Vultures thought to be nearly silent use 16 kinds of calls, and their chicks call from inside the egg

Vultures have a reputation as quiet birds, but Rüppell's vultures turn out to have a rich vocal repertoire, Utrecht University said on 21 October. Researchers recorded about 2,500 calls in roughly 150 hours at two colonies in Dutch zoos and identified 16 distinct call types. The study was published in PLOS One.

Calls that show how excited a bird is

The calls differ in length, pitch and number of syllables, and some blend into one another, which the authors describe as a graded repertoire. That lets the birds “convey not just that they are excited, but also how excited they are,” said Connor Proudfoot, the study's first author.

The recordings were made at Diergaarde Blijdorp in Rotterdam and Avifauna in Alphen aan den Rijn. Because eggs at Blijdorp are incubated artificially, the team could also record eggs and chicks.

Chicks call two days before they hatch

Chicks began calling from inside the egg about two days before hatching. In the wild, parents would hear those calls; when eggs are incubated in a machine, they do not, which could leave the adults less prepared for a chick's arrival, the researchers said.

“Perhaps more chicks could be successfully reared if their parents could hear those sounds.” — Connor Proudfoot, Utrecht University

The team suggests that playing recorded vulture calls through speakers could help stimulate breeding, and that letting parents hear chick calls might improve rearing success in captive breeding programs.

A Critically Endangered species

Those breeding programs matter because the species is in serious trouble. Rüppell's vulture is listed as Critically Endangered, according to The Peregrine Fund. It ranges across the Sahel and East Africa, and one of the main threats is poisoning, when livestock owners lace carcasses with pesticide to kill lions and hyenas, or poachers poison carcasses so circling vultures do not give away their kills.

Zoos are taking on a growing role in vulture conservation. At Prague Zoo, two Egyptian vultures rescued from illegal captivity raised a chick at Prague Zoo. The Utrecht team has not yet tested whether playing calls actually improves breeding.