Birds with the brightest feathers may pay a cost for their showmanship

Males with the brightest plumage are thought to be more sexually attractive to female birds.

But a study of American goldfinches is the first to show that high levels of brightly coloured chemicals in feathers leads to a breakdown in flight muscles, which affects flight performance.

Details are published in the journal Naturwissenschaften. The discovery does not necessarily mean that the brightest birds are also the weakest, or least able to reproduce.

Instead it shows that having bright feathers comes at a real cost to male birds.

That in turn means that bright feathers are an honest signal of quality.

Only the fittest males in the best condition, who are best able to cope with the negative effects, will take on enough brightly coloured chemicals to brighten their plumage.

Yellow health

In many animals, including fish and birds, males gain competitive or mating advantages by ingesting and using large quantities of pigments known as carotenoids.

These yellow, orange and red pigments are found naturally in the bird's diets, and they cannot be made by their bodies.

When eaten, carotenoids are converted to brighten otherwise dull feathers, creating bright plumages.

The beneficial effects of high levels of carotenoids are well documented by scientists: as antioxidants they are thought to improve bird's health, and the resulting bright feathers signal to female birds that males are healthy, have less parasites and a good diet.But until now, scientists have not examined whether there is a downside to eating lots of carotenoids.

To do so, Professor Kristen Navara of the University of Georgia in Athens, US and colleagues at Auburn University, Alabama studied what happened to American goldfinch birds fed a diet rich in carotenoids.

Over two consecutive seasons, they fed wild caught goldfinches a high carotenoid diet for two months, followed by a normal diet for two months.

A control group of birds was consistently fed a diet low in carotenoids.

During the experiments, the researchers collected feathers from the birds to measure how much carotenoid pigment was taken up into the bird's plumage.

In the first year, they also tested for levels of an enzyme that might indicate muscle is being broken down in the birds.

In the second year, they followed this up by directly testing the bird's ability to fly by measuring the performance of the bird's flight muscles.

Muscle wasters

The results were clear.

Birds fed carotenoid supplements were significantly more colourful, having more strikingly yellow feathers.

However, birds fed this high-carotenoid diet also produced high levels of muscle-wasting enzymes, as the carotenoids became toxic, causing tissue damage.

They also performed less well during flight tests.

"The impairments were long-term and occurred two months after carotenoid supplementation had stopped," Prof Navara told.

"In a natural situation, this time period would correspond with the breeding season for male goldfinches."Impaired muscle performance during this time could decrease reproductive output overall."
That means only birds in good enough condition to tolerate these negative effects will take on high levels of carotenoids, and hence have the showiest feathers.

"So when females choose males with bright colouration, they are choosing ones in good enough condition to withstand high levels of carotenoids," says Prof Navara.

"Until now high levels of carotenoids have been regarded as beneficial to songbirds, and out study suggests that other potentially detrimental effects need to be tested in other species to get a full picture."

Climate change 'makes birds shrink' in North America

Songbirds in the US are getting smaller, and climate change is suspected as the cause.
A study of almost half a million birds, belonging to over 100 species, shows that many are gradually becoming lighter and growing shorter wings.
This shrinkage has occurred within just half a century, with the birds thought to be evolving into a smaller size in response to warmer temperatures.
However, there is little evidence that the change is harmful to the birds.
Details of the discovery are published in the journal Oikos.In biology, there is a general rule of thumb that animals tend to become smaller in warmer climates: an idea known as Bergman's Rule.
Usually this trend can be seen among animal species that live over a range of latitude or altitude, with individuals living at more northern latitudes or higher up cooler mountains being slightly larger than those below, for example.
Quite why this happens is not clear, but it prompted one group of scientists to ask the question: would animals respond in the same way to climate change?
To find out, Dr Josh Van Buskirk of the University of Zurich, Switzerland and colleagues Mr Robert Mulvihill and Mr Robert Leberman of the Carnegie Museum of Natural History in Rector, Pennsylvania, US decided to evaluate the sizes of hundreds of thousands of birds that pass through the Carnegie Museum's Powdermill ringing station, also in Pennsylvania.They examined the records of 486,000 individual birds that had been caught and measured at the ringing station from 1961 to 2007.
These birds belonged to 102 species, arriving over different seasons. Each was weighed. It also had the length of its wings measured, recorded as wing chord length, or the distance between the bird's wrist to the tip of the longest primary feather.
Their sample included local resident bird species, overwintering species, and even long distance migrants arriving from the Neotropics.
What they found was striking.
Of 83 species caught during spring migration, 60 have become smaller over the 46 year study period, weighing less and having shorter wings.
Of the 75 species migrating in autumn, 66 have become smaller.In summer, 51 of 65 breeding species have similarly reduced in size, as have 20 out of 26 wintering species.
The differences in size are not big.
"On average, the decline in mass of spring migrants over the 46 year study was just 1.3%," says Dr Buskirk.
"For a 10g warbler that's a loss of just 130mg."
But some species are losing more weight.
For example, the rose-breasted grosbeak has declined in mass by about 4%, while the Kentucky warbler has dropped 3.3% in weight and the scarlet tanager 2.3%.
The trend is particularly noticeable among those birds that winter in the New World tropics of the Caribbean, Central America and South America.
"The headline finding is that the body sizes of many species of North American birds, mostly songbirds, are gradually becoming smaller," says Dr Buskirk.
However, their populations are not dwindling."So many of these species are apparently doing just fine, but the individual birds are becoming gradually smaller nonetheless," says Dr Buskirk.
That suggests that bird species in North America are obeying Berman's rule, by evolving into a smaller size as temperatures increase.
Though this change appears quick, it has taken place over at least 20 generations of birds.
"There are plenty examples of rapid contemporary evolution over much shorter time periods," says Dr Buskirk.
Whether the trend will cause the birds any long-term consequences is unclear.
"In one obvious sense, the consequences are positive," says Dr Buskirk.
"That is, as temperatures become warmer, the optimal body size is becoming smaller."
However, even though the species appear to be adapting to the new climatic conditions, it could still be that their average "fitness" in evolutionary terms, is going down."Evidence from other studies is that some species will benefit and others will be harmed, and it's not always the species we like that will be harmed," says Dr Buskirk.
The jury is still out as to why any species responds to warmer temperatures by becoming smaller.
Originally, biologists proposed that having a larger body surface to volume might help in warmer climates.
But more recent ideas suggest that animals might actually be responding instead to something else that correlates with temperature, such as the availability of food, or metabolic rate.
"It looks like it might take a while before we know," says Dr Buskirk.
His team says much more data is now needed to confirm this trend and to see if it is happening in animals other than birds.
For example, it took an avalanche of data before people became convinced that climate change is already altering when birds start migrating.