Blog #360 Plastic Signals

Young female black kite and chick with a small amount of white plastic in nest, courtesy of F. Sergio. 

Plastic debris is ubiquitous in bird nests worldwide with functions we are only beginning  to understand. Often thought of as silly nest decorations and a sign of urbanization,  plastic debris, especially plastic grocery bags, are a commonly found in the large nests 

The decorated nest of an 11-year-old black kite, courtesy of F. Sergio. 

of crows and hawks like Ospreys. In Spain, white grocery bags have a special,  defensive meaning to black kites (Milvus migrans); it’s a warning to intruders that the nest is fiercely defended. 

Spanish black kites collect white plastic bags by the dozen for territorial defense. The  birds shred the bags into pieces and wedge them into their nests. A breeding pair will start scavenging for the bags about 20 days before laying eggs. Studies have shown that this conspicuous display of plastic warns rival kites not to venture closer into the  

territory. And the more plastic in the nest, the fiercer the professed territorial defense will  be. A nest filled with shiny white, easy-to-see plastic serves as a reliable indicator that  its owner means business.  

Kites, like all birds of prey, are well-armed with deadly talons and combat between black  kites can get quite bloody. Contesting birds will lock talons in midair, twisting,  screaming, and struggling as they plunge toward the ground. Exogenous signals of  strength, such as the white plastic in nests, become important when trying to avoid such  fights. The kite uses its nest to signal just as a male red-winged blackbird indicates his  intentions with his bright, red epaulets or shoulder feathers. 

Amount of plastic in nests built by black kites ages 3, 11, 22 years (left to right), courtesy of F. Sergio. 

In a study of 127 black kite nests, the most white plastic was used by birds 7-12 years  old—the strongest birds, in top physical shape, and in their reproductive prime, able to  fend off attackers that coveted their territory. These are the birds to stay away from.  Younger birds and elderly birds did not use as much plastic, if any, as a territorial signal,  as they are too weak to repel raiders. When the researchers added extra plastic to the nests that had none, the birds took the plastic out, apparently not wanting to send false  signals and fake their toughness. Preference tests with shreds of white, green, and 

transparent plastic shopping bags revealed a clear preference for bright and visible  white bags over the less conspicuous bags.