Latest News - The cat-ematics of colony TNVR (Trap Neuter Vaccinate Return) to protect Scottish wildcats

The cat-ematics of colony TNVR (Trap Neuter Vaccinate Return) to protect Scottish wildcats


In November last year, I had a request to TNVR a colony of ten cats near Boat of Garten. Most were young females of breeding age, which got me thinking: if we hadn’t neutered them, how many cats would there be for me to TNVR this time next year?

Let’s start with a bit of background on colony TNVR. Domestic cats (like most cat species) are solitary by nature. Cat colonies generally arise where you have a reliable food source, such as a constant supply of small rodents around a farm, or where they are fed by humans. Colony TNVR requires a bit more planning than the odd individual cat, because you need both the trapping capacity and the veterinary capacity to get them all done at once, or you risk missing the stragglers. So how do you set about it?

We start with some background information. The lady who called us in this instance was able to provide both good information about the cats and practical assistance with trapping. She was confident there were ten cats, and so we setup ten cage traps in the garden (in sheltered and flat areas) and asked her to pre-bait the traps with their normal food for a week or two until we had the cats reliably feeding inside. Pre-baiting is not essential for individual cats but is a good strategy for colonies, because individual cats will vary markedly in their propensity to go inside a trap, and the longer you pre-bait the more likely you are to get even the most trap-shy cats feeding at the back of the cage (necessary for them to stand on the treadle and trigger the door to close).

The closest vet practice to Boat of Garten is Strathspey Vets in Grantown-on-Spey, and despite limited capacity, they did their very best to accommodate the trapping. They could not possibly take ten cats at once, but they did agree to take a maximum of five cats per day on two consecutive days, which would allow us to trap five cats on the first night, and then keep them inside overnight to recover while we set traps for the remaining five. I had a look at the weather, and with some warmer and hopefully dry (albeit windy) weather on the way, we booked them in.

With everything planned, the last thing to do was call in some expert volunteer help! I was very lucky to have Margaret, a retired vet, volunteering with me in Strathspey last winter and she generously offered to help with the TNVR effort.

On the afternoon of the trapping day, I went down to Boat of Garten to set the traps ready for the night ahead. Understandably, the lady who was feeding the cats was not confident about setting the triggers (it can be a little fiddly), so I decided to set all the traps ready with food inside and then close the zips on the covers (see photo). That way, all she had to do was unzip the covers at 10pm at night when the cats were normally fed, which also meant that the cats would be in the traps for as short a time as possible.

When I returned with Margaret at 7.30am the next day we were excited to have five cats waiting! We actually had two cats in one trap, which I have not encountered before: the dad and one of the juvenile cats. We quickly transferred the cats to an individual ‘crush’ cage (so called because they have red bars that you can pull across from outside the cage to hold the cat still for injection with anaesthetic) and covered them up. I was expecting typical ‘feral’ behaviour but they were all quite calm, most meowing from confusion rather than fear. We got them off to the vets and readied all the traps for night two.

All five of the first cats tested negative for any disease, which was a huge relief because colony cats seem more likely to test positive for common feline diseases like cat flu, or even worse, Feline Immunodeficiency Virus (FIV) or Feline Leukaemia (FeLV). They were at the vets all day because 4-5 of the cats were female, so the operations took a while.

I collected them later that evening and took them nearby to Margaret’s house, where she had kindly offered to keep them overnight before release first thing in the morning. We tied a second crush cage onto the front of each one, and placed food, water and a heatpad inside, so each cat could be warm and fed. We then placed them all side-by-side so they could see each other (they were family, after all) and stood/covered all the cages with cardboard and blankets for further insulation. I then went off to set the remaining five traps for night two.

We checked on them first thing: everybody was fine, and all the food was gone.

We returned them to their garden shelter by 7.30am, only to find we had been very successful again and caught all five of the remaining cats. We let the first five go, and then repeated everything from the day before. These cats were noticeably a little more ‘feral’ than the first five, and there was a little more hissing and spitting during the cage transfers.

A cat being checked by a vet for disease, fleas and ticks before being neutered, vaccinated and returned.

