if you are looking to start with birds, go out with what you have presently and start shooting. i use regularly 2 different set ups, and note, for me, (perhaps you will be different) 16 MP is sufficient for birds if you are close enough, therefore you can crop the middle out of your K1 shots with the lens you have presently and the results will be stunning.
i use and have used and posted often with the SIGMA APO EX 70-200/2.8 and Sigma 2x TC (140-400MM f5.6 EQUIVELENT) and the shots are very good on my k5 even cropped, so do not be afraid to crop,
my new option is the K1 MK2 with the DA560mm which is amazing, but i think you can start with the 150-450.
even 6 MP is enough if you fill the frame, look at this shot
this was shot with an *istD and the SMC K300/4 plus 1.7x AF converter (500 mm equivalent) i used flash to freeze the image, but clearly less than 6 MP is enough so you can crop out of the middle of your K1 MK2 quite a bit