Though it can sometimes seem like everything is available online, historical research often involves visiting a number of institutions that may own photographs, manuscripts, rare books, and other unique materials related to your topic. Keep in mind that you will want to search not only for Birdwood, but for various people, events, and places associated with the Estate through time. It can be helpful to keep a running list of search terms you want to keep in mind. Also, try to think about who/what organization would have cared about the information you're seeking-- land information was recorded in tax records, property information was often recorded in wills, building changes are often indicated in account books (because of the material purchased). A good rule of thumb is to follow the money--historically, records were kept for purchases, sales, inheritance, taxes, etc, more than for any other reason.
Make use of finding aids where available, but remember that most primary resources are not cataloged in high detail. Try multiple search terms to find collections of papers, letters, etc. Family histories and travel accounts can provide information about historic people and places, as well... but they often require an investment of time to hunt and peck. Sometimes you'll end up empty handed... other times you'll hit the jackpot!