Introduction

Like humans, bowerbirds strive to impress their love interest. To do so, male bowerbirds build fancy nests, called bowers, using elaborate decorations that they find in surrounding areas. The male that builds the most enticing bower, or the bower with the most color and unique objects, are able to sexually attract female birds to their nest, where they mate. There are several different bowerbird species and each species has a preference when building their bower. This study follows 5 different species and aims to determine whether brain volume plays a role in bower building

To see a quick video that may help you better understand the behavior of bowerbirds, click here. (Note: not all bowerbirds like blue, the species in this video however, do.)

This is considered a complex nest compared to the nest made by the satin bowerbird in the video above.

Hypotheses

In this study, researchers aimed to study if brain size in bowerbirds might be related to the complexity of the bower that is built. Not only did they study differences across species based on the whole brain but they extended it to study the telencephalon size, a highly developed part of the brain, as well. They also studied the difference in the hippocampus, which is crucial to memory, and the cerebellum, which is crucial to learning through observation.

Method

Five different male bowerbird species were studied, including toothbill, great, satin, golden, and spotted cat. All of these birds have their own method to building the bowers used to attract females, except spotted catbirds. Spotted cat bowerbirds do not build at all. Researchers personally observed and scored birds on the complexity of their nests, based on the size and number of walls and the amount and diversity of ornaments used.

After being observed in their bowers, all of the bowerbirds used for this study were given lethal injections and their brains were extracted. Researches were kept blind to the identity of all birds while testing was being done. Sizes of each region being studied were measured using an imaging system. Each individual brain region was measured and compared using a one-sample t-test, based on body size.

This graph shows the different types of bowers and how they vary in complexity among species.

Predictions

Researchers predicted that variation in bower design might be related to the size of particular brain regions. Meaning, the particular brain functions could be critical for bower building. Researchers predicted that birds with larger whole brains and larger telencephalon are generally able to build more complex bowers. They also predicted that the hippocampus would be larger in birds that use more ornaments and many types of ornaments because their hippocampus would allow birds to remember more locations where ornaments can be found. The cerebellum, which is used in learning by observation, would be larger in birds that have greater structural complexity and precision of bower designs.

Results

Comparing non-bower-building spotted catbirds to the four other species showed that spotted cat bowerbirds had smaller whole brains and smaller telencephalon than the rest of the birds. However, hippocampus and cerebellum were not smaller in spotted catbirds vs. other species. Comparisons that analyzed the volume of each brain region and the bower complexity showed a strong relation between complexity and the cerebellum but not strong relations for any other regions. This means that the cerebellum plays a large role in bower complexity, while other parts of the brain do not.

This chart shows the estimated volume of each brain region in every species based on their body weight.

This graph shows the complexity of the bower vs the size of the cerebellum for each species of bird.

Researchers conclusions

Researchers determined that bower builders in general, had larger brains than non-bower-builders however they did not notice any significant differences in brain size contributing to how complex each species built their own bower. They did not find any significant difference in hippocampus regions and were surprised to find that this could indicate that bowerbirds do not need memory to build more complex bowers. Researcher’s focused mainly on their discovery that cerebellum size and bower complexity were associated. They determined that learning the procedures to build bowers and motor planning were necessary for more complex bower building. They found that cerebellum size increased based on the increase in complexity per bower.

Personal commentary

Humans tend to try and attract a mate by impressing them. Whether they use tattoos, favorite clothes, or a nice house and car. Bowerbirds engage in the same behavior. They try to impress females to attract them into their bowers to mate. Somehow, males are able to go collect ornaments and build elaborate nests solely for females. This behavior is thoroughly interesting, just like this study. Although the researchers used proper permits from wild life services to complete this study, do you think there is a better way to go about studying bowerbirds and their brains?

I wonder if instead of giving the birds lethal injections, they could have just been put to sleep and run through an imagining system. Although I am not sure that method would give clear enough measurements. I wonder if there is better way to go about studying their brains related to bower complexity instead of putting them down.

Citation

Day, L., Westcott, D., & Olster, D. (2005). Evolution of Bower Complexity and Cerebellum Size in Bowerbirds. Brain, Behavior, and Evolution, 66(1), 62-72. dos:10.1159

Posted by: Melissa Mancini