Robin's nest with a brown-headed cowbird egg

Freeloaders of the avian world

Ever felt like some folks are just born jerks? Birds can relate. Take for example the uninvited speckly-white visitor to this robin's nest: That smaller egg belongs to a baby cowbird whose mother sneakily laid it among the blue robin eggs when no one was looking. Once the baby birds all hatch, the cowbird will grow much faster and larger than its robin 'siblings,' soon becoming an only chick by muscling them right out of the nest. And if mama robin ejects the cowbird egg before it hatches, the cowbird mom may take notice and chuck the remaining robin eggs from the nest out of spite.

Don't direct all your righteous ire at the cowbird, though—plenty of critters get by on underhanded tactics (even if you don't count humans!). Check out today's quiz on some of the animal kingdom's other pesky parasites.