After many domestic rabbits starved and the remaining ones were eradicated by biologists in 1923, the ducks began to recover, increasing to a population of about 500 individuals by the 1950s. Congress passed the Endangered Species Preservation Act in 1966, and in 1967 the Laysan duck was declared an endangered species with federal protection. However, population bottlenecks occurred, such as the severe 1993 El Niño drought and food shortage, which reduced the number of ducks to about 100. Today, breeding and survival of the birds are closely tracked. Since 1998, a sample of birds have been fitted with unique leg bands or radio transmitters for monitoring reproductive success and survival, revealing that typically only 30% of ducklings on Laysan Island survive to fledge. In 2004, the population grew to an estimated 576 ducks.