Why is ionisation energy of bismuth lower than lead?

Why is ionisation enthalpy of Bismuth less than that of Lead for it just comes after the latter in periodic table?  

**First ionisation energy of bismuth is 703 kg/mol while that of lead is 715 kg/mol.**


I also found that the covalent radius of Bismuth was more than that of lead, though both try to show me the same cause but I couldn't figure this out.  

Let me give out some reason why Bi should have higher first ionisation energy than bismuth:-  

\* **Bi** falls after **Pb** in the periodic table so it should be more non metallic in nature than **Pb**.(as from the general trend)  

\* **Bi** have half filled p orbitals contributing to its extra stability(as from the general trend).  

\* **Bi** falling down the the group 15 elements show metallic nature but still it should be less metallic than lead as it falls just after it.  

\* Even if we consider d/f orbitals,it would have same effect on lead and bismuth as both falls after elements occupying 5d orbitals and much after lanthanoid series.In fact the only effect I could see would be of increase in 1 electron thus there should be decrease in size of the atom and hence increase in ionisation energy.  

\* I tried to find out whether there might be any relativistic corrections but I was only able to reach to the conclusion that as the nuclear charge increases for atoms the size gets shorter somewhat like in the case of radium so ionisation energy should have increased. 


