Feline Aggression 

   Aggression is advantageous to the survival of both the individual and the species.  By exhibiting aggression an animal can obtain and defend food, territory, shelter and mates, and can also defend itself and its offspring from attack.  Always having to continue aggression to a fighting conclusion would waste energy however, so the animal needs to be able to control its aggressive impulses in response to facial and postural signals from potential challengers.  All of this requires a central pathway for aggression.  Three related areas regulate aggressive response.  These are the hypothalamus, amygdala and the frontal lobe of the cerebral cortex.  Control of agonistic behavior becomes more finely tuned in a hierarchical fashion.  The hypothalamus produces “hardwired” responses.  These are a simple temporal balance between excitatory and inhibitory neurons.  Responses are predictable and are not influenced by experience.  Stimulation of the ventromedial hypothalamus inhibits aggression.  Ablation of this region or disruption of neuronal pathways from the cerebral cortex produces unprovoked aggressive outbursts.  Feline predatory aggression increases with stimulation of the lateral hypothalamus.


  The amygdala exerts regulatory/discriminatory control, which is in part based on previous experience.  Bilateral removal of the amygdala generally makes animals more placid and tame, however previously submissive animals become more aggressive.  Unilateral ablation will usually diminish aggressive responses to stimuli presented to the same side of the brain only.  Feeding and sexual behavior are regulated by similar regions of the brain.  If changes in aggression are due to damage or disease of the central nervous system, these behaviors may also change.  Ablation of the amygdala leads to the consumption of non-food items as well as loss of selectivity in choice of sexual partners.  Animals may attempt to copulate with animals of the same sex, different species or inanimate objects.  Fear is also suppressed.  Damage to the hypothalamus can result in animals which starve to death, as they no longer experience hunger, or which have lost a sense of satiety, and will continue to eat indefinitely.  The cortex offers the finest level of control.  It is here that signals of submission and dominance are interpreted.  Lesions here produce aggressive behavior in response to trivial stimuli, which normal social signals cannot inhibit.  Similarly other social behaviors, such as grooming and mothering, are disrupted.



Medical rule outs for aggression in cats:


  Any condition that affects these areas of the brain would be expected to affect aggressive behavior.  The type of aggression (or by corollary lack of normal aggressive response - although for obvious reasons this usually does not result in a behavioral presentation) would depend upon which area(s) of the pathway was affected.  For the most part these conditions do not normally present as primarily behavioral conditions.  However, some should be considered as rule outs for cases presenting as aggression.


  Of the viral diseases rabies would be the most obvious rule-out.  It typically produces lesions in the limbic system of which the amygdala is a part.  Feline immunodeficiency virus and pseudorabies may also produce aggression.  Protozoal infection with Toxoplasma gondii and Neosporum caninum can destroy central neuronal pathways, and there is at least one report of a boy with toxoplasma whose presenting problem was uncontrollable biting of other people.  Bacteria and fungi could produce abscesses or cysts, but these are not often seen in cats, and have not been reported to produce aggression.  Congenital malformations - lissencephaly and hydrocephalus; thiamine deficiency or thiaminase poisoning; hepatic encephalopathy; feline ischemic encephalopathy can all affect the aggression pathway, and increased aggression may be among the presenting signs.  Aggression is rarely seen in isolation from other signs in these conditions.  We have consulted about two cats that had brain tumors.  One was a phone consultation only, and the referring veterinarian was able to detect neurological signs the owner had failed to mention.  The other cat also showed very clear neurological deficits.  Many toxins will also produce aggression among the presenting signs, but in two instances aggression may be the primary complaint.  The first of these is lead poisoning.  It has been found in humans, and is probably also true in animals, that aggression increases even in children exposed to levels of lead that were not previously thought to cause any toxicity.  Cats are susceptible to lead poisoning, and it should be considered as a rule out, despite their reputedly more fastidious natures.  Lead is thought to produce its behavioral effects by inhibiting adenylcyclase and acetylcholinesterase.  At the cellular level, lead decreases energy production and alters calcium, sodium and potassium ion distribution.  Prolonged exposure produces gross pathological changes, damaging the blood brain barrier, dilating and rupturing cerebral blood vessels and causing scattered areas of necrosis among other changes.


