Do you ever see flocks of birds in your yard that show up in droves one year, but are completely absent the next? Some nomadic species such as Pine Siskins move based on the availability of food and habitat. It’s called “irruptive” migration, and it sometimes leads to backyards full of siskins. While these flocks are a delight for bird watchers, the dense groups can easily transmit diseases such as salmonellosis.More info and transcript at BirdNote.org. Want more BirdNote? Subscribe to our weekly newsletter. Sign up for BirdNote+ to get ad-free listening and other perks. BirdNote is a nonprofit. Your tax-deductible gift makes these shows possible.