What exactly do you mean by intelligence? Is reasoning and problem solving enough or are we looking for avians that will eventually develop space flight or derive the Standard Model? A number of experiments on ravens show them to be quite capable of the former, whilst the latter will almost certainly never happen without the ability to manipulate sophisticated tools (imagine building a particle accelerator with nothing more than beak!).
However, I would argue that it's an exacting critic indeed who wouldn't recognise reading, 'riting and 'rithmetic as signs of intelligence. I see no obvious barrier to raven's developing beak-graffiti to accompany their spoken (tweeted?) language. This is something that could easily occur spontaneously without much (or even any) physical evolution. As far as we can tell human writing only began around 10-30,000 years ago, whilst we have been largely physiologically unchanged since the rise of Anatomically Modern Human about 200,000 years ago. That is to say, we didn't just develop writing when we became smart enough to understand it - it seems to have arisen spontaneously, seemed useful, and stuck around. Human writing began with simple counting systems, and it is easy to conceive the utility of such a system even to ravens. A savvy raven could mark days to decouple themselves from the environmental cues of changing season and get the jump of the competition when migrating.*
An animal with a spoken/squawked language and in the habit of making marks will eventually come to use them for more than numerals. High points might be marked with a "tag" to warn off rivals, strings of landmarks might be marked as sign-posts home, allowing the raven to venture further to find food or mate. Reminders can be left of caches of hidden food. 634
None of this writing needs to be any more complex than their already demonstrated prruk-prruked language skills in order to be useful. Eventually the landscape will be dotted with raven writings, and certain pieces of information will pass from one generation to the next - summer last this many days here, this woodland is good for mice, etc.
If another species got this far, I wouldn't hesitate to consider them intelligent. There doesn't seem to be any reason at all why a raven couldn't do any of this without any further evolution, and we should not be discouraged by the fact that they haven't done so already - modern humans lazed about for 170,000 years without doing it. But what is exciting is what comes next - what might be possible with a little more evolution.
Sign-posts to hidden food might be read by other ravens, encouraging the use of misleading marks and an encryption/decryption arms- (wings-?) race. A migrating bird might add another 30 day-marks to his calendar before leaving, so that next years rivals postpone their own migration. When a technology proves useful then in time it typically becomes essential - if farming can support a larger population, then in time there will be a population that can only be supported by farming, etc. In this way literacy will eventually become an essential skill among ravens, as the underfed, exhausted illiterates are passed over by potential mates attracted to the new breed of well-fed, glossy-feathered renaissance ravens. And one day, they will be calculating their latitude and performing spherical trigonometry to plan their migrations...
(* yes, I know most ravens don't migrate, but some do, and this is world-building so I can have ravens migrate if I want. Ravens are the birds most commonly associated with intelligence, and migration seems a behaviour in which intelligence might be advantageous.) 802