  Other readily available toxins that may produce aggression as the primary problem are the organophosphate pesticides.  Acute toxicosis is due to irreversible inhibition of acetylcholinesterase.  In one rather classic case, a man liberally applied a carbaryl-containing flea powder to his cat.  He noticed that the cat, which had never shown any prowess as a hunter, was catching large numbers of mice and birds.  His long-term partner noticed that the owner’s temper had become unbearable, and he was becoming so violent that the partner felt forced to leave their home.  Within a week of the owner stopping treatment of the cat with the flea powder, the cat stopped hunting, and the owner’s behavior returned to normal so that his friend could move back into the house.


  These are more oddities than normal presentations.  One more frequently presented condition that may produce aggression is feline hyperesthesia syndrome.  Location of this condition under aggression or compulsive disorder has been disputed.  It appears to have many similarites to episodic dyscontrol - more often known as rage - in the dog, and may also be the result of partial seizures.



Partial seizures and Feline Hyperesthesia Syndrome


   While this condition may be presented as one of aggression, it is generally considered to represent an obsessive-compulsive disorder (see article on compulsive behaviors).  It has been likened to episodic dyscontrol in the dog, and it is possible that some cases at least are related to a partial seizure condition.  The temporal lobe has the lowest threshold for kindling in the brain, and abnormal brain waves frequently originate here and may or may not spread to other parts of the brain.  Partial seizure disorders can certainly produce aggression in cats, just as they do in dogs. One cat, a 10 year old spayed female, I treated showed characteristic signs of redirected aggression.  The initial attack on the owner occurred when a cat was seen outside the window.  The owner screamed at the cat to stop.  Subsequently, the cat attacked the owner as she played back a phone message from her sister who was speaking in a high-pitched voice, something like the owner’s scream.  Treatment with first clomipramine and later fluoxetine was unsuccessful.  The cat was switched to phenobarbital, and the other medications were stopped.  Subsequently, although the cat was reported to seem slightly anxious when other cats appeared in the yard, it was never aggressive to the owner again.  It is probable, that the stress of the other cats’ presence was sufficient to induce partial seizures that resulted in the cat’s aggressive behavior.





  The psychiatric disturbances caused by thyroid imbalances have long been recognized in human medicine.  Hyperthyroid patients may exhibit anxiety, nervousness, irritability, dysphoria, emotional lability (lose temper easily, burst into tears, become hysterical etc), hyperkinesia (can't sit still, tap fingers, feet, shift position frequently), insomnia, cognitive problems, phobias, panic, visual disturbances and psychosis.  Clearly, many of these problems are hard to recognize in animals.  Hyperthyroid cats are frequently restless, hyperexcitable, hyperactive, irritable and aggressive.  They may resist being held, touched and restrained.  The stress of visiting a veterinarian may precipitate these problems, and make it difficult to examine them.  Pacing, circling or aimless wandering may indicate a similar confusion or anxiety to that reported in human patients.  The cats generally sleep less deeply and are more easily aroused than euthyroid cats.  They are also more prone to focal motor seizures.  Elevated levels of thyroid hormones decrease the firing threshold for cerebral neurons.  Over 80% of hyperthyroid cats are reported to be hyperactive, 40-50% are easily stressed, and 10-25% are aggressive. 


  In contrast, about 10% of hyperthyroid cats, as well as humans, with hyperthyroidism are apathetic, lethargic and depressed (obtunded) on presentation.  While most cats have a ravenous appetite, these cats may be anorexic.  Some of these cats will become hyperexcitable, and behave in a more characteristic fashion as the disease progresses. 


  Surprisingly few cats hyperthyroid cats present for behavioral problems directly resulting from hyperthyroidism.  Perhaps this is because the disease is fairly insidious in onset.  The increased activity is attributed to changes in the weather, the decrease in affection, occurring slowly, may not be thought of as a major problem, particularly in a busy family.  The behavior problems are more likely to be manifest when the cat is stressed, aggression is primarily seen when the cat is restrained, so that veterinarians are more likely to experience this aspect of the disease than the cats’ owners.  Most cats have shown signs of hyperthyroidism for a year before veterinary help is sought.   When weight loss in the face of increased appetite has become appreciable owners will seek help.  Other cases may be picked up at the vaccination and annual check up visit.  At this time owners may mention an increase in vocalization, ravenous appetite, frequent vomiting and other signs that may be suggestive of thyroid disease. If behavioral changes are mentioned, they generally will include increased vocalization, loss of litter box training - particularly fecal as hyperthyroid cats defecate more, and possibly diminished affection.  95% of hyperthyroid cats also have a palpable cervical mass that together with elevated heart rate makes diagnosis relatively straightforward.  The majority of cases of hyperthyroidism result from bilateral adenomatous hyperplasia, and only 1-2% from thyroid carcinoma.  While hyperthyroidism is primarily a disease of older cats and the mean age of onset is 13 years, it has been reported in cats as young as three or four years of age.


