The below graph shows a reduction of catch in December compared to November. This was due to fewer ship rats and weasels being caught. Perhaps our baiting programme is starting to work – with weasels being secondarily poisoned? Hedgehog, stoat ,and non-specified rat numbers increased a little. Unfortunately, the January catch, shown here to the 8th, is on track to easily exceed that of November, so the December reduction might not be significant. We have not caught a feral cat since October last year. Geoff has started cutting out the backs of Timms traps and replacing them with mesh; this hopefully will make these traps more successful.
The next graph shows that catch from the two colony lines – 191 and 192 – has declined from the previous month and is now comparable to that from the permanent riverside lines. (If something looks incorrect, please let me know). Unexpectedly Lines 193 and 194 (at a black-billed gull and black fronted tern colony and BFT colony respectively) still haven’t caught a single predator. Lines 191 and 192 are at a BFT colony and BFT & BBG colony respectively. The only predators caught out on the gravel adjacent to nests are still Norway rats, and these currently seem to be the main land-based predator danger to nesting birds – especially BFT and banded dotterel. There appears to have been zero BFT fledgling success from the Railway colony (about 50 nests), with more than half of the nests being predated before hatching, and chicks disappearing at an early age. There have been only 2 -3 fledgling BFTs seen at the Thomas/G9 colony. The G4 colony looks like being more successful – very likely because of lack of predators. The Toppings BFT colony was very small with only a few nests found. Some eggs hatched, but chicks haven’t been seen lately.
A few young gulls have perhaps also been caught by Norway rats at the Railway colony, evidenced by piles of feathers near the colony. The main predation here, though, has been by harriers. More than 20 piles of fledgling gull feathers (see below) have been found on the southern river bank so far. This could well increase to more than 50 as it will be some time before all the chicks can fly away. Nick has captured an image of a harrier carrying a gull on a trail camera. As with last year, the main danger to the gull chicks seems to be when fewer adults are around to protect them from harriers. This predation is of course a natural process.
Nick has captured an image of a harrier carrying a gull on a trail camera. As with last year, the main danger to the gull chicks seems to be when fewer adults are around to protect them from harriers. This predation is of course a natural process.
The following map shows where the various types of rat have been caught since August last year, when we first started distinguishing the different species. Pie chart size represents the number of rats caught in a trap with the largest catch being 5 and the smallest 1. The colony lines 191 and 192 out in the gravel have caught by far the most Norway rats. However, 13 in total have been caught along the south bank, generally in traps very close to the river. Some of these traps have caught both Norway and Ship rats. By contrast only 2 Norway rats have been caught on the north bank, on Lines M and E. The Norway rat on Line E was very close to the colony. Perhaps the south bank is a source of Norway rats; we maybe should be increasing our baiting efforts there.
Catch in December continued to increase with a total of 22 predators compared to 17 in November. In December 2018 total catch was also 17, however Lines I and J were only installed in January 2019.
Rats were once again the main contributor but this month Ship rats outnumbered Norway rats. Lines A and B failed to catch a predator in December. A hedgehog was caught on Line G – the first since May 2019 and only the 10th caught since trapping began. No cats have been caught since August 2019.
The following map (the smallest pie chart represents 1 rat per trap, the largest 3) shows that Norway rats have essentially been caught in the period August 2019 to 8 January 2020 only along both banks of the river above where the river meets the estuary. It appears that the river margin is more of a habitat for them than the estuary margin. Ship rat distribution was more widespread. More bait stations are needed, especially on Lines I and J.
– Grant Davey, 09 January, 2020