Two Black Headed Gulls (IMAGE) University of Bristol Caption Photos of two black headed gulls viewed against the clear sky showing how when viewed in intensity (ie brightness) [left] the bird can appear either lighter or darker than the background but when viewed in polarisation [right] the contrast is always the same thus providing a more reliable source of information Credit Sam Smithers, University of Bristol Usage Restrictions The image is for single use, should be credited and should not be archived License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.