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 censuses being conducted at the peak of egg laying as established by the CCAMLR Ecosystem Monitoring Program (CEMP) (Scientific Committee for the Conservation of Antarctic Marine Living Resources 2004). Additionally, the historical literature, necessary for establishing baseline conditions, also 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 1) how the census is timed relative to the peak of egg laying and 2) how nest numbers change through the breeding cycle. In this paper we present an analysis relating to both of these two challenges. Clutch initiation dates for four penguin breeding sites are examined (Cape Shirreff, Livingston Island (62°28′ S, 60°46′ W), Admiralty Bay, King George Island (62°10′ S, 58°30′ W), Humble Island (64°46′ S, 64°06′ W), and Petermann Island (65°10′ S, 64°10′ W)) in relationship to potential drivers of clutch initiation (e.g. temperature, precipitation, sea ice, sea surface temperature, etc.). We find 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, gentoo, and chinstrap). We present a statistical model for determining the peak of clutch initiation for any given year and site and, along with a simple estimation of species-specific nest attrition rates, we use this model to step through the procedure for correcting off-peak census counts.