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The trend in breeding pair numbers for this species is obtained via the Black Duck Joint Venture helicopter survey. The number is expressed as "indicated breeding pairs". For example, when we observe a lone male, we assume that there is an incubating female nearby that is invisible to the observers. The vertical line extending above and below the average (mean) number of indicated pairs is the standard error (SE). This line represents the precision of the annual data.
Would you like more information? Consult the BDJV helicopter survey 2003 report (PDF format).
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