I followed up on a report on ebird that an adult Parasitic Jaeger was seen chasing gulls at Timnath Res. in southeast Larimer County at midday. I arrived at 6:20 pm, and was joined by 3 other local birders. We found the jaeger in flight chasing a Ring-billed Gull out over the lake (at several hundred yards). The jaeger's wingspan was shorter than the gull, so immediately we ruled out Pomarine Jaeger. It landed on the water, and we could study the plumage for several minutes. Solid black cap on a white head/neck, blackish upperparts, and white underparts confirmed adult age. Later, in flight we observed a broad white patch at the base of the primaries. This feature and a dark smudge at the sides of the upper chest ruled out Long-tailed Jaeger. Eventually we got a good view of the jaeger's tail and outer wing from above after it successfully robbed a Ring-billed Gull of its meal in mid-air. The central rects were pointed and longer than other rectrices, and the outer 5-6 primaries had white around the base of their shafts. These features are consistent with Parasitic, and rule out Pomarine and Long-tailed, respectively. I was able to snap some poor quality photos as the sun was setting behind me.

Parasitic Jaeger is the rarest of the three jaeger species in Colorado. They occur during fall migration from September to November. This would be the fourth that I have seen in Colorado in 17 years of birding here.