  While behavioral concerns are not the usual presenting complaint, there are exceptions, and such was the case with Roger. This ten-year-old castrated male tabby cat was confined to the house, but not within the house.  Most of the time Roger behaved much as he always had, but every so often he would start to tear around completely out of control.  At such times he was most likely to attack people, both the ladies of the house and visitors.  These attacks were quite vicious, and on several occasions he had drawn blood.  The only other abnormality the owner had noticed was that his appetite seemed to have increased, although he was not gaining weight, and recently she suspected he may even have been losing it.


  On physical examination, Roger was sleek but lean.  He tolerated physical examination well provided he was not overly restrained.  Abnormalities included bluish gums, tachycardia and pansystolic murmur.  An enlarged thyroid gland was palpable.  In addition, severe tartar build up was noted.  Serum T4 was 115 nmol/L.   An electrocardiogram revealed left ventricular hypertrophy, thickening of the interventricular septum, left atrial and ventricular dilatation and myocardial hypercontractility   A diagnosis of hyperthyroidism was made, and as the owner declined surgery or radioactive iodine therapy the cat was treated with methimazole (Tapazole) at a dose of 10mg q 12h.  Within 2 weeks, Roger’s appetite and behavior had returned to normal.  His gums were pink and no cardiac abnormalities were detected on examination when he presented for dental prophylaxis.  Aggressive behavior has not been observed subsequent to therapy


  Treating the underlying thyroid problem generally resolves the behavioral problems however, and because onset is often insidious, it is only after the endocrine imbalance has been addressed that some owners appreciate the deterioration in their animal’s behavior.



Behavioral aggression:


   Cats tend to relate to the humans with whom they live as if they were members of the same species.  This results in their rubbing and lying against us, and licking or grooming us. Cats’ relationships with humans show as much diversity as those they have with other cats.  After house soiling, aggression is the second most common behavioral presentation of the cat.  Intraspecies aggression, rather than human directed aggression, however, is more frequently the problem.


   The most common type of human related aggression in the cat is play or predatory aggression.  While it can happen in cats of any age, the most frequent perpetrator is a young cat that is confined to the house.  It is usually an only cat or else there is no other cat near to it in age.  Owners will describe the cat as psychotic or schizophrenic, or more poetically as demonic or possessed.  They describe a cat lying in wait behind corners to leap out and attack their - preferably bare and exposed - feet and legs.  If the cat is spotted (usually by another person), it will be in the crouch of the hunter, still, the only movement being the madly twitching tail tip.  Another common scenario has the cat suddenly attacking the owner’s hands as they reach to pet it.  The cat’s ears flatten against its head, it gets a “demonic” look, actually a fixed stare, in its eyes, and may throw itself on its back and kick out at the offending hand with its back legs, or wrap its forelegs round the owner’s wrist or leg, and sink its “slavering” jaws into the unprotected hand or ankle.  The cat which rolls on its back as a sign of submission, usually holds its back legs in readiness to attack if the other cat should take advantage of the situation, and this behavior may be a corruption of that.   Another variant, which my own cat enjoyed, is the attack upon the toes in bed.  Even if said toes are protected under the covers the cat goes on a search and destroy mission.  Our cat used to sneak into the bedroom if she possibly could, while the dogs and I left for our morning walk.  If she succeeded, even before we were downstairs, a shriek and curses from my husband announced that she had been successful.  Noses can also be bitten in the pursuit of this sport.  Normally the aggressive response is inhibited, but cats have a layer of protective fur we lack, and we end up with painful scratches and bite wounds, which can be extremely dangerous for those with compromised immune systems.


