Exactly when and where Isaac was born is a mystery. He listed his birth date on the naturalization documents as June 5, 1881, but he told the WWI draft board that he was born on April 10, 1881 and he told the WWII draft board that he was born on June 5, 1882. On his marriage record, Isaac lists his father as Elias Burk and his mother as Necke Burk--but both names have been written over with some corrections, so the exact spelling isn't known.
When Isaac entered the United States in May 1904, he said his last permanent residence had been Gerst. My guess is this was a mangled version of Gorsk, known in Lithuanian as Gargzdai. This is 11 miles east of the Baltic port of Klaipeda, Lithuania. Before World War I, Gorsk was in Russia, Kovno province, Telsiai district.
Isaac and his wife Henrietta Mahler went back and forth between New York City (where they were married in 1906) and Montreal until about 1915, when they settled in the Bronx to raise their four children: Mildred, Harold (my Dad!), Miriam, and Sidney.
PS: I found Isaac and Henrietta in the 1940 Census, right where they should have been: 3044 Valentine Avenue in the Bronx. Both of their sons, Harold (age 30) and Sidney (age 25), were living with them. Isaac's occupation was "manufacturer, dress forms" and son Harold's occupation was "clerk, baggage room." Now here's an interesting detail: Isaac's 1939 income had been ZERO but Harold's had been $1,000 and Sidney's had been $600. I have a suspicion that when Harold and Sidney went into the Army for WWII, they had their pay sent home to Isaac and Henrietta, who had no other income that I know of.
2022 update: Isaac was no longer alive for 1950 US Census but Henrietta, his widow, was alive and living with one of her sons, Sidney, in the Bronx. She died in 1954.