Watching a foster pet leave the nest can be rewarding, but you probably are going to cry your eyes out.

A foster home is a place where pets get a chance to heal or come of age or learn how to be social. It leaves room at the shelter for another dog or cat. But knowing you are raising and training a pet for someone else can be equal parts gratification and heartbreak.

Beth Stern, spokeswoman for the North Shore Animal League America, has fostered several litters of cats.

She and husband, talk show host and "America's Got Talent" judge Howard Stern, had planned to take only one litter.

"My husband and I had a ball. He named every one of them, photographed them and started talking about them on his radio show," Stern said. She shared photos and stories about them on Facebook and Twitter and on North Shore's website. Adoption applications started arriving at North Shore in bunches.

When the day came for the new owners to be notified, and the kittens returned to the shelter to be spayed and neutered, Stern said it "was one of the hardest days of my life." But while she said goodbye to six kittens, she ultimately said hello to 10 new ones.

The American Society for the Prevention of Cruelty to Animals makes more than 1,000 foster placements a year, said Gail Buchwald, the ASPCA's senior vice president of adoptions. Fosters are an indispensable component for shelters, but there's a lot of turnover, she said, because of so-called foster failures - people who fall in love with their foster pets and adopt them.

When a foster becomes an adopter, no one minds, Buchwald said, but it does leave shelters on a constant lookout for new fosters. Without foster homes, more animals will have to be euthanized.