   Play for cats as well as many other species is a means to hone and practice skills needed for the real world - in this case hunting and self-defense.  In the case of the domestic cat, it may never be thrown upon its own devices in the real world, but it still has an innate need for practise.  If left with its siblings and/or mother the kitten or young cat would work on these skills with them, and then gradually apply them to its survival when it was launched into the world on its own.  Humans and monkeys raised in isolation and deprived of play become dysfunctional adults frequently indulging in self-mutilative behavior.  This knowledge however, should not spur owners to gird their loins and accept the situation.  Often there is an element of dominance in this behavior, and we will address this later and separately.  The cat needs to play.  A plethora of static toys on the ground just may not do anything for him, however.  Favorite toys for this group are the feathered and beaded dancer toys on the ends of fishing rods.  Dragging a piece of cloth or a stuffed animal tied to a length of string gives the cat something it can really launch an attack upon.  Clockwork mice, balls of aluminum foil, ping pong balls or balls with bells inside - anything that moves, particularly erratically, will get their attention.   The application of catnip can help to get the cat interested in the new entertainment.  Perhaps the best cure for the problem is getting another cat of similar age.  The two can then play together and the owner is out of the loop.  Allowing the cat to go outside, if possible, will usually help resolve the problem.  Swatting at the cat to push it away or smack it will be interpreted by the cat as your contribution to the game, and he’ll swat back.  If you do find yourself subject to attack an indirect deterrent is called for.  Sounding an air horn is a popular one; another is the loaded water pistol.  Carry one at all times, or leave several loaded weapons around the house.  On the whole the latter is preferable.  Loud noises can trigger fear responses in cats that can lead to their own aggressive responses.  Some owners are prepared to garb themselves in commando gear - boots, stout pants and long sleeves, and wrap double sided tape around their lower extremities.  They then go about their day-to-day activities unafraid of injury if the cat attacks them.   When it does it encounters the tape.  For whatever reason most cats find this extremely aversive and rarely continue their attack games after a few encounters of the sticky kind.  In most cats play aggression will self extinguish with time - usually before the cat’s second birthday.  Its persistence in some older cats has been attributed to early neutering or breed, although free-living cats continue to play together as adults.


   Cats that indulge in play aggression are usually self-confident animals, and at least a bit dominant aggressive.  The most frequent expression of dominance aggression in cats has been described as petting induced aggression.  However, this descriptor does not describe the whole gamut of presentations in the cat, any more than protective aggression does for the dog.  The problem responds very well to a behavior modification program, with or without adjunctive pharmacological therapy, very similar to that used to treat canine dominance aggression.  

   Frequently owners report that the cat will initiate a petting session, but then sometimes with no apparent warning, it will lash out and bite or scratch the owner.  Closer attention usually shows that the cat glances sideways at the owner, exhibits tail twitching and/or emits a low growl shortly before launching into the attack.  If the owner attempts to restrain the angry cat, and make up with it, violence can escalate, and if the owner tries to leave at this point the cat may pursue him and continue the attack.  If the owner observes the warning signals and stands up forcing the cat off his lap, the attack is averted.  Some cats will typically tolerate petting of no longer than a set time (usually 2-5 minutes), and provided each session is shorter than this the problem is avoided.  Cats that exhibit petting induced aggression may just be hypersensitive souls who can only take so much stimulation before it drives them nuts, as some behaviorists have suggested.  However, we generally have found that these cats show aggression in other situations that mirror those which can arouse the ire of the dominant aggressive dog - which may also respond irascibly to “too much” petting.   Puberty is usually the age at which dominance aggression is first noted in both species.


   These cats are not shrinking violets, if they want your attention they tend to get it.  They’ll jump in your lap, swat you to get you to notice them, and attack the newspaper if it gets between the two of you.  They will not be ignored.  Food and valued possessions may also elicit aggressive protection.  One cat even guarded the refrigerator aggressively.  Cats, even more than dogs, can be very impatient with their caretakers.  They do not let their owners sleep in late when it is time for their breakfast, and bite at noses, fingers and toes in particular to get the owner up.  Some persist in harassing their owner through the food preparation to try and hurry it along.  These cats frequently adopt furniture as their own, and may not tolerate others using it.  As with dominant aggressive dogs one owner may be allowed in bed and not the other. 


