Zoological facilities strive to continuously improve the welfare of animals under their professional care. This requires staff to utilize positive and negative indicators of animal welfare to monitor an animal’s current state. Brookfield Zoo Chicago currently has over 500 species, many of which are quite rare, and additional research is needed to better understand what indicators of welfare are best for each species.

Animal Welfare Science staff recently completed a project examining sex bias as a potential indicator of animal welfare for birds. Typically, species will produce offspring at an equal sex ratio of 1:1, meaning on average there is one male for every female. However, research would suggest that if that ratio is skewed with more males or more females than there may be some type of concern surrounding animal welfare.

Research has shown that corticosterone, a hormone produced in response to a perceived or actual threat, can skew the ratio of hatchlings in birds. Depending on when the hormone is produced it can lead to either a skew in males or females. For this project, we analyzed a large dataset containing records for over 1 million zoo-housed birds. The sex ratios of offspring from 19,867 zoo-hatched females were calculated to determine if species ratios differed from what would be expected. Only one species had a significantly biased offspring sex ratio. Results would suggest that birds within zoological facilities have good welfare and thrive under human care.


Lance J. Miller, Ph.D.,
Vice President of Conservation Science and Animal Welfare Research
Published April 29, 2024