Spectrometry is for the birds

Gary Boas

About 10 years ago, a pair of studies reported that the small birds known as blue tits have ultraviolet-reflective crown feathers, which likely play a significant role in mate choice. Not long after, another group of researchers demonstrated how it manipulated the reflectance of male blue tits and furthermore described the influence of its manipulations on the proportion of male and female offspring in the brood.

Following these studies, researchers with the Centre for Ecological and Evolutionary Sciences at the University of Groningen, in Haren, the Netherlands, launched an investigation into the relationship between male plumage and female reproductive tactics. “We decided that the blue tit’s UV plumage would provide an ideal opportunity to study the effect of male attractiveness on female reproductive decisions, such as the relative investment in sons and daughters and the occurrence of extra-pair mating behavior,” said Peter Korsten, whose PhD thesis is based on the research.

They knew that the literature already was replete with theories about female reproductive tactics and strategies and the ways in which females relate to the attractiveness of the male partner. “The blue tit’s UV reflectance gave us a grip on male attractiveness, because it appeared to be easy to quantify this trait in wild and live males,” Korsten said, noting the additional benefit of being able to perform controlled experiments by manipulating reflectance.

Studies have shown that the small birds known as blue tits have UV-reflective crown feathers and that these figure prominently in mate choice among the birds. Researchers more recently have used a USB spectrometer to study the relationship between male plumage and female reproductive tactics.

To study blue tits in the field, the researchers used a USB-2000 spectrometer made by Avantes of Eerbeek, the Netherlands, with a deuterium-halogen light source and a bifurcated optics probe. The setup proved ideal because it was affordable, relatively simple and lightweight, allowing easy setup in field stations. Finally, the device enabled the scientists to measure both visible and UV spectra, which was necessary for quantification of the birds’ crown plumage reflectance.

The company since has introduced a new generation of USB spectrometers, including the AvaSpec-3648-USB2. This device covers the UV/VIS/NIR range, from 200 to 1100 nm, and offers an integration time of 10 μs to 10 min, in 10-μs increments.

The USB spectrometer was lightweight and relatively simple to use, allowing easy setup for field measurements.

The research yielded findings about female reproductive adjustment to male UV coloration and about UV coloration as a signal in interindividual competition (two of the three major sections in Korsten’s thesis). Perhaps most interesting, though, were the findings about inheritance of plumage UV coloration. “This is very important because most of the theoretical models that predict female investment in sons versus daughters and female extra-pair mating behavior in relation to the attractiveness of the male partner assume that attractiveness [that is, UV reflectance] is heritable,” Korsten said.

Contact: Peter Korsten, The University of Groningen; e-mail: p.korsten@rug.nl; Caroline Bach, Avantes; e-mail: info@avantes.com.