Choosing postdocs In this period I have been applying for some postdoc positions. One of the main difficulties is that there is no central organization, so the times and practices for applying vary considerably even in the same country. Even being able to start a postdoc exactly when the previous one ends is not always simple.
Here I have a question about etiquette. What is people expected to do when moving from one postdoc to another one? A typical example would be a position which starts a few months before the previous one is finished. Or it is conceivable that one may have to accept a position in a short time, only to find later that he has won another position (for which he had already applied) which is "better" for various reasons, for instance it lasts longer or is in a place which is more favourable for personal reasons. An extreme case would be that on getting a permanent position while still doing a postdoc.
What is the expected etiquette in such cases? I'm interested mainly in what happens in Europe, if the opinion changes worldwide, but it is probably useful to hear from anyone about this matter.
 A: What usually makes it much easier in Europe is that most postdoc positions are without attached teaching, so there is no pressure of that sort when you are considering leaving. Otherwise, everything seem to mainly depend on whether the postdoc position in question is an individual research grant given for your own research proposal or it is a "research associate" position attached to a bigger grant which has already started. In the first case, the starting date is often very flexible, and the leaving date can be easily changed if you have done enough to demonstrate they were paying you money for a good reason; if you want to leave MUCH earlier, say after a year with a 2-year position, then you have to convince them that it is what is crucial for your career growth - but in all cases I know the postdocs did not have problems either. In the second case (research associate), the starting date is usually supposed to be very soon (the grant is given for a fixed period, so they want postdocs to start ASAP to not fail their first year joint report), whereas with leaving early people tend to be rather understanding (but (1) it depends on the grant PI a lot, clearly; (2) my information in that respect is mostly about Ireland and the UK, that might be seriously different in Germany, for example). 
A: All in all, I had 4 postdoctoral position outside the US, and my experience is very similar to that of Yiftach Barnea. For instance, I had absolutely no problem making a German postdoc start 3 months sonner than planned so that I could join another position afterward. The two other european postdoc positions I had, I could simply choose the starting and ending date at will (within a one year frame). 
At any rate, and especially if the position you are leaving has no teaching duties, I think everybody will understand if you have to leave a postdoc because you got a permanent position. In my case, this was in fact positively welcomed by the institution financing my postdoc (I was cheaper than thought and I boosted their "Got a permanent position thanks to us" stats).
A: I think the discussion in Jordan's blog is more towards the American market. In any case, in my view there is no clear answer and it really depends on the circumstances. However, it seems to me that in Europe the dates when postdoc positions start and end is almost random. In lots of cases postdocs have no teaching obligations so the starting and ending dates could be quite flexible. Also most people are quite understanding of the difficult situation postdocs find themselves. So I think the best thing to do is just to talk to your current supervisor / employer and ask for their advice. Also if you get an offer from some place you can ask them how flexible they are willing to be about the starting date. In many cases they will be very flexible.  
A: Just so we have a complete picture here: all of the other answers I've seen describe a type of postdoc which is common in Europe (I'm not really sure about the rest of the world), but unheard of in the US.  In the US, basically any postdoc will fit into the standard academic calendar (with a few exceptions that are 1 semester); sometimes you can have flexibility about starting a year later (this is actually quite common with tenure-track positions).
I think the generally accepted standard in the US is that you should only accept one job in a given year (i.e. if you accept one job, you shouldn't just drop it and take another one a month later), though I think people violate this rarely enough that I'm not sure we have reliable data on how it really affects one's career.  I think it's generally considered fine to look for jobs after that (say after 1 or 2 years of a 3 year position).
A: From my experience, the starting dates of post doc positions are flexible, though the degree of flexibility depends upon the funding source. Employers will be aware that you will be concurrently looking for other positions and that it is likely that you will need to start late or leave early. I think the key is to be open about the fact that you may be considering various positions at the same time. Try to avoid leading on one party, though, because it may have negative consequences later in your career. One case that anyone will understand is if you are offered a permanent faculty position. As such jobs are hard to come by, people will understand if you leave a post doc in order to take on such a position.
