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The animal on the cover of Getting Started with Varnish Cache is Montagu’s harrier (Circus pygargus); its name pays homage to British naturalist George Montagu.

The harrier’s plumage differs based on sex: adult males are covered in a pale grey plumage with black wingtips, and adult females have pale yellow underparts with longitudinal stripes and uniformly dark brown upper parts. As a juvenile, all harriers resemble adult females but with uniformly red brown under parts.

The Montagu’s harrier is known for its graceful flight and powerful wingbeats. This harrier is small compared to most raptors; its wingspan ranges 38–45 inches and it weighs between 265 grams (males) and 345 grams (females). The female of the species is larger in order to produce eggs.

This species is found in temperate climates as well as in the Mediterranean and boreal zones. It tends to nest in lowlands, mostly broad river valleys, plains, and areas bordering lakes or the sea. When desperate for habitat, the Montagu’s harrier will nest in agricultural farmlands, making it vulnerable to early harvesting. This harrier eats mostly small rodents, birds, bird eggs, reptiles, and large insects, taking whatever prey is available in the area of its nest. It catches prey while flying at low heights and low constant speeds along fixed routes. During the breeding season, males provision food for the females and later for the young. Males tend to hunt over a large area—within 7.5 miles from the next. Females often hunt closer to the nest, within 0.62 miles away, and only after the young have hatched.

Many of the animals on O’Reilly covers are endangered; all of them are important to the world. To learn more about how you can help, go to animals.oreilly.com.

The cover image is from British Birds. The cover fonts are URW Typewriter and Guardian Sans. The text font is Adobe Minion Pro; the heading font is Adobe Myriad Condensed; and the code font is Dalton Maag’s Ubuntu Mono.