The National Biological Information Infrastructure (NBII) has updated its search engine to search multiple databases at the same time, and now also returns clustered results. You can try it at NBII.gov. This search form is pretty basic; just enter a search term. I did a search for raptors. The search results look like this:
As you can see the search is powered by Vivisimo. The top of the page shows that you’re getting results for Birds of Prey as well as raptors. (But you do also have the option to search for raptors.) Results are provided in clusters, but not all of these seem to be on-topic. “Bald Eagle” and “Raptor Propagation”? Great! “Diurnal”? Well, okay. “Canada”? Probably a bit too general.
In addition to the clusters you also have the option to have your search results come from different sources. Each source has a blue i that will provide you extensive information on it when you click it (handy for me because I didn’t know about any of these sources.) The search results themselves has a section for images with the option to see more (looking at images alone I found 82 of them, lots of hawks, eagles, and buzzards.) The text results include a bit of context from the page itself (I wish there was more of this) as well listed source and publisher. There’s also a cache and page size as well.
The materials here are pretty overwhelming — general searches are going to bring you a lot of results. On the other hand, many of the clusters are useful and the quality of information you get here is great. If you’re doing any research on American wildlife definitely make this a stop.