Why do foxes kill all the chickens in a hen-house?

A Fox with a Magpie. Photo Credit: Tony McLean

 

The red fox (Vulpes vulpes) is a highly adaptable species, found across Britain, but absent from Scottish Islands (except Skye), in all habitats from salt marshes and sand dunes to the tops of mountains. In Britain, more so than elsewhere in Europe, foxes have also adapted to life in urban surroundings.

The red fox has red-orange fur over much of its body and thick bushy tail with, white fur on the neck and belly and brown/black legs, an average male measures between 67-72 cm long and females 62-67cm; the tail is about 40cm. Males weigh about 6-7 kg for and females 5-6kg. Despite its name, the species often produces individuals with other colourings, including leucistic and melanistic individuals.

The fox feeds on small rodents, though it may also target rabbits, game birds, reptiles, invertebrates and young sheep and deer. On salt marshes they eat crabs and dead seabirds, while in upland regions carrion may be important, particularly during the winter months. In lowland rural areas small mammals, especially field voles and rabbits, are the major source of food, with earthworms, beetles, fruit (particularly blackberries) and small birds also being eaten. Urban foxes glean large amounts of foo by scavenging from dustbins, bird tables and compost heaps. Those living in some urban areas eat many small birds and feral pigeons.

Foxes have been recorded as living up to 9 years in the wild, but most survive only one to three years.

Foxes are territorial. The size of a fox’s territory depends on habitat; they can be as small as 0.2 square kilometres in urban areas or up to 40 square kilometres in hill country. Each territory is occupied by a fox family group. These often consist of a pair (dog fox and vixen) and their cubs. However, in areas where foxes are not persecuted and where there is a plentiful supply of food, a family group may contain several adults. The young of the mated pair remain with their parents to assist in caring for new cubs.

Usually only one vixen in a group produces cubs, once a year, in the spring. Litters average four to five cubs which are born blind and deaf in a den (called an earth). The earth may be dug by the foxes, or they may enlarge a rabbit burrow or use holes made by other animals. In urban areas, cubs are often born under garden sheds. A vixen stays in the earth with her cubs for the first two weeks of their lives. At about four weeks old, usually in late April or early May, cubs begin to come into the open.

Foxes are not protected legally. For many years they were hunted for their fur, and as part of countryside tradition. The Hunting Act 2004 outlawed hunting with dogs in England and Wales, from 18th February 2005. This also applies to the hunting of deer, hares and mink.

 

Surplus killing, excessive killing, henhouse syndrome, or overkill

A highly controversial aspect of fox behaviour is that they are often observed to kill more prey animals than they consume at the same time, this has resulted in their being labelled cruel or greedy or even evil.

However, the results of a study by the EU, “Global food losses and food waste: Extent, causes and prevention” (2019) suggest roughly one-third of human food produce is lost or wasted globally, amounting to about 1.3 billion tons per year, and yet we perversely profess that taking more than needed is wrong, and wasting it, worse.  We extend these standards, anthropomorphically,  to other species and so when we view the carnage of a chicken coop devastated by a fox’s killing spree with ‘wasted’ chicken corpses scattered around wen act as judge and jury condemning the fox for its actions.

Michael Chambers sums this up in his book Free Spirit (1990)

“… man spends so much time in his relationships with his fellows, and particularly with the animal kingdom, demanding that they behave in a manner he prescribes, and often killing them if they do not.”

In the role of defending advocate for wildlife, I would first demand recognition of the basic tenet: it is not a pre-requisite that wild creatures shall benefit man in order to qualify to be left unmolested.”

Predatory or prey drive

So, why do foxes kill more prey than they can consume? One suggested factor is predatory or prey drive.

Predatory drive is the instinctive inclination of a carnivore to find, pursue and capture prey. If, in responding to this drive, a fox gains entry to a coop (chicken wire was designed to keep chickens in, not to keep foxes out) this may result in a panic reaction with confined, excitable birds flapping and squawking, reinforcing the prey drive until the stimuli are removed i.e. all the birds are dead, resulting in what we term overkill

This is supported by reports where a fox has broken into a coop and taken only a single bird because the others have remained calm and quiet on shelves above, suggesting it is the noise and movement of live prey in the immediate vicinity that causes this overkill.

