Esophageal diverticula in Common Redpolls

food visible in the 'E.D.'

The days of great weather have continued. Our capture rate improved today with the capture of three Common Redpolls; each of the three individuals varied in the volume and type of food stored in their esophageal diverticula. 

Esopha-what?! 

Esophageal diverticula are small pouches attached to the esophagus (the tube that runs from your throat to your stomach). This is not necessarily the same thing as what is commonly called a 'crop'; in other words, all crops are esophageal diverticula, but not all esophageal diverticula are crops (for example, many grouse species have esophageal pouches that they inflate as part of courtship display). There are three categories of esophageal diverticula: 1) 'real crops' are well developed, round and bilobed , 2) 'false crops' are typically long, narrow and small in size, and 3) 'rudimentary crops' are small and simple (i.e., they lack complex function or much supporting anatomy). 

 A small number of birds (mainly birds in the finch family that occur at Northern latitudes) have esophageal diverticula that are used as simple storage pouches and is a 'false crop'. This simple pouch's primary purpose is to temporarily store food for acute and energetically demanding events when food is not available. For example, consider how energetically demanding it is to maintain your body temperature outside in the middle of an Interior Alaska winter. Esophageal diverticula are an elegant solution to a complex problem for residents like redpolls. They are able to forage and fill their esophageal diverticula during the short days, and then continue to eat all night long. (Perhaps, like eating a midnight snack without leaving your bed?) These extra calories that can be consumed during periods when they can't forage allow them to survive conditions that would cause most birds to perish. Brooks (1968) suggested that this single adaptation extends the survivable temperature extreme from -30C to -68C. Pretty rad, huh?

At CFMS, we monitor how "full" the esophageal diverticula of redpolls are. Sometimes we are even able to determine what the bird has stored in the pouch, because their skin is translucent and the tissue between the pouch and skin is very thin (check out our picture from today). If larvae have been eaten recently we can even see them moving!