Throughout Spidersilk the novel there are a few birds used to symbolize deeper meanings. One of these birds is the crow. Although in popular culture the crow tends to be used to signify darker motives. For instance, in many films you'll see them in cemeteries and even old folk legends associate the crows with death. There is an alternative though. In Native American symbolism the crow is told to reveal itself when a revelation must be made. The crow is a sign for the person seeing it to open up the sixth sense and see what is not actually visible to the naked eye. When the crows appear in Spidersilk it is like they're asking Mark to see what is not yet visible, when he does everything else becomes more apparent.
For more on crows, check out this link:
http://www.universeofsymbolism.com/symbolic-crow-meaning.html