Risky Business

Don't be fooled! It's not only males who advertise their genetic quality to mates, females can too (although it is less researched).

Once a female had build a nest, it was removed from the bird box and enlarged or reduced in size. Control nests were also taken out of the box but replaced without changing size. Aromatic plants (lavender) or control plants (grass) were then added at random. Nests were placed back in the box for egg laying.
Image created using Word Clip Art 2007

A paper published last month suggested that males are prepared to take greater risks for females with larger nests that are well-stocked with aromatic plants (to deter parasites). It is known that females without parasites or with better feeding resources build larger nests, so these features may advertise a healthy female.

Risk-taking behaviour was determined by males behavioural responses after being trapped and tagged inside the bird box. Using a little aluminium door propped open with a stick, both males and females could be trapped inside the nest box. The researchers insist that returning to the nest once this had occurred was "risky" behaviour, because it could happen again:

"... [It] may show the risk birds are willing to incur to feed their offspring".

Blue tit entering its nest box with a beak full of moss
Image credit: Frank Boxell

Reproductive success was improved by males who were willing to take greater risks to provision for their offspring. However, there was no difference between male risk taking in large and control nests. Perhaps there is a limit to male risk taking that is not explored. 

How "risky" this behaviour was and whether most blue-tits would act this way is questionable. 

Although this study was done in a woodland with little human disturbance, many blue tits are found throughout urban areas and should be used to humans. Human presence is probably not considered a great threat as we are not a natural predator of blue tits. Being trapped inside a bird-box or having a radio-tag are also unlikely natural situations. It would have been interesting to see how they respond to a real predation threat in the area. 

Females who build larger nests and add more plants gain higher investment from males in the form of 'risk-taking'. 

Find Out More:

VIDEO: Cigarettes in the Nest  - Urban birds have found a new way to deter parasites

Nest size changing in response to climate change - Really interesting read about what else may affect nest size of blue tits, by ScienceDaily 

Sources:

• TOMAS, G., MERINO, S., MARTINEZ-DE LA PUENTE, J., MORENO, J., MORALES, J., & RIVERO-DE AGUILAR, J. 2013. Nest size and aromatic plants in the nest as sexually selected female traits in blue tits. Behavioural Ecology [online] Available at: <http://beheco.oxfordjournals.org/content/early/2013/03/25/beheco.art015.abstract> [Accessed 28 March 2013]