In the news

Cat/pigeons, walking into a road not at the crossing and without due care and attention for traffic is also reckless, no?

http://www.cps.gov.uk/legal/h_to_k/homicide_murder_and_manslaughter/#unlawful

For involuntary act manslaughter, four points need to be established:

1. that the accused has committed an unlawful act;
   
2. that the act was the cause of death;
   
3. that the accused intended to commit the act;
   
4. that a sober and reasonable person would recognise that the act carried some risk of harm, however slight
   
   

So, they're trying to prove

1. brakelessness is unlawful
   
2. that brakelessness was the cause of death
   
3. that the defendant intended to be brakeless
   
4. that brakelessness was obviously dangerous
   
   

So arguing for contributory negligence could maybe perhaps be a defence to point 2. Maybe. I studied law donkey's ago, never practised, so I don't really know what'd stand up in court.