Body mass, Morphological Measurements, Flock Counts, and Age-Ratios from Black Brant Overwintering in Three Lagoons Along the Baja California Peninsula, Mexico

BODY MEASUREMENTS: Sport harvested brant were examined during bag checks at the Old Mill boat dock in Bahia de San Quintin during 14 hunting seasons (Nov-Feb: Fri, Sat and Sun) between 1992 and 2013. Culmen and tarsus were measured with calipers and body mass was measured with electronic and Pesola spring scales. Age was determined by plumage characteristics: adults have dark wing coverts and an obvious white collar, while juveniles have white-tipped wing coverts and may lack a white collar early in fall. Sex was determined by cloacal examination. Birds were processed on the same day they were shot.

CENSUS: Counts of brant were conducted biweekly by one or more observers in Bahia de San Quintin from the top of Mt. Ceniza, a 175 m-high cinder cone situated between the two lobes of the bay, during the study period, which varied across years. Brant were identified and counted with a spotting scope (10–120x) and categorized into one of three areas of the bay: Back Bay, East Bay and Bahia Falsa, except in 1997 and 1998 when estimates in Back Bay were combined with East Bay. Counts represent best estimates for the number of brant in the bay during each survey.

AGE RATIO SURVEYS: Surveys were conducted from shoreline locations that provided unobstructed views of nearshore geese as they fed on eelgrass or came ashore on sand bars to roost and grit. Observation locations were selected for maximized visibility of geese and accounted for vantage point and weather conditions, such as sun glare and wind. Geese were aged while viewing a flock through spotting scopes (10–120x) and tallying adult and first-year birds on a hand counter. Juvenile brant were distinguished from adults by the presence of white edging on the distal ends of secondary wing coverts. Flock scans started at one end of the flock and moved through the flock from closest to the distant edge of the flock with a goal of aging ≥30% of the flock. The observer took care to age birds throughout the flock and not at the edge of the flock, where juveniles tend to concentrate. During a flock survey, observers recorded the location, date, time of observation, number of adults and juveniles aged, and approximate total flock size (Ward et al 2018). Juvenile age ratio equals numbers of juveniles aged divided by number of juveniles plus the number of adults aged.

BEHAVIOR: Selected flocks were visually scanned, the number of brant engaged in different behaviors during scans were recorded. During a flock survey, observers recorded the location, tide status, date, time of observation, and total time per each behavior.