The cover shows a thermal image of the carbonaceous (C-type) asteroid 162173 Ryugu. C-type asteroids formed some 4.6 billion years ago and are made up of primitive materials that could shed light on the origin of the Solar System. But even though they are the most common type of asteroid, little is known about their physical properties. In this week’s issue, Tatsuaki Okada and his colleagues describe their analysis of the thermal data captured by the thermal imager TIR on the spacecraft Hayabusa2 when it encountered Ryugu. The results reveal that the asteroid is probably a collection of small, remarkably porous rocks held loosely together. The authors suggest Ryugu may represent a missing link between the fluffy dust that pervades much of space and dense celestial objects such as planets.

The cover shows a royal flycatcher, one species in the rich diversity of bird life in Costa Rica. In this week’s issue, Nicholas Hendershot and his colleagues discuss the long-term changes in avian biodiversity brought about by agricultural practices and climate change. Over the course of 18 years, the researchers repeatedly surveyed 48 sites in Costa Rica that represented a range of land use from natural forest to diversified and intensive agriculture. They found that long-term biodiversity changes were greatest in the most intensively managed landscapes, effects that were magnified under drought conditions. The species diversity within diversified agriculture remained high over the long term, indicating that this form of land management could play a powerful role in biodiversity conservation. Diversity in intensive agriculture, however, declined progressively.

The piece of amber on this week’s cover is just 31.5 millimetres across. Found in Myanmar, it contains the complete skull of what is believed to be the smallest known dinosaur from the Mesozoic era. Described in this issue by Jingmai O'Connor and her colleagues, the skull is around 99 million years old and is from a primitive bird-like species the researchers have named Oculudentavis khaungraae. The skull itself is a mere 14.25 mm long, meaning that the creature was similar in size to the bee hummingbird (Mellisuga helenae) — the smallest living bird. The small aperture of the eye suggests that O. khaungraae would have been active in well-lit daytime environments, and the long row of teeth on the jaws hint at a predatory diet that consisted mainly of invertebrates. The diminutive size of the fossil hints that miniaturization may have evolved earlier than was previously thought.

Tropical forests, such as the one in the Democratic Republic of the Congo pictured, are important carbon sinks, removing about 15% of anthropogenic carbon dioxide over the course of the 1990s and early 2000s. In this week’s issue, Wannes Hubau and his colleagues examine the rates at which such forests in Africa and Amazonia have taken up carbon between 1983 and 2015 — and find marked differences between the two regions. They reveal that the ability of forests in Africa to act as a carbon sink was stable until the 2010s when it began to decline, in contrast to the previously documented decline in Amazonian forests since the 1990s. They conclude that both continents show a pattern of carbon-sink saturation and decline, with asynchronous timing and different rates of reduction. The researchers extrapolate their findings to predict that by 2030 the carbon sink in Africa will have shrunk by 14% compared with 2010–15, and that the Amazonian sink will reach zero in 2035. This decline has significant implications for the goal of limiting global warming to below 2 °C.