Thursday, April 22, 2010

Blue-gray Gnatcatcher migration

The Blue-gray Gnatcatcher, according to the authoritative Birds of North America, "may be a diurnal migrant" (Ellison 1992).

Having spent some time at various hawk watches in North America, this is also my impression.

Ellison cites a single tower kill record over multiple years of study from Leon Co., FL, and an absence of the species among lighthouse casualties from another study (Ellison 1992), as circumstantial evidence for the bird's status as diurnal migrant.

Yesterday morning, I went out birding before the start of the raptor count at Sandy Hook, because I anticipated encountering the results of a good passerine flight overnight, and indeed I was not disappointed. Large numbers of Palm Warblers, Yellow-rumped Warblers, Ruby-crowned Kinglets, Hermit Thrushes, Savannah Sparrows, White-throated Sparrows, and smaller numbers of Black-and-White Warblers, Common Yellowthroats, Prairie Warblers, Blue-headed Vireos, and Gray Catbirds were found around the hawk watch platform, where only a day before very few of these birds could be seen.

But I didn't come across large numbers of Blue-gray Gnatcatchers yesterday morning.

There were some around, but not noticeably more than the day before. As diurnal migrants, they probably weren't passengers on the previous night's warbler and sparrow express train heading northeast.

In this part of the range, the Blue-gray Gnatcatcher is an April migrant. The first Blue-gray Gnatcatcher for the Sandy Hook Migration Watch this year was seen on April 6. They're still coming through - another small wave was noted today - but the bulk is probably past. This season I've already counted 112 individuals. The graph below shows my day counts so far. The numbers here represent individuals or small groups of birds seen flying over the platform as they head to the northern tip of the Hook.

Graph 1: daily totals of Blue-gray Gnatcatcher at the Sandy Hook Migration Watch, spring 2010. n=112

A strong peak around the 11th, coincidental with a surge in overall diurnal migration across several avian taxa, and a more even distribution around a smaller peak on the 18th, are apparent. Today's count forms a tertiary peak, and it will be interesting to see how many gnatcatchers I will still get.

Graph 2: cumulative hourly totals of Blue-gray Gnatcatcher at the Sandy Hook Migration Watch, spring 2010. n=112

The spring Migration Watch is carried out daily from 9 AM to 5 PM. As Graph 2 illustrates, most gnatcatchers are seen in the morning. The first hour of the count is usually the best, while no gnatcatchers have yet been seen this season during the last two hours of the count. The graph hints at the possibility of strongest gnatcatcher migration occurring between sunrise and the start of the count.

This pattern may be typical for Blue-gray Gnatcatcher migration in this part of their range.

In the fall of 2006, I was one of the counters at the Smith Point Hawk Watch, a peninsular site on the east shore of Galveston Bay in Texas. There, co-counter Heidi Trudell and I witnessed a couple of days with strong Blue-gray Gnatcatcher migration. We didn't keep an exact count of them, but I seem to remember we saw several hundreds a day, sometimes moving in flocks of 15 or 20. The BNA account mentions a peak on the Gulf Coast for late September, and gives as an example a count of 1,000+ on 23 Sep 1989 Nueces Co., TX (Ellison 1992). Our wave of migrants was earlier in the season: I remember there wasn't much else flying except Mississippi and Swallow-tailed Kites. I think they went all day. In Veracruz too, I have seen Blue-gray Gnatcatchers migrate, though not in the same numbers. In fall, it is certainly one of the most common species there. In Veracruz, like Sandy Hook, most gnatcatchers pass through in the morning.

Cited reference:
Ellison, Walter G. 1992. Blue-gray Gnatcatcher (Polioptila caerulea), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

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