   For the simple case of petting aggression many behaviorists have reported success using desensitization.  The cat is ignored for several days.  Then if the cat initiates petting the owner pets it for a brief period, gives it a tasty treat for permitting the intimacy, and ends the session.  Sessions are gradually extended.  The owner should always terminate a session if the cat starts to show signs of intolerance.  While the owner is taking some control with this method, he is deciding when the session ends, it seems to us that it reinforces the cat as dominant as he is being the one who initiates the petting in the first place, at his convenience as it were.  Instead we suggest a program similar to that used to treat the dominant aggressive dog.


   Confrontations are avoided.  The cat is not to be disturbed while it is eating or sleeping.   The cat will be kept out of the bedroom, off the furniture and banned from any part of the house that it is ‘guarding’.  Cats usually have greater furniture privileges than dogs.  Booby trapping furniture with upside down set mousetraps under paper or aluminum foil, scat mats or Snappy trainers (basically mousetraps in plastic sleeves) can be used without the owner having to be directly involved in policing the furniture.  The cat should be fed at strict meal times.  If it does not respond to its name or the sound of the can being opened or banged with a spoon, it does not get fed until the next meal.  When it comes the food is put down and the cat is left to eat.  After 15 - 20 minutes, provided the cat has left the vicinity of the bowl, any remaining food is removed, and the cat is not fed again until the next meal.  The owner can also control toys and playtime as they would with a dominant aggressive dog.  If the owner wants to play, he takes out a toy and calls the cat.  If the cat responds play is initiated and continues provided the cat doesn’t get too aggressive or demanding.  If he begins to do so, or when the owner wants to stop playing, a verbal termination or an unusual sound (a duck call works well) signals the end of the game.


   Cats can be taught obedience, although not usually to the level we can reasonably expect a dog to attain.  Like the dominant aggressive dog, it is probably harder to teach the dominant aggressive cat than it is most of his species.  Using the clicker-treat and shaping methods are most likely to bring success.


   When presented with such a behavior modification program, many owners are going to find it too demanding.  While we expect dogs to show some obedience, owners tend to like cats for their independence.  They do not want a subservient pet.  Encourage them to adopt as much of the program as they can.  However, pharmacological intervention is more likely to be requested to treat dominance in the cat than in the dog.  Serotonin stabilization is the goal of treatment, and the three drugs most frequently recommended are: fluoxetine is 0.5mg/kg q 24 h; clomipramine 0.5 - 1 mg/kg q 24h, and buspirone 2-4mg/kg q 12h.  In general, appreciable behavioral effects are seen earlier in cats than they are in dogs.  There have been some reports that while buspirone increases owner affection in the cat, fluoxetine or clomipramine may make the cat more aloof.  This has not been our experience, and if owners comment at all, it is to say that the cat seems more affectionate and content on any of the drugs.  Side effects may include a paradoxical increase in aggression although this is extremely rare.  Cats are quite likely to become anorexic and/or vomit, but this is usually transient, in my experience.  In general, I would be more inclined to try fluoxetine or clomipramine for the treatment of dominance aggression in the cat, and reserve buspirone for the cat whose aggression stems primarily from fear, as it does in the two other forms of feline owner directed aggression.


   Fear aggression is quite common in the cat.  One of the most common stimuli is a trip to the veterinarian’s office.  Other cats may respond fearfully to loud noises, visitors, new cats or dogs, or particular individuals.  Most cats will try to flee from the source of the fear, and will only become aggressive if escape is blocked.  Restraining the cat will cause its aggression to escalate.  The fear response is usually sufficiently dramatic that the owner recognizes it.  The pupils dilate, the ears are flattened against the head, the legs are tucked under the body, which is held low to the ground.  The cat may strain away from the source of its fear if it cannot flee; while hissing, growling and batting at it with its fore paws.  Fears can generally be overcome by systematic desensitization to the object that aroused the fear.  Unless the owner tries to restrain the cat while it is too close to the source of its fear, he will usually not be attacked.  Sometimes the owner can be the object of redirected aggression.


   The initiator for this is a powerful, external stimulus over which the cat has no control.  Other animals as well as humans can be the recipients of redirected aggression.  Generally, there is no history of any animosity towards the object of the cat’s aggression prior to the initiating event.  These cats are usually described as anxious, high strung or sensitive.  Diagnosis however, can be complicated if the trigger event was either not witnessed or its importance was not appreciated.  As a further complication, if no suitable object for aggression is present at the time the cat is aroused, it may stay in a state of arousal for several hours until the owner or someone or something else does present itself.  If a person is involved in the inciting event, the cat may redirect its aggression against someone who resembles that person.  Likewise, if a cat caused the arousal, another cat may be more likely to be attacked.


