Researchers analyzed 209 fossil bird bones (mostly humeri) from stratified layers (CLL7–CLL9) in Cueva del Llano, a volcanic tube cave in northern Fuerteventura near Villaverde.

These layers date to the Middle Holocene (roughly 9,000–5,000 years ago, with calibrated radiocarbon dates around 8,521 ± 938 yBP and 5,786 ± 479 yBP), prior to human arrival in the Canary Islands.

The bones were primarily accumulated by predators (no human or fire marks).

The assemblage includes a diverse set of species, dominated by Common Quail (Coturnix coturnix, 149 bones) but featuring many small passerines and others whose modern ecological preferences point to habitats incompatible with Fuerteventura’s current hot, arid climate (low rainfall, xeric vegetation, influenced by the Azores High and trade winds).

The authors conclude the early-to-middle Holocene climate in the Canary Islands was much wetter than today. Warmer global temperatures during the Holocene Climatic Optimum (~10,000–7,000 years ago, 3–7°C above present per Greenland ice cores) likely altered the Azores High, enhancing rainfall and supporting diverse vegetation and avian life.

Direct quote from the paper:

“The ornithological record from Cueva del Llano suggests that in the early stages of the Holocene, the dominant climate in the Canary Islands was much wetter than it is today. […]

Higher global temperatures than the present ones may have led to changes in the annual displacements of the Azores High and promoted a more intense rainfall regime, which fostered the maintenance of more diverse habitats and, consequently, a significantly more diverse avian fauna than today.”

As the Holocene continued, aridification caused these moisture-dependent species and habitats to decline.

In the context of the Middle Holocene bird fossils from Cueva del Llano (Fuerteventura), the HCO explains the “unexpected” wetter conditions.

Warmer global (especially Northern Hemisphere) temperatures during the optimum likely weakened or shifted the Azores High subtropical anticyclone southward or reduced its intensity.

This allowed more humid westerly air masses and enhanced rainfall over the subtropical eastern Atlantic, supporting bodies of water, riparian vegetation, dense forests/shrublands, and moisture-dependent bird species (e.g., warblers, nightingales) that are absent today in the now-arid islands.

The Holocene Climatic Optimum (HCO) illustrates natural millennial-scale climate variability driven by orbital changes rather than CO₂ (levels were ~260–280 ppm vs. today’s ~420+ ppm).

It is not a perfect analog for current anthropogenic warming, which is faster, more globally uniform, and primarily greenhouse-gas driven.

Recent studies (2020–2023) confirm the HTM via expanded proxy databases, while highlighting the role of vegetation feedback in resolving model-proxy mismatches.

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Here’s the complete, direct summary of the open-access paper at https://doi.org/10.3390/quat9020020 (published 1 March 2026 in Quaternary Volume 9, Issue 2, Article 20).

Full Title

Unexpected Climate Revealed by a Middle Holocene Avian Assemblage from Fuerteventura (Canary Islands)

Authors & Affiliations

• Antonio Sánchez-Marco (corresponding) — Institut Català de Paleontologia Miquel Crusafont (ICP-CERCA), Universitat Autònoma de Barcelona, 08193 Barcelona, Spain
• Ricardo Sánchez-Sastre — Facultad de Geografía e Historia, Universidad Complutense, 28040 Madrid, Spain
• Carolina Castillo — Facultad de Ciencias, Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Spain

Full Abstract

“A group of avian species, mostly small passerines, allows us to reconstruct the landscape and general climate of an area of Fuerteventura prior to the arrival of the first humans. Many of the bird species are typical of forest environments and the edges of bodies of water, conditions incompatible with the current hot and arid climate. The record of a high number of quail as well as small flying passerines surely implies the concurrence of two types of diurnal birds of prey, hunters on the ground and in flight, respectively. No trace of the abundant Puffinus holeae has been found, which evidently occupied a habitat very different from those in the north and interior of the island.”

Study Site

Cueva del Llano (Ramal Nuevo branch), a volcanic tube cave in northern Fuerteventura (La Oliva municipality, near Villaverde; UTM 607,237.98–3,170,403.41).

The cave segment is ~25 m long, 1.5–3 m high, ~2 m wide. Bones come from stratified layers CLL7–CLL9 (Phases IV–V).

aphonomy: accumulated by avian predators (no human, fire, or cut marks). One Haliaeetus sp. (sea-eagle) skull fragment was also found.

“Higher global temperatures than the present ones may have led to changes in the annual displacements of the Azores High and promoted a more intense rainfall regime, which fostered the maintenance of more diverse habitats and, consequently, a significantly more diverse avian fauna than today.”

These moisture-dependent species and habitats declined with progressive Holocene aridification (xeric conditions). The cave records pre-human conditions and highlights the paleontological value of volcanic tubes.

Here’s the complete, direct summary of the open-access paper at https://doi.org/10.3390/quat9020020 (published 1 March 2026 in Quaternary Volume 9, Issue 2, Article 20).