Camera monitoring is another method you might consider. Helpful to minimise the risk of disturbing nesting birds (if your species is sensitive to human approach) and reduce time and travel costs needed for visual observation.

If you can say what species and what aspects of their breeding ecology you want to investigate, then people could offer more helpful suggestions.

Yes , in general, it is possisbsle. Depend from group ( raptors, passerines, waterbirds) you will modify  your research.

As others have mentioned, it is indeed possible to do a breeding ecology project without marked birds.  You can answer many questions regarding a species breeding biology without having marked individuals such as nest survival, nest-site habitat selection, nestling provisioning rates (sex specific if the sexes are dimorphic), and life history information (e.g., clutch size, # nestlings, # fledglings, etc.).  However, marking individual birds allows one to look at different aspects of breeding ecology such as influences of adult age on nest survival, return rates to specific breeding sites (if the species of interest is migratory), and presence of helpers or documentation of extra-pair copulations.