The animal on the cover of Visualizing Data is a Northern hawk owl (Surnia ulula). They are called Northern hawk owls because they are largely found in the boreal forests of North America and Eurasia, and because of their behavioral similarities to the hawk; they fly, hover, and soar low over open areas searching for prey. Also, more like hawks than owls, they predominantly use their sight, rather than their hearing, when hunting.

They are widely dispersed geographically, found from Eurasia to Norway, Sweden, and Finland; east through Siberia to Kamchatka; and in North China and Central Asia as far south as Tien Shan. In North America, they can be found from Alaska east to Labrador, Canada. They breed wherever food is plentiful; when food is scarce, they (most the young owls) may fly south of their normal distribution.

Medium-size owls, they are usually between 36-41cm long with a wingspan between 22-25cm wide. Typically, males weigh between 273-326g and females weigh more, between 306-392g. The sexes are very similar in appearance and can be most easily distinguished by their calls. The typical male call is a fast, melodious, purring trill; although the female call is similar, it has a higher pitch and is less clear.

Their heads are round and their faces whitish, bordered on each side with a thick black stripe. The upper half of their body is generally dark gray and black, with a densely spotted forehead and crown. Their tails are long with white stripes. Their bills are yellow and they have pale yellow eyes. (Young owls have golden yellow eyes that turn paler as they age.)

Northern hawk owls--unlike most other owls--are typically diurnal, meaning they hunt during the day. They feed on small mammals, such as lemmings, voles, and rabbits. Other prey includes birds, frogs, and even fish. Often, they sit on a perch--in a conspicuous spot--scoping out potential targets. Once prey is spotted, they quickly take flight and swoop down to attack. When food is plentiful, they catch an excess and hide it for later.

Their nesting period begins in April and lasts through the first of May. Males scope out potential nesting sites, and females select the spot. (They are monogamous throughout the mating season.) Potential sites include empty woodpecker holes; abandoned squirrel, crow, and hawk nests; and rotting trees. They lay between 3-13 eggs at 1 to 2 day intervals, and incubate the eggs for approximately 25 to 30 days. The males feed the females during the incubation period. Chicks leave the nest after about 25 days and can fly well by the time they are 6 weeks old.

More research is necessary to better understand Northern hawk owls--one of the least researched bird species in North America--and their habitat needs and migration patterns. Habitat destruction is a major threat to them. The removal of dead trees and stumps deprives these owls of the nesting areas they need to procreate. Other concerns include poaching (even though they are protected under the Migratory Bird Treaty Act, which makes it illegal to harm or kill certain migratory bird species) and collisions with power lines and vehicles.

The cover image is from Johnson's Natural History. The cover font is Adobe ITC Garamond. The text font is Linotype Birka; the heading font is Adobe Myriad Condensed; and the code font is LucasFont's TheSans Mono Condensed.