Let the young make mistakes

This morning, a pair of woodpigeons dropped into the garden. It led me to much biological speculation.

I tried to take some photos:

The flash was on

There's a robin

The key picture

Some readers may see a problem - these do not look like wood pigeons. There are a few differences probably beyond the scope of this questionable photo, like their big dark eyes and drab beaks, but it's clear that they lack the flashy collar. Wood pigeons have a bright white patch on the sides of their neck, with green iridescence around the back. This is because these birds are juveniles.

A typical reason for a juvenile to look different to an adult is sexual maturity: it makes no sense for a bird to have flashy colours to show off to potential mates if it's not interested in finding one, especially if the colours will interfere with its camouflage. This could definitely apply to these pigeons, even if pigeons are not known for camouflage (despite today's prevalence of grey concrete). But the adults aren't sexually dimorphic - males and females look the same - and flashy colours are almost always associated with just one sex. And watching them, it does seem to be females accepting or rejecting males rather than both having to look attractive. So what else could flashy colour be used for?

I've forgotten where, but I think I've read that another species of bird has a white patch near its head just like wood pigeons. It lives in groups, and the patch is a signal to its flockmates: when the bird suddenly takes off, the movement of the patch is very visible, so the rest of the flock can quickly follow. This makes sense if the first bird has spotted a predator, and it's in the interest of the flockmates to escape too. The white tails of deer and rabbits probably does the same job.

But importantly, the juveniles of this bird species also didn't have the patch. It only grows in with the adult feathers. So, a juvenile could take off and the other birds wouldn't necessarily feel the urge to follow. This might be selected for if juveniles are very prone to false positives: flying away from predators that don't actually exist. Ignoring the inexperienced younguns would save the adults (mostly family, if this speculation is going to make biological sense) lots of energy. Vervet monkeys, known for their different alarm calls that refer to different types of predator, will also usually ignore calls from juveniles. So, maybe these juvenile pigeons lacked the flashy collar not because they're pretty young to be searching for that kind of fun, but because it pays for their parents to ignore them. Anthropomorphise from that what you will.



Note: robins (photo two) also have very different colouration as juveniles. They're mottled brown, lacking the bright white and orange of adults. Adults of both sexes use their colours in competition over territories and can be very aggressive. Another unsourced factoid: 10% of robin deaths are caused by other robins. Lacking the colours might save the juveniles from being attacked by bigger birds before they need to find a territory. Maybe anthropomorphised robins would wait until voting age to reveal their political leanings.