How does a pair of Lesser Black-back Gulls end up with 6 chicks to take care of, knowing that the standard clutch size is 3 eggs with a maximum of 4?
That is the question that we asked ourselves when we observed this scene in the gull colony of IJmuiden Forteiland, the Netherlands, 11 June 2018:
As can be seen in the video, the male and female Lesser Black-backed Gull are clearly looking after 6 chicks.
What is striking is that they are all of roughly the same age: probably a week or so old. They very much behaved as a family with the chicks actively following both adults and begging them for food.
During the few hours that we observed them, they were being fed with a predated chick while a second chick was predated and prepared to be fed to the chicks later.
At the end of the above video, the adult is regurgitating a predated chick (unfortunately this is obscured by the gull standing in the foreground).
Here you can see the chicks feeding on the dead chick:
(Note that in the image above, the 7th chick in the foreground on the right is not part of the family.)
Image above: one of the chicks trying to swallow the legs of the predated chick.
One of the 6 chicks was smaller than the others and also very weak: it was laying down most of the time and was much less active than the other five. It didn’t get to eat much and at one point one of the adults was showing a lot of interest in it, almost as if it had plans to eat it:
Here’s another seen, taken from a different angle:
The parenting behavior did not seem to have changed: one of the adults tried to take all 6 of them under the wing for some shade, but with so many chicks this is not easy:
Here, a second predated chick is being prepared to be fed (this video contains no audio):
We had never come across this before, nor were we familiar of any such sightings elsewhere.
After asking around, it turns out that this is very likely a case of adoption in which chicks that are abandoned or that are left alone (perhaps because one parent has died and the other is away), somehow end up being accepted by another pair.
It could even be possible that these chicks return to their parent(s) when they see them return to the colony.
Many thanks to gull researchers Eric Stienen (INBO) and Kees Camphuysen (NIOZ) for providing us with this information.
A quick search on the Internet showed that adoption is indeed not unheard of in gull species.
Here a reference to Common Gulls:
“In common gull colonies on islands of the Vistula River, Poland, adoption of chicks is common. In 1997, we observed 81 chicks from 35 nests.“
I also found a reference to Herring Gulls in Scotland that adopted chicks:
“Adoption of young by unrelated adults is expected to be rare in natural populations, yet a high incidence was observed in a population of Herring Gulls on the Isle of May, Scotland.“
And here is a reference to adoption in Ring-billed Gulls:
“Of 47 ring-billed gull,Larus delawarensis, chicks that departed their natal broods over four summers of field work, 34 remained in recipient broods for 3 or more days and received parental care (i.e. were successfully adopted), and 13 remained in recipient broods for less than 3 days (‘runners’).”
(More references are welcome. Any updates will be added here.)
What will happen to this strange family? Will the adopted chicks return to their own parents? Will they all survive? Will the current parents be able to keep feeding all of them (the colony won’t have bite-size chicks to predate for long…)?
It will be interesting to see what happens over the coming week or weeks…