A subject I am extremely familiar with.
Being one of four I can relate to the struggles of chicks within a nest – fighting for your parents attention, trying to get the best resources, being the better person…. the usual stuff.
As my thesis looks at the effects of stress on development in dippers one of the easiest things to compare between nests is brood size – number of chicks per nest. The greater the number of chicks within a nest the more stress each chick faces in regards to competing for food in order to succeed.
Whereas I would like to think my sisters wouldn’t stand on top of me in order to be fed first, chicks on the other hand have very little regard for one another in the nest. It is a dog eat dog world and every meal counts, and when one of them is ready to fledge the rest of them also have to go. Usually parents stop feeding at the nest once the first chick is out and so chances of survival start to drop.
Some of the nests I have visited over the last few weeks have had chicks with very similar weights and body measurements, however occasionally you can see a stark difference in sizes. Usually the chicks that hatch first are given the advantage, they get fed first, they can grow stronger quicker and in turn they grow bigger and more able to physically push their siblings out of the way.
It is true that in some species parents will be choosy or coordinate feeds to make sure all chicks are fed equally. But this can mean hanging round the nest for longer and the chances of predation start to increase.
In the photo below are two eleven day old chicks from the same nest. It is hard to tell but the one of the right is the smallest out of the brood (of 4) weighing in at 42.9g and the one on the right weighing 49.4g. You might think, 6.5g isn’t that much of a difference, but the small chick is 15% smaller than it’s sibling. This could have a huge impact on the rest of this chicks life. Whilst this bird was physically smaller, it had a very similar wing size which could be due to the aforementioned readiness to fledge, investing what little resources you are getting into wing length is a wise choice. This enables quicker getaways when a predator is about, which is crucial for fledglings.
I measured a nest of chicks that were five days old on Saturday and the difference in mass of the smallest to the largest was 7g to 13.5g – which is almost double in size! ( I was hoping to get a photo but the little chick was getting a bit too cold to keep it out of the nest any longer). Sometimes as the nestling stage progresses the sizes of chicks can even out, but in large broods of five or six individuals it’s these birds that are the ones who don’t even make it into the second week.
I am hoping to be able to relate the stressors of early life to their adult phenotype aka song complexity, body size, reproductive success etc. So we are currently measuring growth rates in the nest, which can be quite fiddly but mostly adorable.
Next time you think your sibling is getting special treatment at least it’s not a matter of life and death. And you can crush them next time.