   In a typical presentation, the cat will be sitting in the window when a strange cat strays into its view.  (This stimulus would be most likely to arouse a male, territorial cat.)  The cat watches the strange cat, but cannot get at it.  The owner may try and pick the cat up, pet it, move it from the window or just pass by while it is aroused (the offending cat may long since have disappeared from view), at which point the redirecting cat launches a vicious attack on its owner.  This is likely to be particularly frightening to the owner, coming as it often does, apparently completely out of the blue.  The attack is usually uninhibited, and can cause quite extensive injuries.  Sometimes the owner can escape to a different part of the home where the cat is no longer aroused - it has lost its connection to the initial stimulus.


   Stimuli may be visual, often violence begets violence - cat and or dog fights; people beating dogs; stray dogs; auditory - a car backfiring, sirens; or olfactory - catnip, delicious food.  A secondary stimulus may also become associated with the primary one at initial presentation - the owner screams as the cat attacks when it sees the cat fight.  Subsequently, the owner or another woman screams in delight or astonishment at something, and the cat again launches an attack.  The cat was by the coffee table when it was aroused and attacked.  Now it only attacks when the owner approaches the coffee table.


   If the initiating stimulus can be identified the cat can be desensitized to it in some instances.  This is not always possible.  The other cat may not be available, or its owner not prepared to go through the slow, tedious process of gradually moving carrier contained cats closer to each other, while the cat displaying redirected aggression is rewarded for not reacting to the other cat.  In this case, it would probably be necessary to repeat the process with other cats, too.  In some cases, where each time the cat sees the owner he repeats the attack, and particularly if attacks only occur in a particular location (e.g. near the coffee table) desensitization can be much more successful.  Here only one person and/or place causes the response.  By gradually moving the person closer, rewarding the cat for each advance it tolerates without aggression, quite rapid progress can be made.  Unfortunately, usually this was someone the cat liked before the initiating event, and there’s no guarantee that a similar trigger wouldn’t produce a repeat performance.  Castrating intact male cats may reduce their territoriality and their frustration.  Usually it isn’t practical to block all the windows so that the cat can’t see outside stimuli that could trigger arousal and the redirected aggressive response, although access to outside stimuli can be reduced.


   Because most of the behavioral approaches to treatment are time consuming and frequently extremely impractical, we are again left with the use of pharmacological control.  This, in conjunction with behavioral modification, is most likely to be successful.   Again fluoxetine, clomipramine and buspirone would be the first drugs I would consider.   If, as is frequently the case, the cats’ response seems largely motivated by fear, the latter would be my first choice.


   Owners often become fearful of their cat, although they want to try and resolve the problem.  Their fear is often more than justified.  Carrying a water pistol or air horn may discourage the cat from attacking, although the stimulus may intensify the arousal and make the attack worse in some cases.  Separating the cat from the owner when a third party is not present to work on desensitization may be prudent, or resorting to the commando gear and double sided tape outfit described earlier.


   In the clinic the presenting signs may not be as clear-cut.  Aggression can frequently be a compound response to both dominance and fear.  It may also be the response to pain or disease. 


Territorial aggression: This is cat-cat aggression generally, although may be expressed as aggression to people, especially the new boy/girl friend who the cat sees as a rival.  Gradual desensitization is the best way of addressing the problem.  Introduce animals to each other (or cat to person) in such a way that they can see each other but the cats can’t attack each other. (For human directed aggression, approach the problem in the same way as described for cats that are afraid of people, see article on feline.)  Feed cats in each other’s presence so they associate other cat with good things.  If aggression continues, it may be necessary to treat one or both cats. In general, the undercat is given buspirone and the dominant aggressor fluoxetine.  It is necessary to determine if one or both cats are causing the problem.  If it’s a struggle for the role of top cat, giving both fluoxetine may be necessary.


Predatory aggression:  this is hard-wired, and very difficult to extinguish in the cat.  As mentioned above acetylcholinesterase inhibits this type of aggression.  Anticholinergic agents that cross the blood brain barrier should diminish this type of aggression, but this has not been investigated.  Belling the cat, or keeping it in, are probably the most effective deterrents at this time and much favored by bird watchers.