Very impressive set, especially the first one handheld and at F1.8

The sensor kind of generates its own shake because it can't find any that its expecting to find for it to counter.

IMHO, if nothing on the bird shot is in focus, it sort of suggests camera shake of some sort or perhaps a lens requiring a service. If you were closer I'd suggest we meet and you try my example.

If the stabilization is left on, it can introduce shake when on a tripod

As you were on a tripod, did you turn off the stabilisation ?

Try taking the ISO right up, say 1000 / 1600 or more to get your shutter speed above 1/750th sec

You can obviously have it serviced by Sigma, if I was you, I'd go through some images I've shot with it and being quite brutal with myself, look hard at the shutter speed, was it handheld ? how fast was the subject moving and then refine the technique accordingly.

I found with it a monopod was very useful as the light wasn't brilliant and I didn't wish to boast the ISO to far.

I've just recently acquired a sigma 150 - 500 from a fellow pentaxian on this forum

Used it properly for the first time a couple of days ago, and its razor sharp as shown below

K5iis, Sigma 150 - 500 @ 500mm 1/125th sec F8 ISO 200 with monopod

The lenses can be had for decent £££ if your prepared to bring them in from the Far East and the US via Ebay,

I picked up a 45-85 F4.5 for around £360 from Japan and a 80-160 F4.5 for around £460 from the US both within the last 12 -15 months including Duty, both were described as used but unmarked and boxed with instructions, both were as described, in full perfect boxed condition even with Pentax branded bags and the silica gel and perform as a new lens would do.

Plus with the good Paypal protection, bargains can be had. The saving was over £2k on UK dealer new prices,

It would be possible to tidy the hair a little in PS or whatever software you wish to use, but its much easier to do it before the shot is taken, the model should have done her best to be as prepared as possible, for me the sloping window sills and floor detract more than the dancers hair being slightly out of place.

Overall you've done a good job in the circumstances, the lightings good, looks like you've used some fill in flash, which is balanced well against the daylight, the compositions good and everything else is as it should be.

I would suggest that the head is fixed, i.e Non Removable

I've only been able to look at an image HERE

Given that the price is £84.95 inc vat and generally you'd pay that and more just for a Manfrotto set of Legs with a head starting at £50+

Get yourself a cheap 4GB or 8GB SDHC card, use that for the update and keep as a spare once you've got the other one working.