This data package includes 2 child items with data from Pacific Black Brant (Branta bernicla nigricans) during winter and migration periods. Child Item 1: "Body mass, Morphological Measurements, Flock Counts, and Age-Ratios from Black Brant Overwintering in Three Lagoons Along the Baja California Peninsula, Mexico". The data include: (1) body mass and morphological measurements from sport hunter harvested brant, and (2) census counts of brant in Bahia de San Quintin, (3) counts for flock age ratio determination, and (4) behavior. Child Item 2: "Black Brant (Branta bernicla nigricans) Radio Detection and Migration Timing Alaska to Mexico". Data from brant tagged with VHF radio transmitters in July 1999 on the Yukon Kuskokwim Delta, Alaska and detected in the following two years during their annual migration. Data include: 1) one table with attributes of the tagged brant, 2) one table with estimated dates of migration events, and 3) seven tables of detection data from radio tracking scans at coastal sites between Alaska and Mexico.