Where are the warblers? Usually by this time of year there will have been days upon days with warblers indistinguishable from raindrops, being both numerous and falling out of the sky to rest in the trees.

The silence of the woods has not been complete, though. I've noticed lots of Indigo Buntings, plenty of Scarlet Tanagers, and a delightful afternoon where White-throated Sparrows were practically attacking me. Others have noted a paucity of short-distance migrants, from phoebes to bluebirds to Hermit Thrushes, all possibly linked to severe cold weather in the south this year.

But overall, warbler abundance seems especially low this year. It's an interesting question, one that thirty years ago would have taken a monumental effort to comb through and tally records. But today, with the power of eBird, we can test these observations across multiple counties, across five years, and across almost every migrant species.

Some of the graphs summarizing this information are shown below. The purple line is 2021, which is most often the lowest line. This data spans Jefferson, Walworth, Waukesha, and Dane counties, March through May, and 2017-2021.

Ovenbird

The Ovenbird, one of the most common birds of northern Wisconsin, has seen its lowest migration abundance since at least 2017. I've heard its lively and repetitive song just once this year.

Golden-winged Warbler

The Golden-winged Warbler peaked in early May, but has dipped since. Its buzzy song has been missed. While I've typically only seen single birds during migration, the absence of any birds stands out.

Northern Parula

The Northern Parula’s trilling song and the soft, beautiful pastels of its plumage peaked in early May, and I saw good numbers then too, but haven't seen or heard any since.

Yellow-rumped Warbler

Finally, the Yellow-rumped Warbler, an abundant migrant, didn't seem to move through in quite the density typical for the species. In the past, I could grow bored and frustrated by the sheer number of Yellow-rumped Warblers. But this year, looking at high counts, its numbers are way down.

Unfortunately I don't have an overarching explanation for the apparently low warbler abundance this year. Some large studies have shown that an asynchrony in leaf-out and bird migration (due to climate disruption) could cause population declines because the birds have missed the insects exploding abundance during leaf-out. If anyone has theories for these apparent declines I'd love to hear them!

Written by Drew Harry, Faville Grove Sanctuary land steward

Cover photo by Andy Reago & Chrissy McClarren