Having killed more birds than it can consume on the spot or carry away at once the fox is confronted with the dilemma of what to do with all the birds. One solution would be to abandon the excess bodies but as in the case of many predators the solution is to bury the leftovers for use later – a process we call caching.  It should be noted that this activity is not confined to predators e.g. squirrels

Caching can take one of two forms, either storing everything together in one place (larder caching), or burying individuals separately or in small clusters (scatter caching) in different locations. Larder Caching it makes it easier to recover everything at a later date, but at the same time if another animal discovers the hoard, the fox may lose everything.

Conversely, scatter caching minimises losses if the cache s discovered by another carnivore but the fox must remember the location of each cache. Foxes, it seems, show a general tendency towards scatter caching, although there are exceptions.

In his book “Free Spirit: A Brush with a Fox” (1990), Michael Chambers described how his hand-reared vixen, Ferdi, broke into the pantry while he and his wife were watching TV to demolish a packet of chocolate biscuits, or so they thought. Upon ascending the stairs, a curious crunching sound alerted them to the young vixen having cached the biscuits under the carpet running up the middle of the stairs; two per stair. Most biscuits were intact, suggesting the operation had been carried out with considerable care and precision, despite her never having been taught to cache food.

In “Running with the Fox” (1989) David Macdonald observes that caching tends to involve a fox digging a hole with its front paws, placing the object into it and then pushing the soil and vegetation on top with the snout.

Observations of wild and captive foxes have shown that the desire to cache food appears at about six weeks old and that the appearance of the cache varies with age (adults being more proficient at concealing a cache than cubs) and how hungry the fox is – satiated foxes tend to make rather haphazard caches. An item may be cached even when the fox is still hungry and it seems that preferred foods are more likely to be eaten on the spot, while less palatable morsels are cached. Various other factors, including season, age, social status and whether the fox thinks it’s being watched may also have an impact on caching behaviour.

During the spring and summer, when a vixen is rearing cubs, less palatable food items (that might normally have been ignored) are cached as an insurance policy.

David Macdonald also observed that when surrounded by other adults (during which squabbling was likely) foxes were more likely to cache food, including items that might otherwise have been avoided.

Generally foxes can carry only one chicken at a time, particularly if the entrance by which they gained access to the coop is narrow. If the fox removes a single bird, much can happen while it is gone – it may be killed, displaced from its territory, or simply delayed from returning, allowing time for the unfortunate owner to find and remove the dead bodies.Circumstances may conspire against the fox to prevent it from removing all, or even any, of the birds. Th ere is evidence to suggest, however, that under most circumstances the birds will be removed as illustrated by the zoologist Martin Hughes-Games who related an experience in 2010 (Autumnwatch) where a fox attacked his chicken coop and killed all the chickens leaving behind many corpses, Martin left the bodies to see what would happen and, over the course of several nights, all the bodies were recovered by a fox. We cannot be certain it was the same fox that came back for them all, but given that foxes are strongly territorial it seems probable.

There are observations to suggest that how neatly a fox caches food can depend on how hungry it is, with caches made by satiated foxes being more haphazard. If this is representative, perhaps the superabundance of easily-caught birds had in some way impaired the foxes’ caching response. Alternatively, with several hundred birds killed on some nights perhaps there were physically too many to handle, like being confronted by a very messy room and not knowing where to start to clean it.

The Dutch biologist Hans Kruuk, documented an example of surplus killing (Behaviour, 1964), where black-headed gulls killed by foxes at the Ravenglass reserve in Cumbria during the early 1960s were killed in large numbers and not cached

Kruuk found that although the foxes cached many of the eggs they took, they seldom cached gulls that they killed (typically burying fewer than 5%), which meant that nearly 1,500 adult gulls, and many more chicks, were killed and abandoned by the foxes during the three years of the study.

