Nazca boobies are ground-nesting colonial seabirds that exhibit unique pseudomothering behaviour.
In any given year, there are reproductive females whose nests have failed. While busy parents are off foraging for food for their nestlings, which can take a long time, depending on the availability of resources in a specific year, these females, and sometimes males too, pay “visits” to unattended chicks.
These visits can last for varied amounts of time, from one minute to over an hour.
Newly hatched chicks and almost-fledged chicks are not visited by these pseudomoms because biological moms almost always guard new hatchlings, and chicks that are ready to leave the nest are large and strong enough to defend themselves.
It’s the chicks that are in the middle that receive the visits, they are the “goldilocks” age for vulnerability – too young to defend themselves but too old for their mothers to remain at their side.
During pseudomom visits, chicks assume a submissive posture with their heads inclined forward and the dorsal surface of their beaks on the ground.
Some of these visits are friendly and peaceful. Females will stand beside or preen young chicks or, in some cases, even provide them with gifts of pebbles or feathers.
Unfortunately, the bulk of the visits involve aggression by the visitor, who will scratch or bite the chicks, leaving them with bloody wounds on their necks.
These wounds aren’t fatal, but they serve to attract the attention of blood-sucking ectoparasitic birds like finches or mockingbirds. These parasites feed from the cuts, which deepens them significantly – often leading to the death of the chicks.
This strange behaviour is very common in Nazca booby colonies. It’s observed across different populations in different areas, and therefore cannot simply be dismissed as erratic behaviour of a few females.
Up to 25% of booby chicks die as a direct or indirect result of these unwanted visits.
This post is an adapted excerpt from my book “Wild Moms: Motherhood in the Animal Kingdom“