We delivered them off to the vet first thing (crossing all fingers for more male than female cats!) but there was some bad news this time. One of the male cats had to be euthanized for health reasons, leaving one other male and 3 more females to be returned. They were kept in overnight as before, and released first thing the next morning.

Apart from the one individual that had to be put down, we were pleased with the success of this joint trapping effort. But the number of female cats had certainly made it quite challenging, especially for the vets: 7/10 cats were female. That actually makes sense, because the cats had reached dispersal age, and male cats are more likely than females to disperse and find a new territory. But it did make me wonder: how many cats would 7 female cats have produced by this time next year?


To estimate our cat colony size this time next year, we need to make a few assumptions and we need some basic cat biology. I have taken the following stats from Cats Protection’s very useful and informative website

A few facts about cat reproduction:

• Cats can become sexually mature from 4 months of age.
• Cats will mate with relatives (yes: brothers and sisters, mothers and offspring: they aren’t picky).
• The peak breeding season typically runs from February to August [but domestic cats can potentially breed at any time of the year].
• Cats can have 1-9 kittens in a litter but usually between 4 and 6.
• Pregnancy lasts approximately 9 weeks.
• Cats can potentially have between 1-3 litters per year.

If we start with 7 female cats, and assume they all get pregnant between November and February, and assume an average litter size of 5 kittens that gives us 35 new cats in the garden by mid-April (plus the 10 that were already there) = a total of 45 cats. We probably need to factor in some mortality: the juvenile females may not all survive the winter, and then they may not survive the pregnancy/birthing process, although they are being fed so mortality will be lower than for truly wild-living feral cats. Let’s assume a bad winter, and 3/7 females don’t make it to reproduce in April: that reduces our first estimate to a more conservative 27 cats (4 x 5 = 20 plus the 7 original cats that remain). If we assume an even sex ratio, then half of the kittens born to the 4 female cats will be male, which leaves us with ten new female cats. Some of those kittens won’t make it to reproductive age, which again reduces the number by a few (let’s say we lose three). By fairly conservative estimates, we have around 14 female cats by August who are all ready to breed again (the 7 new ones plus the 7 from the previous year onto their second litter). If all 14 females have a litter of 5 kittens that would be 70 kittens by October.... and a lot more traps required by November 2019.

Obviously, this is pretty ‘back of the envelope’ cat-ematics. We have not included migration of cats from other surrounding colonies, or the impact of the additional male kittens. The upward trend would not continue forever, because as the number of cats grew, the resources available would be lower per individual cat, and the health and condition of those individual cats would lessen, and mortality would increase while reproduction would decrease. But it does help to illustrate just how quickly a cat colony can get out of hand, and how early intervention and neutering can leave behind a stable and healthy little colony of cats. These cats have also been vaccinated, so will be less likely to contract some common diseases and to spread those further amongst the wild-living cat population.

What could have improved the TNVR of the colony above is if we had been called in sooner, when the juvenile cats were still young kittens. Kittens can be socialised and rehomed if they are caught and taken in to care before 7-8 weeks of age. If you see young kittens, don’t wait! Ask for help now, and those kittens have a chance of a loving home instead of a lifetime outdoors.

If you have some wild-living cats that need neutering:

Hopefully the cat-ematics has shown how quickly a few cats can escalate into something unmanageable. This is the scenario many people find themselves in, and sometimes they do not ask for help for all kinds of reasons, but a key one can be the feelings of shame or embarrassment that they ‘let it get to this state’, particularly when they care about the cats, and their welfare was what motivated feeding a few cats in the first place. But you shouldn’t worry about that, because the people that will come to help you understand how these things happen! So contact us for help with TNVR if you live within a Wildcat Priority Area, or contact your local branch of Cats Protection, or the SSPCA, if you live elsewhere.

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Image for Dr Keri Langridge

Keri is a research scientist with PhD in Animal Behaviour. She is our Monitoring Officer & also responsible for Northern Strathspey & Morvern Priority Areas. Keri has extensive experience working as an ecological consultant in the Highlands and has also worked as Education Officer for Cats Protection.

email  07920 181949

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