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Today's birds are smaller and face more breeding challenges than prior generations, because the number of insects available to feed on has dropped in recent decades. 

According to a new study led by the University of Michigan School for Environment and Sustainability (SEAS), the number of insects at Canada's Long Point Bird Observatory has dropped by more than 60% since the 1970s. The researchers were focused on tree swallows, a bird species that's rapidly declining and that feeds on flying insects. 

“Tree swallow clutch size is really tightly tied to insect availability,” said Charlotte Probst, lead author of the new study and a doctoral student in SEAS. “When there’s fewer insects available, the birds are smaller and the birds also produce fewer young.”

The study is one of the first to consider the role of resource availability alongside climate change in understanding how these pressures are reshaping bird biology. 

Brian Weeks, an associate professor at SEAS, was senior author of the new study. The research team's SEAS cohort also included Professor Inés Ibáñez; Scott Yanco, who was a SEAS postdoctoral researcher; Isaiah Clark (MS '25), who is currently a field technician with Detroit River-Western Lake Erie Cooperative Weed Management Area; and Mark Ziebell who worked in the Weeks Lab as an undergraduate researcher. The co-authors were Matthew Fuirst and Stuart Mackenzie from the Long Point Bird Observatory.

Read the full press release on the Michigan News website

Study: Resource declines shape phenological and morphological responses to climate change (DOI: 10.1073/pnas.2607714123)