The breeding population of Bermuda's endemic national bird, the critically endangered Bermuda petrel, or Cahow (Pterodroma cahow), returned to the sole nesting sites on the Castle Harbour Islands Nature Reserve during late October 2024, staying through the month of November for the courtship & nest-building phase of its lengthy nesting season. The first Cahows were confirmed back in their nest burrows on the very early date of 14th October and have been surveyed through late October and November during regular visits to Nonsuch Island and the other 5 smaller nesting islets and rocks. The combined area of all 6 nesting Islands, including Nonsuch, totals only 22.2 acres, of which Nonsuch by far is the largest of these at 16.5 acres.
However, the weather conditions during much of November have been characterized by lengthy windy periods, making it too dangerous to travel out to and land on the nesting islands except on just 1 or 2 days of each week, rather than the more usual 3 to 4 days. I normally try to check nests on each of the nesting islands during this period to briefly remove any adult birds present, which are checked for weight, wing chord (outer wing length), and most importantly, the number of the band or ring fitted to the leg of Cahows when they are chicks in the nest. The birds are then returned to their nest burrows after these brief checks. The general body condition and health of the returning Cahows can be determined from their weight and wing chord length, and their band/ring number can be referenced to identify the bird's age, nest of origin, parent birds in many cases, and which birds are nesting with which mates in which nests. This basic information has confirmed that Cahows are overwhelmingly monogamous, with the same birds nesting with the same mates in the same nest burrows for in many cases over periods of 24 years or more.
During this courtship/nest-building period, I normally try to process or handle & measure at least 100 adult Cahows, but due to the windy conditions this year, I have only been able to handle about 80 birds. As of this writing (in late November) I still have about another week to try and get out to carry out checks and hopefully process some more Cahows, but the population will then return back out to the open ocean to feed intensively during the whole month of December, the females to develop their single large eggs, and the males to build up fat reserves to carry out most of the early egg incubation. After the birds return in the new year to lay their eggs in their underground burrows and begin egg incubation, I have an objective of processing at least another 100 Cahows during the egg incubation period of the nesting season in January and February.
So far this season, I have been able to confirm that almost all adults are in good body condition and at good weights; I have also been able to confirm at least 4 first-return birds, that is, young birds that have been out at sea for 3 to 5 years after fledging as chicks, and are now mature and returning for the first time, the males to choose new nest burrows and attract potential mates, and the females to choose mates with good nests. As we move into the December, pre-egg-laying exodus period, I will try to finish several new artificial nest burrows, in time for the return of the breeding adults for egg-laying, an event that I always look forward to!
Jeremy Madeiros | Principle Scientist - Terrestrial Conservation | Dept. of Environment and Natural Resources