It’s World Seabird Day today, so let’s see what is happening at our gull colony.
Back in the late 60s, the colony of Lesser black-backed and herring gulls at South Walney was the biggest in Europe, with about 45,000 pairs of these gorgeous characterful birds nesting. When my colleague and I completed this year’s transect back at the end of May, there were just over 3,500 pairs. This is a huge decline, but actually a slight improvement on last year, however that could just be stats as we count quadrats and multiply up, and I had readjusted some of the quadrat locations.
Still, we would like to not get any smaller, and I hope through predator deterrence we could increase and be stable at about 5,000 pairs, possibly recruiting urban gulls driven out from nearby towns.
Ideally, gulls lay 3 eggs, and most can usually get at least one fledged, if all goes well; sometimes if all goes very well they get two away, I don’t think they ever manage to raise all three. When the colony was at its height, it was estimated to have a productivity rate of 0.7 chicks fledged for each pair. When predation became a problem (I am told foxes only arrived on Walney in the 1990s) this fell to about 0.2 or 0.3 chicks fledged per pair.
We have electric fences, which now are mains powered and can be maintained at over 7,000 volts – a nasty shock to deter even hungry foxes. The colony was also incredibly dense, with nests at the centre only a foot away from each other, and this density of gulls in a colony can easily drive away aerial predators, such as magpies or kestrels.
So we were confident that we might have an exceptionally good year – except by mid June, it was clear that something had gone horribly wrong. Instead of thousands of fluffy gull chicks, there just didn’t seem to be any at all. As the end of June was reached, the BTO and Natural England arrived hoping to ring several hundred chicks; they found only 9 big enough to ring. It was devastating but seemed that the colony had failed.
We found 9 fresh dead chicks and one dead adult, and took them to the Animal & Plant Health Agency’s lab for a post-mortem. I had seen a colony collapse like this before, working a few years ago at a gull colony in the north of Cumbria, also managed by Cumbria Wildlife Trust, which has failed for nearly ten years running. Post-mortem there revealed that the chicks were being pecked to death by adults, who were presumably stressed, although it’s difficult to say why. The gulls nesting on a nearby industrial estate had no problem fledging young.
At that site, it was difficult getting any further. Here at South Walney, however, we are very lucky that we have hosted gull research for decades, and despite the dwindling numbers, it’s still a nationally important site for these birds.
The post-mortem of the gull chicks showed that all 9 had starved; their stomachs were empty. Conversely, the adult bird was in good condition and had actually eaten itself to death gorging on earthworms. I emailed these results to the BTO researchers working at South Walney this year, and also to our regional Natural England contact, expecting just commiseration. Instead, a proper investigation into what has happened is being driven forward – why have none of the gulls gotten enough food? Has a food source collapsed somewhere?
This really highlights how our noisy neighbours can act as an “early warning” for a marine ecosystem that’s so often out of sight. Has a mussel bank disappeared? Has a fish population crashed? This morning myself and two colleagues went back to the colony, to count chicks and collect food pellets.
After a walk through of the whole colony, we counted just 53 chicks, all about the same age – 3ish weeks – and no fresh corpses, which suggests that all the other chicks died at roughly the same time, but that the chicks alive now should hopefully make it as the parent birds were clearly finding food somewhere.
Like owls and other birds of prey, gulls eat food whole and then regurgitate anything they can’t digest in a tightly packed pellet. We collected 38 pellets this morning, and dissected them to see what the gulls had been eating.
This isn’t a robust method, because the gulls will eat lots of soft things that won’t be represented in a pellet. But it gives us some idea. By far, the majority of pellets contained a fair bit of vegetable matter, and plastic, sadly typical. There was a low percentage of mussels, but that’s because mussel shells don’t form into pellets (they just sort of scatter everywhere) so I hadn’t collected many. We found the remains of a small rodent, and beetles, but what we didn’t find were very many fish bones or scales.
Next, I want to go back and collect more pellets, and make more note of how many piles of mussel shells there are. Because the BTO have been tagging birds at South Walney with GPS trackers, we know where they are going to feed – the next step would be physically going there ourselves and see what is going on.