Winter sportswomen capturing a thousand male seabirds (8)

'seabirds' is the definition.
(skimmer is a kind of seabird)

'winter sportswomen capturing a thousand male' is the wordplay.
'winter sportswomen' becomes 'skiers' (skiing is a winter sport).
'capturing' means one lot of letters goes inside another.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'male' becomes 'm' (common abbreviation).
'm'+'m'='mm'
'skiers' enclosing 'mm' is 'SKIMMERS'.