Evolution News - How the Environment Made a Difference with Penduline Tits

Eurasian penduline tit, Remiz pendulinus

Chinese penduline tit buries eggs to prevent them from blowin’ in the wind | Science Linx News | Science LinX | University of Groningen

First off, if you came here attracted by the title, hoping to see something about large busty substances, you're in the wrong place. This is about a couple of closely related birds known as the Eurasian and Chinese penduline tits and about how subtle differences in their environments can cause differences in behaviour that act as barriers to hybridisation and so reinforce the evolving divergence into different species. If you're so shallow as to be looking for the former on line, you're probably too shallow to be interested in the latter.

The Eurasian penduline tit, Remiz pendulinus, is so called because males build a relatively large, dangling (penduline) nest on the end of thin willow branches or reeds, often over water. He then tries to attract a female to use the nest and, if she approves, he mates with her, and she begins to lay a batch of eggs over the next 14 days but won't permit the male to enter the nest. This choice of location is probably to make it difficult for predators to reach the nest.

The strange thing is each day she covers the eggs in the bottom of the nest before going off to feed herself. This is thought to be due to inter-sex rivalry because either one or the other parent is going to take responsibility for the incubation of the eggs and rearing the chicks and the female needs the male to hang around to mate with her and the male needs her to stay and use his nest so his offspring are in the eggs she lays.

When the batch of eggs is complete, the female, more often than not, incubates them and the male leaves to find another mate, but frequently, it's the female who leaves the male to do the incubation and rearing. Very occasionally, neither parent takes responsibility, and the nest and eggs are abandoned. But the male won't start to incubate the eggs until the batch is complete. Burying the eggs is thought to be an evolved strategy for keeping the male interested until the batch is complete.

Chinese penduline tit, Remiz consobrinus

However, a closely related species, the Chinese penduline tit, Remiz consobrinus, has a different pattern of behaviour. The make builds the next in the same way and in a similar position to his Eurasian cousin, and attracts a female to use it. However, she doesn't exclude him from the nest but they both roost in it together at night, and both sexes share responsibility for incubation and rearing the chicks. But, like her Eurasian cousin, the female Chinese penduline tit still assiduously buries her eggs every day. The question is, if this isn't due to inter-sex rivalry as with the Eurasian species, what is it due to? The answer has turned out to be due to subtle environmental differences.

To investigate this difference and the lack of aggression between the sexes by which the Eurasian female keeps the male from entering the nest while she is laying her batch of eggs, Jia Zheng, who had completed her master’s degree under Professor Zhengwang Zhang, from Beijing Normal University, and was now completing her PhD at the University of Groningen, conducted a number of experiment to test out various hypotheses.

The Groningen University News release explains how she went about it:

Parasitism
When eggs were buried because of sexual conflict, the female should be the one to rebury the eggs to hide them from the male. Yet, video observations showed that both females and males reburied eggs.

Sexual conflict seemed unlikely, as there was no aggression between male and female partners. Zheng confirmed this by removing the cover of feathers and other material in which the eggs were buried at the bottom of the nest and observing what would happen. Furthermore, the male was allowed in the nest during the night, so he would know when there were eggs in the nest.

Jan Komdeur, co-author Behavioral and Physiological Ecology,
Groningen Institute for Evolutionary Life Sciences,
University of Groningen, Groningen, the Netherlands

Zheng also ruled out nest parasitism, by depositing an extra egg into the nest during the day, on top of the covering material. ‘These new eggs, either an old penduline tit egg, or a dummy egg, were simply buried by the tits. This showed that they accepted the new eggs as their own eggs, and she concluded that burying is not a measure to prevent [brood] parasitism.’

And finally, Zheng tested the temperature hypothesis. Komdeur: ‘This was done by moving half of the eggs to an old nest that was suspended about two meters away from the real nest during egg laying. This was only done during the daytime, when both birds were active in and out of the nest.’ Conditions were the same in both nests but the moved eggs were not buried. ‘By using a fake egg with a temperature sensor, we found that the eggs in the original nest were slightly warmer but this had no effect on the development of the embryos and the hatching success.’

Wind
This left one—slightly more exotic, hypothesis. Chinese penduline tits often build their suspended nests in trees that are situated in windy places, near lakes or at the edge of forests. Zheng remembered an experiment that she executed in 2018 to study temperature regulation. This involved experimental nests in which the eggs were not buried. After checking these nests in the evenings she found that some eggs had fallen from the nest onto the ground. Komdeur: ‘Zheng then hypothesised that egg burial may serve to prevent the wind from blowing eggs out of the nest.’ Therefore, she designed a wind experiment in 2019.

To test this, she placed eggs in several nests and placed an ink cushion in the nest entry holes. Any eggs that would roll near to the nest entry hole would touch it. After one night, she found that 45 percent of the eggs in these experimental nests had ink stains. ‘Looking back, the number of broken eggs during the earlier incident was just about half, statistically the same,’ says Komdeur.

So, the difference appears to be due to subtle climatic differences which gave an advantage in retaining a behaviour inherited from a common ancestor (or evolving anew, which seems less likely). A behaviour pattern that originally evolved for one purpose was probably re-used for another unrelated purpose, so acting as a barrier to hybridization and reinforcing emerging differences as the species diverged. An alternative explanation is that the behaviour originated in a common ancestor for the reason it’s used by the Chinese species today and it's the Eurasian species that has exapted it for a different purpose.

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