PIGU Shatters Record

April 28, 2020

For the past 49 years, we have been systematically checking natural and artificial crevices used as breeding sites by a charismatic seabird called the Pigeon Guillemot, or PIGU for short. This species breeds throughout the west coast of North America, from Southern California to Alaska, with one of the largest breeding colonies here on Southeast Farallon Island. Recent tagging work by Point Blue biologists has revealed this population of PIGUs migrates north to British Columbia during the winter months, and returns to the Farallones in early spring to find their mates and begin the breeding cycle. Over the years, data collected from followed breeding sites show considerable inter-annual variability in the timing of when pairs will lay their first of two eggs. Just like the Cassin’s Auklets that were highlighted in an earlier post, reproductive timing for PIGUs is strongly linked to oceanic environmental condition. Lay dates are delayed during poor years with low ocean productivity, and advanced in years when the spring winds kick up early and spark upwelling along the coast.

The figure above is a summary of our long-term record of lay dates for PIGUs on the Farallones. Sites are checked every five days starting on April 27th. Cooler shades represent fewer sites where eggs were discovered, and warmer shades represent more sites with eggs. The solid black line shows the trend in median lay date over time, and the dashed line the earliest date eggs have ever been discovered. This year, so far one PIGU has shattered that record! A single bird incubating a complete clutch of two eggs was discovered by Farallon research assistant Paul Ruiz-Lopez, in box 5 at Garbage Gulch during the first check of the season on April 27th. If this bird is representative of the population as a whole, we can assume the median lay date will be equally record breaking this year. Yet another indication of a strong seabird breeding season ahead.