CAN OCEANIC EFFECTS ON GROWTH AND TIME TO FLEDGING MEDIATE TERRESTRIAL PREDATOR LIMITATION OF AN AT-RISK SEABIRD?

Seabirds commonly nest colonially on cliffs or inaccessible islands, so that oceanic effects on the quality or quantity of prey fed to chicks more often determine nest success than does terrestrial predation. However, when predators can access nests, impacts can be dramatic. In Kittlitz’s murrelet (KIMU), a rare and recently declining seabird, nests are widely dispersed in poorly accessible areas, growth rates are exceptionally high, and nestling periods are very short. This unique strategy may offset the vulnerability of their ground nests to predators but demands adequate deliveries of high-energy prey. We investigated whether variations in energy content of prey fed to chicks could alter growth rates and resulting duration of exposure to predators, and whether extending predator exposure had important effects on nest success. From 2009‒2016, we measured fish length with cameras, modeled prey energy content from chick energy needs, and measured chick growth rate and nest survival. We monitored an average 17 nests/year (139 total), of which 49% were depredated (almost all by red fox) and 25% survived to fledging. Prey were mainly Pacific sand lance (80%) and capelin (19%), with capelin having 2.3× higher energy per unit length. In a year of slow growth, sand lance energy density needed to increase by 31% from 4.29 to 5.64 kJ/g (within published values), or the proportion of capelin in the diet needed to increase from 5.6% to 27.2%, to achieve maximum chick growth. Adults could supply the energy required for maximum growth by delivering only 1.9 capelin/day versus 5.5 sand lance. Slow growth increased time to fledging by 5 days. This extended exposure to predation could decrease overall nest survival from 14.2% to 13.1%, a relative decrease of 7.7%. With reported breeding propensity of KIMU averaging only 20% and sometimes much lower (range 5 to 45%), even small effects on nest success may be consequential. Although direct predation was the main limitation to nest success, effects of ocean conditions on prey quantity and quality can have overriding effects in some years, and apparently small but substantive indirect effects in other years by mediating exposure to predation. Continued climate warming that adversely affects availability of high-energy fish and allows range expansion of terrestrial predators may have disproportionate effects on species such as KIMU with accessible nests and demands for energy-rich prey