Field guides use range maps to show the seasonal occurrence of birds throughout the area covered by the guide. Many different symbols are used in field guides - such as a variety of colors - to indicate where bird species can be found during different times of the year.
For example, maps in "The Sibley Guide to Birds" are shaded with purple to indicate the permanent, year-round range of birds. If you were to look at a range map for House Finches, you would see that all of the lower continental 48 states are colored in purple, indicating that House Finches are year-round in all 48 states.
However, many wild bird species are migratory - spending the breeding season in one part of the country, and wintering in a different part of the country. In the Sibley Guide, this is illustrated by using different colors on the map. For example, the breeding range of birds is shown in peach and the wintering range is shown in blue.
The color yellow is used to show the migration route used by migratory birds. Looking at the range map for Rufous hummingbirds, you would see that the area between their summer range and their winter range is shaded in yellow.
Range maps are an invaluable tool for bird watchers - old and new. They provide birders with a greater understanding on where birds should be by season.
Other symbols used in some field guides are dotted lines, representing the extent of an irregular range for either spring or fall migration or the extent of an irregular or 'irruptive' range in winter.
Sometimes bird species "irrupt"- or move beyond - their "normal" range. This behavior is usually a result of a scarcity of food in their normal wintering habitat, or it can be because of severe winter weather, or both. In irruptive years, birds at northern latitudes move significantly further south compared to a normal year.
The National Geographic Field Guide to the Birds of North America uses these dotted lines in their range maps to show the potential boundary limits of migratory birds. These boundary lines are helpful to birders and provide insight on the possibilities of where birds might show up - even if it is with great irregularity.
Recently, in the Prescott area, there have been several bird sightings of species that are often associated with "irruptive years." Birders have recently observed Cassin's finch, Evening grosbeak, Lewis' woodpecker, pinyon jay and Clark's nutcracker in and around Prescott. Normally, we will go years without seeing some of these species.
Are the birds telling us something by this behavior? Will it be a hard winter? It will be interesting to continue to monitor the bird activity we are seeing to try and determine whether there is any correlation with the occurrence of these irruptive bird species and our impending winter weather.
On another note, I would like to personally invite you to join our family this Saturday, Oct. 26, between 11 a.m. and 2 p.m. as we celebrate the ten-year anniversary of Jay's Bird Barn (now The Lookout) with an Open House. There will be a variety of birds of prey on display, and at noon, we will be announcing the winners of our wild bird photo contest.
We will also be providing a wonderful lunch, and Brad Newman, Executive Director at Yavapai Exceptional Industries (YEI), will provide live music. The event is free and open to the public. Grab a friend or neighbor and please come join us.
I hope to see you Saturday. Until next week, Happy Birding!