Black-throated Blue Warbler

The Black-throated Blue Warbler is one of the most-studied migrants in North America, and much of this research has taken place right here in the White Mountains of New Hampshire. Since 1969 biologists from several institutions have been monitoring this species’ numbers and breeding success at the Hubbard Brook Experimental Forest, and for a time they were also doing work in the Greater Antilles where this warbler winters. The results of all this work have informed our thinking about the ecology and conservation of long-distance migrants.

Some of the most recent work has focused on climate change, while still drawing on what we’ve learned over the preceding decades. As a northern species, Black-throated Blue Warblers are often assumed to be on the front lines as warming temperatures potentially shift their habitat northward or up the slopes of mountains. To test this hypothesis, ornithologists studied warblers over a 2000-foot range and found reproductive success was highest at the highest elevation. This might mean populations lower on the mountain are already experiencing negative effects, and it will be interesting to see what happens as studies on this species continue.

Another climate-related study looked at when Black-throated Blue Warblers initiated their nesting attempts in relation to the timing of spring. In warmer springs leaves emerge earlier on trees but the warblers have not advanced their migration date. This could lead to a mismatch between the nesting cycle and food availability, which has resulted in lowered reproductive success in other species. In this case, the warblers are still arriving prior to leaf-out, and deal with the varying gap by altering how long they wait before starting to nest. Again, they can currently adjust to a changing climate, but this adjustment window is only likely to get smaller over time.

Perhaps one of the major contributions of this long-term study has been our increased understanding of what influences bird populations year to year. For Black-throated Blue Warblers at Hubbard Brook, two key factors are food supply and predator density, with the latter (primarily chipmunks and squirrel) in turn driven by seed production the following year. When multiple factors are combined, one can see how consistent changes to any one factor (e.g., slowly increasing predator populations over time) might result in changes to population trends. In the case of the Black-throated Blue Warbler, populations gradually increased both at Hubbard Brook and regionally over the course of the study, so a more recent downward trend may in fact be an indication that one or more threats – perhaps including climate change – is now exerting a greater effect than previously.