a possible birth date from merges is April 1450

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a short summary from Wikipedia:

Elizabeth Tilney, Countess of Surrey

Spouses:

1) Sir Humphrey Bourchier

2) Thomas Howard, Earl of Surrey

Issue:

John Bourchier, 2nd Lord Berners

Anne Bourchier

Margaret Bourchier

Thomas Howard, 3rd Duke of Norfolk

Admiral Sir Edward Howard, Lord High Admiral of England

Lord Edmund Howard

Lady Elizabeth Howard

Muriel Howard

Henry Howard

Richard Howard

Sir John Howard

Charles Howard

Father: Sir Frederick Tilney

Mother: Elizabeth Cheney

Born Before 1445

Ashwellthorpe Manor, Norfolk, England

Died 4 April 1497

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"Elizabeth Tilney, Countess of Surrey (before 1445 – 4 April 1497) was an English heiress and lady-in-waiting to two queens. She became the first wife of Thomas Howard, Earl of Surrey. She served as a lady-in-waiting to Queen consort Elizabeth Woodville, and later as Lady of the Bedchamber to the Queen's daughter, Elizabeth of York, consort of King Henry VII of England. She stood as joint godmother to Princess Margaret Tudor at her baptism. She was the mother of Thomas Howard, 3rd Duke of Norfolk. Through her daughter Elizabeth she was the maternal grandmother of Anne Boleyn, and through another son, Edmund, the paternal grandmother of Catherine Howard, both queens consort of King Henry VIII. Elizabeth's great-granddaughter was Queen Elizabeth I of England. Elizabeth was commemorated as the "Countess of Surrey" in John Skelton's poem, The Garlande of Laurell, following his visit to the Howard residence of Sheriff Hutton Castle."

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Links

http://en.wikipedia.org/wiki/Elizabeth_Tilney

http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=53751141

http://histfam.familysearch.org/getperson.php?personID=I217&tree=Norfolk

http://histfam.familysearch.org/getperson.php?personID=I206620&tree=Welsh

http://thepeerage.com/p10299.htm#i102982

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Citations / Sources :

1. [S6] G.E. Cokayne; with Vicary Gibbs, H.A. Doubleday, Geoffrey H. White, Duncan Warrand and Lord Howard de Walden, editors, The Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, Extant, Extinct or Dormant, new ed., 13 volumes in 14 (1910-1959; reprint in 6 volumes, Gloucester, U.K.: Alan Sutton Publishing, 2000), volume II, page 153. Hereinafter cited as The Complete Peerage.

2. [S3409] Caroline Maubois, "re: Penancoet Family," e-mail message to Darryl Roger Lundy, 2 December 2008. Hereinafter cited as "re: Penancoet Family."

3. [S6] Cokayne, and others, The Complete Peerage, volume II, page 363.

4. [S37] Charles Mosley, editor, Burke's Peerage, Baronetage & Knightage, 107th edition, 3 volumes (Wilmington, Delaware, U.S.A.: Burke's Peerage (Genealogical Books) Ltd, 2003), volume 1, page 356. Hereinafter cited as Burke's Peerage and Baronetage, 107th edition.

5. [S6] Cokayne, and others, The Complete Peerage, volume II, page 154.

6. [S2] #623 The Visitation of Norfolk in the Year 1563 (1878-1895), Harvey, William, (2 volumes. Norwich: [s.n.], 1878-1895), FHL book 942.61 D23ha; FHL microfilm 990,432 items., vol. 1 p. 15.

7. [S2] #623 The Visitation of Norfolk in the Year 1563 (1878-1895), Harvey, William, (2 volumes. Norwich: [s.n.], 1878-1895), FHL book 942.61 D23ha; FHL microfilm 990,432 items., vol. 1 p. 15, vol. 2 p.

8. [S2411] #11915 British Genealogy (filmed 1950), Evans, Alcwyn Caryni, (Books A to H. National Library of Wales MSS 12359-12360D. Manuscript filmed by the Genealogical Society of Utah, 1950), FHL microfilms 104,355 and 104,390 item 2., book 6 p. F10.