However, the birds were killed on nights when the weather conditions were very dark or stormy which caused the birds to remain on their nests – Kruuk and his co-workers reported that they could walk right up to the birds and pick them without the bird protesting. Consequently, even when one gull was killed there was no panicky prey-like behaviour from its neighbour to distract the fox and re-trigger its prey drive. The reason for this curious behaviour remains unexplained; but it is interesting to note that it only happened during very particular weather conditions, which seemed to cause the birds to lose their antipredator response and affect the behaviour of the foxes.

One of Kruuk’s co-workers, Nico Tinbergen, the zoologist and ethologist, considered that the prey drive may be more sensitive than described and may not even require the flapping panic. In his book” The Animal in its World (Explorations of an Ethologist, 1932-1972) (1974), Tinbergen explains:

“It is furthermore quite possible that a hunting animal such as the fox can hope to be successful (even on favourable nights) only if it possesses an unquenchable urge to kill on sight, to kill immediately whenever opportunity offers. It is quite possible that it cannot afford to curb its instant reaction towards ‘moving-prey-within-reach’ under any circumstances whatsoever.”

This plausibly explains the predation of so many gulls, but not help why Caching behaviour failed in this case. All we can say is that, based on the available evidence, the lack of any desire to cache adult birds seems to be aberrant behaviour that occurs very infrequently in the wild. Most foxes do cache surplus food, even from a very early age.

In “Free Spirit”, Michael Chambers noted how a rescued vixen, Ferdi would never cache food if his dogs were watching. If she left the garden with her prize and was followed by one of the dogs or cats she would return carrying it – only when she got away unseen would she return without the food.

This accords with observations of a group of foxes feeding in Guildford, West Sussex; subordinate animals (as determined by submissive behaviour towards others in the group) would enter the area, seize food and disappear into the darkness, returning a few seconds later for more. The period that the foxes were absent, apparently, was too brief to permit consumption of the items taken, suggesting that they were being cached close by. 

Broadly-speaking, foxes recover their caches within a day-or-so, although Nico Tinbergen (1965) examined how a fox Vulpes vulpes scatter hoarded gull eggs. A seaside colony of black-headed gulls Larus ridibundus was only accessible to the fox if it crossed sand dunes that surrounded the colony. By checking these dunes regularly for tracks, he could monitor the fox' visits to the gull colony over a whole summer. By following the fox' tracks, he could also find buried food such as gull eggs, dead gulls and rabbits. In order to examine the significance of scatter caching, he positioned 100 chicken eggs 10 m apart in a line across the dunes. He buried the eggs, just as the foxes, below 3 cm of sand. To control that his experimental manipulation or smell did not attract foxes, he also made 100 fake caches, with no egg buried. As fake caches were never examined by the foxes, he could show that the fox located cached eggs by their smell, not needing to remember exact caching positions. 

Tinbergen found that some of the gull eggs cached on his dune study site were recovered up to two months later. Studies on Arctic foxes in Russia suggest that snow goose eggs buried in the ground have lost only 8% of their nutritional value two months later, suggesting caching is a good strategy for such robust food items.

Stephen Harris, Professor of Environmental Sciences at the University of Bristol, has studied urban foxes for nearly half a century, found that most caches were excavated on the following night, with other studies showing almost all were recovered within a week of burial. 

One fox’s cache may occasionally be raided by another fox (or other species), although the ‘cacher’ is often very careful at concealing the cache site — even to the extent of walking backwards brushing paw prints away as they go —and David Macdonald found that other foxes were generally unsuccessful at finding caches that weren’t their own. 

Highly-prized food items may be cached more diligently than lower-value ones. At the same time, several naturalists have noted how well-fed foxes tend to be rather careless when it comes to achcing, leaving wings and legs protruding and sometimes not even bothering to cover the food! Interestingly, toxoplasmosis may also affect a fox's caching behaviour, reducing/removing scent-marking behaviour and making them more tolerant of cache pilfering.

