Timing of clutch initiation in Pygoscelis penguins on the Antarctic Peninsula: Towards an improved understanding of off-peak census correction factors

Penguin censuses on the Antarctic Peninsula are often subject to logistical challenges that preclude nest counts being conducted at the peak of egg laying. Additionally, the historical literature includes many census counts with non-standard timing. The challenge is, therefore, to correct 'off-peak' census counts to make them comparable with current standard methods. Census correction involves knowing (i) how the census is timed relative to the peak of egg laying, and (ii) how nest numbers change through the breeding cycle. In this paper the authors present an analysis relating to both these challenges. Clutch initiation dates for four penguin breeding sites are examined (Cape Shirreff, Admiralty Bay, Humble Island and Petermann Island) in relation to potential drivers of clutch initiation (e.g. temperature, precipitation, sea-ice etc.). It is found that mean October temperatures constitute the most consistent significant factor related to the timing of clutch initiation in all three of the penguin species examined (Adélie (Pygoscelis adeliae), gentoo (P. papua) and chinstrap (P. antarctica)). A statistical model for determining the peak of clutch initiation is presented and, along with a simple estimation of species-specific nest attrition rates, is used to illustrate the procedure for correcting off-peak census counts.