If you have ever been anywhere near a seabird colony, the first thing you will probably notice is the sound of hundreds of birds yelling and calling at each other. The next thing is the sight of an enormous number of seabirds squashed onto ledges, sitting on nests, circling in the air or bobbing in rafts on the water surrounding the colony. The final thing you will undoubtedly notice is that there is a certain smell associated with large numbers of birds nesting together. Not just the smell of several tons of guano and rotting fish discards but often a certain indefinable muskiness. If you picked up a bird and put your nose to it you would certainly be able to distinguish this smell. Generally however, this isn’t recommended; you may end up minus a nose.
Is there a purpose behind this smell? Is it just a byproduct of the production of the oil which waterproofs their feathers? The unfortunate side effect of an all fish diet? Or do the birds just like it?
In other species, such as mice, smell signals are used to help distinguish between mates and avoid inbreeding. While it has been speculated that smells in birds may serve a similar function, very little experimental work has been done to test this speculation. This is possibly because birds have such an abundance of other, more obvious signals such as their calls, so that the less obvious smell (or olfactory) based signals are largely ignored. Recent studies have shown that the chemical profiles of olfactory cues in several species of seabirds are remarkably similar from year to year and are thought to contain information such as species, gender and individual identity.
Testing if this is indeed the purpose of these signals are used is more difficult. The studies which demonstrated the similarity of olfactory cues over the years performed detailed chemical analyses using gas chromatography. Collecting enough data from related birds to test for things such as the avoidance of inbreeding using these methods is problematic. The seabird species likely to rely on such signals tend to only have chick a year, and these will not breed themselves until they are about seven years old.
So how can scientists test their theories about olfactory signals? A recent study by Celerier et al using a rather novel method to test for information contained in bird odours. The study was carried out on the blue petrel Halobaena caerule, which live in burrows within dense colonies on remote islands, to which they return every year. Making the correct mate choice, recognising individuals and avoiding inbreeding is therefore of utmost importance to them. As such, olfactory signals would be extremely useful to help them make the best choice.
In order to see if information about relatedness or identity is contained in the olfactory cues of blue petrels, a “biological olfactometer” was used: a mouse. The study essentially used the mice’s noses to test for similarities in odours between related and non-related birds. To do this, a mouse was first allowed to get used to a certain individual’s smell. A mouse will always investigate a non-familiar odour, and so the mice were deemed to be familiar with a smell once they showed less interest in it. These mice were then used to perform three experiments.
The first experiment investigated if the odours contained information about an individual’s identity. The mice were presented with two separate odours, the one they had been habituated to and an odour from another random blue petrel. The mice spent most of their time investigating the new odour, showing that blue petrel odours are different enough between birds that they could be used to identify individuals.
The second and third experiments examined odours for information about relatedness. In the second experiment, the mice were presented with the odour from a chick related to their familiar odour and an odour from an unrelated chick. The third experiment presented them with the same choice, but with odours from older chicks. It was found that the mouse spent a more or less equal amount of time investigating both odours. However, when presented with odours from older chicks, the mice spent far more time investigating the unrelated odour. This suggests that the odours produced by blue petrels can be used to determine if two individuals are related, and thus avoid inbreeding but that this information is not present in younger chicks. This is likely to be related to the development of feathers and of the gland that produces feather oil.
If blue petrels have the ability to get this information from odours, then it this explains several things about their behaviour. For example, how they can recognise their mate from the previous year after several months at sea as has been demonstrated in previous studies. It also has potentially interesting implications about how they might go about choosing mates. The avoidance of related odours can assist in preventing inbreeding, exceedingly important in the largely closed island communities that blue petrels live in. It is also likely that individuals will not only avoid their own, related odours, but that they may develop preferences for certain odour types. All of this presents interesting possibilities for future research. These may, or may not use the noses of mice to get answers.
Reference: Chemical kin label in seabirds www.rsbl.royalsocietypublishing.org/content/7/6/807.abstract