Corticosterone is a steroid hormone found in the blood of all birds. We have developed a way to use corticosterone levels in free-living seabirds to document the recent history of their food availability.

Levels of corticosterone in the blood are direct indicators of nutritional or energetic stress - corticosterone rises when food availability decreases. This rise in corticosterone helps mobilize stored energy and activates foraging behavior. However, if corticosterone is elevated for a long time, it has numerous detrimental effects on reproduction and survival. Thus we have found that levels of corticosterone in the blood can be used to predict survival and the likelihood that birds will produce chicks.

The first thing we do when we catch a bird is to try to collect a blood sample within three minutes of capture to measure corticosterone. This can be tricky, but is critical to ensure that the hormone levels we measure in the blood reflect the birds pre-disturbed state. For baseline measurements, we want to measure the birds physiological state, not its response to us. It takes 3 minutes for blood levels of corticosterone to rise in response to a stressful event like capture. The strength of this rise is another parameter that we can use to assess birds nutritional history.

To take a blood sample, we expose the alar, or wing vein on the inside of a birds wing. After brushing the feathers aside, a small sterile needle is used to puncture the vein. A small amount of blood forms a drop that can be collected in very thin-diameter glass tubes (capillary tubes). The blood is then centrifuged to separate the heavy red-blood cells from the lighter plasma, which contains all the hormones. The plasma is frozen and shipped back to the University of Alaska, Fairbanks, where it will remain frozen until analysis.