Mid Season Review of Bermuda petrel (Cahow) 2015 Breeding Season

"At this time, there appears to be a total of 57 or 58 chicks for 2015 from a new record number of 112 nesting pairs (compared to 108 pairs in 2014, and only 17 to 18 pairs when the recovery program began in the early 1960s). Although this sounds like a large increase, it is still a dangerously tiny number as it is the entire population of the species on Earth." 4-16-2015 | Jeremy Madeiros Senior Terrestrial Conservation Officer Department of Conservation Services

  Jeremy Madeiros holds Cahow chick from the 2015 season.

Jeremy Madeiros holds Cahow chick from the 2015 season.

The 2015 Cahow Breeding Season (which actually began in late October 2014 with the return of the first adult Cahows to their nesting islands) is now in it’s final phase as the  downy chicks are reaching anywhere from one-third to one-half in their growth before they are ready to fly out to sea, between late May and late June.

This is a good time to review the breeding season to date and look at the successes and challenges of the Cahow Recovery Program to date this season, as it attempts to help our National Bird, which nests no-place else on Earth except Bermuda, recover from nearly becoming extinct.

Many people may think that the Cahow has already been rescued from extinction, and so we really don’t need to spend as much time with it anymore. Nothing could be further from the truth, the Cahow is still one of the very rarest seabirds on the planet, and still faces numerous very real threats to its survival, some of which can be managed for on its breeding sites, and some of which cannot.

One of the biggest of these threats is the impact of powerful hurricanes on the tiny nesting islands, some of which are mere rocks only a half acre in size. These islands are so small and low that the huge waves and storm surge experienced in hurricanes can completely submerge these islands, tearing off huge sections of rock, causing massive erosion and damaging or destroying even the solidly built artificial concrete nest burrows. Hurricane “Fabian” in 2003 submerged and eroded most of the islands with waves reaching over 35 feet in height, and damaged or destroyed many of the burrows, but fortunately was early enough in mid-September that none of the birds had yet returned for nesting. This enabled the Government Conservation Crew to rebuild many of the burrows before the nesting season began, but even so, the number of both nesting pairs and Cahow chicks fell steeply the following year.

  Eye of Hurricane Gonzalo passes over Bermuda

Eye of Hurricane Gonzalo passes over Bermuda

Hurricanes “Fay” and “Gonzalo” made direct hits on Bermuda within 5 days of each other in mid-October, 2014, causing widespread damage as winds reached 145mph as the eye of “Gonzalo” tracked directly over Bermuda. Unfortunately, this hurricane was late enough in the year that some of the male Cahows had already returned to claim and clean out their nest burrows. Cahow pairs mate for life, returning every year to the same nest burrow, and the males consistently return 7 to 10 days before the female birds. After checking and repairing all active nest burrows after the hurricane, I was able to confirm that at least four to five males had disappeared during the hurricane, and had evidently been drowned and washed out of their burrows as huge waves submerged two of the nesting islands. The female Cahows, returning and finding no males at the nests, then abandoned the sites, causing a net loss of at least four to five nesting pairs,

Despite this, at least eight new nesting pairs of Cahows colonized burrows and laid their single eggs for the first time this season, bringing the total nesting population up to a new record number of 112 nesting pairs (compared to 108 pairs in 2014, and only 17 to 18 pairs when the recovery program began in the early 1960s). Although this sounds like a large increase, it is still a dangerously tiny number as it is the entire population of the species on Earth. The Cahow is still listed as Critically Endangered, and it is a sobering fact to realize that a species is not considered to be out of the immediate risk of extinction until it reaches over a thousand breeding pairs in number, when it can be down-listed from “Critically Endangered” to “Threatened”.

  Wooden "Baffle" used to keep Longtails out of Cahow nests.

Wooden "Baffle" used to keep Longtails out of Cahow nests.

Other critical threats to the Cahow include rats occasionally swimming out to the nesting islands and killing and eating the Cahow chicks or eggs, and nest competition with the numerous Longtail (White-tailed Tropicbird), which can enter Cahow burrows, kill the chicks, and take over the nests. The Cahow chicks are defenseless against the more aggressive Longtails as their parents are usually out to sea during the day to gather food to feed their chicks. This problem has been largely managed for many years by fitting the entrances of nest burrows with wood “baffles” with a precisely shaped entrance hole, which allow the Cahows to enter and leave, but normally prevent the Longtails from doing so because they have a different shape and slightly larger body. However, in early April, I unfortunately recorded the first death of a Cahow chick by a Longtail in many years as an evidently smaller than normal Tropicbird managed to squeeze through a baffle and kill the chick. I immediately built and installed a smaller baffle, but this demonstrates the fact that if management of the Cahow population was reduced, that the population would immediately plunge again, possibly to the point of extinction.

At this time, there appear to be a total of 57 or 58 chicks for 2015, which are now reaching between 4 and 6 weeks of age. It normally takes 12 to 14 weeks (84 to 98 days) after hatching for a chick to fully develop, fledge and leave the nest. After flying out to sea, the chicks will not return to the nesting islands to choose mates and nesting burrows until they are 3 to 5 years old.

Our video star chick, “rainbow”, hatched on the 4th March, 2015, and weighed only 46 grams at its first weighing 2 days later. By the 15th April, its weight had already reached 324 grams, and should continue to increase until it reaches its peak weight in another month or so. During the last two to three weeks, chicks actually loose weight again as  they convert fat reserves to bones, muscles and feathers before flying out to sea.