Gentoo share trends were positive and combined nest count trends were negative at four of 19 Antarctic sites, 1980–2025

A rise in species share need not show population growth. We tested this distinction with gentoo and chinstrap penguin nest counts from MAPPPD version 5.0. We matched species by site and breeding season in CCAMLR subareas 48.1 and 48.2. The primary panel had 241 matched seasons at 19 sites within 1980–2025. Each site had at least three matched seasons across at least 15 years. We fitted site trends in raw gentoo share and log combined counts. The share trend was positive at 17 sites. At four of these sites, the combined count trend was negative. Their first-to-last combined count losses ranged from 55% to 82%, while gentoo share rose. The equal-site mean share trend was +6.91 percentage points per decade. Its spatial block bootstrap interval was +1.77 to +8.99. The mean combined count trend was uncertain and changed sign with site weights. The same four cases remained with matching survey methods for the species. Three remained with ground counts only. A strict date check left 10 sites and one case, but none of the four original cases had enough fully dated records. Thus, close-date counts could not confirm those four trends. The result is limited to selected seasonal survey records. It does not show competition, a current regional trend, or ecosystem health. The package provides unchanged source files, source-row links, all panels and trends, figures, sensitivity checks, independent arithmetic checks, and code.

penguins · species composition · paired surveys · Antarctic Peninsula · MAPPPD · abundance · monitoring bias