Mangrove Finch: the rarest of Darwin's finches

Darwin’s finches are among the most cited examples in biology: a group of closely related species on one archipelago, differing chiefly in the size and shape of their bills according to what they eat. They appear in every textbook account of adaptive radiation.

One of them lives in thirty hectares of mangrove.

Thirty hectares

The Mangrove Finch Camarhynchus heliobates occupies coastal mangrove on the north-west coast of Isabela, and its entire world range is two stands of it. Caleta Black is ten hectares, a block of red and white mangrove roughly 500 by 300 metres with a beach on one side and bare lava on the others. Playa Tortuga Negra, a little to the south, runs to about twenty hectares in two blocks. That is the species’ distribution, and it can be walked across in an afternoon.

It was not always this. The finch was recorded historically from five mangrove forests on Isabela and from Fernandina, and surveys have failed to find it on Fernandina at all. A third Isabela site, Bahía Carthago, some 70 km to the south-east, held a remnant that was already tiny when it was last looked at properly.

The habitat was never extensive to begin with. Mangrove on a volcanic archipelago occupies a narrow coastal fringe, so a bird restricted to that fringe starts from a low ceiling. What has happened since has taken a small range down to a residual one.

What the bird actually does

The finch itself is easy to under-describe, because a plain brown bird with a stout bill invites it.

It is an insect-eater working dead wood. The bill is strong enough to strip bark from a dead branch and to peck into the wood beneath, and it also lifts the loose scales of living bark to take what is underneath, and probes through leaf litter on the mangrove floor. It uses tools, holding a cactus spine or a twig in the bill to winkle prey out of cavities it cannot reach. That habit is shared with its nearest relative, the Woodpecker Finch Camarhynchus pallidus, whose range meets its own at the landward edge of the mangrove.

The other relevant trait is that it barely moves. Dispersal is poor even by the standards of a small island passerine, which is why the two occupied stands do not function as one population, why the bird has not recolonised the mangrove forests it used to hold, and why moving birds by hand to unoccupied mangrove within the historic range has been on the table as a conservation option. A species that cannot travel ten kilometres of coast on its own is not going to repair its own range.

How anyone knows how many there are

The counting is worth describing, because it is harder than the number suggests and because the difficulty is informative in itself.

Birgit Fessl and colleagues, publishing in Philosophical Transactions of the Royal Society B in 2010, put the combined population of the two sites at 93 birds in 2006/07, 120 the following season and 99 the season after that. The confidence intervals on those figures are wide, 48 to 182 on the first of them, which is what a population estimate looks like when the animal is small, quiet and living in a tangle.

The detections behind those surveys are the striking part. Across three seasons the fieldwork logged 215 males and 12 females. That is not a sex ratio. Male mangrove finches sing and display and are therefore findable; females are silent and cryptic, so the chance of detecting one is a fraction of the chance of detecting a male. Any count of this species is really a count of singing males with an estimate bolted on, and the same fieldwork noted that 18 per cent of displaying males that had built a nest never found a mate at all.

Recent figures come from the Charles Darwin Foundation, which put the population at 62 individuals on 2023 data. The IUCN assessment of 2021 gives 20 to 40 mature individuals. Those are not in conflict; they are counting different things, and both are small enough that a single bad season matters.

The fly

The immediate threat is not habitat loss, and that is what makes the case unusual.

Philornis downsi is a parasitic fly, introduced to the Galápagos at some point around the 1960s, whose larvae live in the nest and feed on the blood of nestlings. The infestation is not marginal: the 2010 study found a mean of 40.8 larvae per nest in the nests it examined, and attributed 14 per cent additional mortality during the nestling phase to the fly in a season when rats had already been dealt with. Two nests that had failed outright were found full of larvae.

Introduced black rats take eggs and chicks on top of that, and the numbers there are unambiguous because they were measured either side of an intervention. Before poison was deployed, artificial nests at Playa Tortuga Negra were predated at 77 per cent, and rats were trapped at 20 per 100 trap nights. Two months after baiting, predation was 28 per cent and the trapping rate had fallen to 1. Nine months later, after a break in the baiting, it was back to 28 rats per 100 trap nights. Real nest success moved with it, from 18 per cent of nests producing fledglings to 37 per cent.

A population does not have to be killed to be lost. It only has to stop replacing itself, and a parasite that removes chicks does exactly that.

Head-starting, and why it was not the answer

For three seasons from 2014 the response was head-starting: eggs and very young chicks taken from wild nests, reared in conditions the fly could not reach, and released back into the mangrove once independent. The Charles Darwin Foundation ran it with the Galápagos National Park Directorate, San Diego Zoo Global and the Durrell Wildlife Conservation Trust. It worked as far as it went. Thirty-six birds were reared and released between 2014 and 2016, against roughly sixteen chicks that fledged naturally in the same period, and initial survival after release was 97 per cent.

Then the difficulties. Re-sighting a small quiet finch in mangrove is close to impossible once the radio tags come off, so the fate of most released birds is simply unknown; through 2014 and 2015 the team relocated exactly one. Access to the nests means boats and difficult terrain. And the method treats the symptom. Every year it is not done, the fly is still there.

The project’s own account from that period said plainly that there was no viable way to protect nestlings from the fly in place, which is why chicks were being carried out of the mangrove instead.

Self-fumigation

That has changed, and it is the most interesting thing to happen to this species in decades.

The birds now fumigate their own nests. Cotton and sisal fibres treated with a slow-release insecticide are tied to branches at dispensing points in the mangrove early in the breeding season; the finches collect the material, as they would collect any nesting fibre, and build it into the nest themselves. The insecticide keeps the fly larvae out without the chicks being handled at all. Trials began in 2017 and the approach came good in 2023.

The seasons since read as follows. In 2023 the team recorded 16 pairs and 18 fledglings from nine nests, the best result since the project started in 2007. The 2025 season was thinner: 12 pairs, 19 nesting attempts, six successful nests, eight fledglings, with 19 dispensing points and no handling of chicks at all. The 2026 season, running December to April, produced 20 breeding pairs and 25 fledglings, the most on record.

María Igual Beltrán, who leads the work, has been careful about what that means: sustained reproductive success at a scale that makes hope realistic, in her words, not a species out of trouble. Rakan Zahawi, the foundation’s executive director, put it as one of the most encouraging advances in decades of work on the bird, which is not the same as a recovery.

Why this one is difficult to raise money for

Sixty-odd birds in two mangrove patches, threatened by a fly, on an island most people cannot visit, in a group of species that are hard to tell apart and unremarkable to look at. Every element of that works against attention.

The argument for it has to be made on different ground. The finches are a reference case in evolutionary biology, studied continuously for generations, and losing one of them removes a branch from the example, not merely a species from a list. That is a genuine reason and a harder one to put on a poster than a parrot.

It is also worth noticing what the successful version of this looks like. The Spoon-billed Sandpiper programme took the same head-starting logic and put a captive population behind it, in a bird with a flagship’s profile and the funding that goes with it. The Echo Parakeet and the Cahow got thirty years of a permanent local organisation each. The Mangrove Finch has had a fly problem and a mangrove nobody can get to.

What it now has instead is a method that costs a boat trip and some treated fibre. The detail that made the difference was not the insecticide, which had been trialled for years, but where the fibres were put: tied to branches, early, before the birds had started building.