was still legal in the UK in about 1957. I carried the miniscule egg home using an old egg-collector's trick when climbing back down a tree - placing it carefully under my tongue and keeping the tip of my tongue pressed against my lower teeth - while riding home on my bike. I even succeeded in blowing it! I have even taken an incubating bird out of the nest and put it back in again and it continued to sit tight.

All this is thankfully illegal now but we really weren't aware of conservation issues then and, as a ten year-old inveterate collector of everything from birds eggs, to skulls, fossils, pressed leaves, and bird wings (taken from road and rail kills), adding another egg to my collection was probably uppermost in my mind. It's now illegal to disturb a nesting bird or to take wild bird eggs in the UK.

Anyway I digress.

Part of a winter flock (Japan)
What we have in the long-tailed tit is a lovely example of evolved altruism - something that creationists purport not to be able to understand, even claiming it's impossible for 'selfish' Darwinian evolution to create altruistic behaviour. This is either because they genuinely don't understand evolution or more probably because they are playing to an ignorant audience who like to imagine the only reason for altruism is because of the rewards their imaginary magic friend will give them later.

The obvious fact that altruistic and cooperative behaviour frequently evolved simply because that gives greater success than non-cooperation and selfishness, seem incomprehensible to them, but then they do earn their living selling lies, misinformation and spurious confirmation of their bias to ignorant and self-important people for money. Their failure, feigned or otherwise, to comprehend anything good being done if not for a reward, speaks volumes of creationist pseudo-scientists.

Helpers

Due to high predation, there is a high nest failure rate. If nest failure occurs after the beginning of May, failed breeders will not try to re-nest, but may become helpers at a nest of another, usually related, pair. In one study, around 50% of nests had one or more helpers. By helping close relatives, helpers gain indirect fitness benefits by increasing the survivability of related offspring. Helpers may also gain greater access to mates and territories in the future. Helpers also gain experience raising young and therefore their future offspring have greater survivability rates.

Males and females are equally likely to become helpers. Parents may allow the care of helpers to be additive to their own efforts, or on the other extreme, they may reduce their efforts with the care of the helpers. Juvenile males have a higher survivability than juvenile females, although the survival rate for adults of the two sexes is the same. Offspring that were raised with helpers have a higher survivability than offspring raised without. Failed breeders that became helpers have a higher survivability than failed breeders who did not. This may be because of the reduced energy expenditure from sharing a nest. This is similar to Acorn Woodpeckers and Green Wood Hoopoes. However, failed breeders that did not help are more likely to breed successfully in subsequent years, so there may be a cost of helping. This may be due to helpers having relatively poorer body conditions at the end of the breeding season, similar to Pied Kingfisher and White-winged Chough. Successful breeders have a survivability rate around the survivability of failed breeders who became helpers1.

  1. Mcgowan, Andrew, Ben J. Hatchwell, and Richard J. W. Woodburn (2003). "The Effect of Helping Behaviour on the Survival of Juvenile and Adult Long-tailed Tits Aegithalos Caudatus". Journal of Animal Ecology 72 (3): 41-99

Next season's altruistic gene carriers
The only requirement for altruistic behaviour to evolve is that it results in more copies of the genes for it than alleles of those same genes which do not support it. In the case of the long-tailed tit, the 'helpers' are almost always the siblings of the parents which have either not bred or have lost their brood to a predator. Long-tailed tits are extremely gregarious and sociable animals with family groups normally banding together into small flocks in the winter. Walk through any UK woodland in winter and you will frequently become aware of a lot of small birds around you, keeping in touch with high-pitched tweets and foraging for small insects.

Because these flocks still stay in contact during the spring and summer, those helping to rear another pair's young will have a high probability of carrying the same genes as the young they are helping to rear. It has been shown that broods reared with helpers will be larger and more successful than those reared without help. There is no mystery to it because one thing follows inevitably from the other. Altruistic behaviour produces more copies of the genes for altruistic behaviour than non-altruistic behaviour produces copies of those genes for non-altruistic behaviour. There is no morality involved and no conscious decision to be kind. The genes for altruism are no less 'selfish' than they ever were.

Selfish genes produce altruistic behaviour if this results in more copies of themselves. In an otherwise weak and vulnerable highly predated species, like the long-tailed tit and humans evolving in leopard and lion country, this is usually the case. This is simply an example of evolved moral behaviour, something and fully expected of 'selfish' genes.





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2 comments :

  1. Beautifully written! And I agree totally when you write the following sentences: There is no morality involved and no conscious decision to be kind. The genes for altruism are no less 'selfish' than they ever were.

    I really like your clear-sightedness, Rosa.

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