Macdonald observed that foxes may dig up and relocate a cache if they think another is aware of its location and some particularly fascinating observations from naturalist and fox rehabilitator Kelly Kilfeather suggests that, foxes can be very ‘creative’ in order to maintain the security of their caches. Kelly described the behaviour observed between two foxes, a dominant male “Gus” and subordinate “Sage”, visiting her allotment. Gus made a habit of raiding caches made by Sage on a daily basis, within a couple of hours of it having been buried; however Kelly observed:

“… one day, Sage cached one of these treats in front of me… nice cooked sausage… and he spent ages and ages covering it and patting it over, until visibly you would have no idea it was there… and then he did something that shocked me. He went and took a bit of the tinned food, which was mixed with anchovy, so it really whiffed… and he took a tiny morsel of it, dug a hole 1ft from where the sausage was, and dropped it into the hole. Did a half-hearted nose push of the dirt to cover it, and then spent ages, rubbing his muzzle all over the ground, all around the hole and all around where the sausage was hidden…. and off he went. Gus came… sniffed around where the anchovy scent was, found the badly hidden morsel, ate it, and went off on his way… the sausage never got spotted.”

Gus had been deceived by Sage's dummy cache. Furthermore, Sage continued to make dummy caches everywhere. Thus resulted in Gus not only benefiting from these low value caches, but he became more tolerant of Sage’s presence

When it comes to relocating a cache, it seems that foxes rely heavily on their memory, rather than just remembering the rough area (based on landmarks) and then letting their nose lead them to the exact spot. 

Macdonald found that when he cached another mouse within three metres of his vixen’s cache she found only about 20% of them (but recovered 90% of her own caches), and when he dug up her cache and moved it one metre (3 ft) away she only found one-quarter of them. If she was relying on her nose for the last stages of the recovery she should surely have found both the additional and transplanted caches.

Several authors have noted how foxes not only seem to remember what is in a particular cache, but seldom return to an empty cache. It is possible that foxes remember which caches they have excavated and which they haven’t, in his book, Red Fox: The Catlike Canine (1986), the biologist J. David Henry, proposed an alternative explanation, suggesting that foxes urinate on caches once they’ve excavated the contents indicating the smell of urine means there’s no food left and it’s not worth wasting time digging – in other words, the foxes use this scent-marking as a kind of cache book-keeping system. Henry noted that, for the most part the system seemed to work, but in some cases the smell of food must have been so strong that the fox dug anyway.

In conclusion I must suggest that foxes are highly evolved and predators that are both finely attuned to their environment and capable of responding flexibly to it. Overkill is an established phenomenon but it is arguable that this is a response to unusual environmental conditions. By caging chickens we have made a significant change to this environment and where we have failed to ensure the chicken coops are secure we should accept the responsibility for the failure and remedy rather than excusing ourselves by projecting our own values onto the fox which has just as much right to exist on this planet as do we, if not more.

 

By Jim Pearson

 

References

The mammal society: https://www.mammal.org.uk/species-hub/full-species-hub/discover-mammals/species-fox/

Wildlife Britain: http:/ ww.wildlifebritain.com/redfox.php

The Natural History Museum: The secret life of urban foxes

https://www.nhm.ac.uk/discover/the-secret-life-of-urban-foxes.html?gclid... 

Chambers, M. (1990) Free Spirit: Brush with a Fox, Methuen Publishing Ltd

European Union (2019) Global food losses and food waste: Extent, causes and prevention”

https://ec.europa.eu/knowledge4policy/publication/global-food-losses-food-waste-extent-causes-prevention_en

Henry, J. D. (1986) Red Fox: The Catlike Canine, Smithsonian Nature Books No 5

Kruul, H. (1964); Predators and Anti-Predator Behaviour of the Black-Headed Gull (Larus ridibundus L.) Behaviour, Brill publishing

Macdonald, D. (1989) “Running with the Fox”, HarperCollins Publishers Ltd

https://www.wildlifeonline.me.uk/questions/answer/do-foxes-kill-for-fun

Springwatch 2010 https://www.bbc.co.uk/programmes/b00sm50z

Tinbergen, N. (1974), The Animal in its World (Explorations of an Ethologist, 1932-1972), Harvard University Pres

Surplus killing, Wikipedia

https://en.wikipedia.org/wiki/Surplus_killing