Many species time their breeding to coincide with a brief peak in food availability. When the elements of that sequence advance at different rates, the coincidence stops holding.
The sequence depends on separate triggers
Trees leaf out mainly in response to accumulated warmth, and the caterpillars that eat those leaves emerge in response to warmth as well, so the two track each other closely.
Migratory birds, by contrast, begin their journey in a distant wintering area where the cue is often day length, which does not change from year to year.
Because one part of the chain responds to temperature and another to a fixed astronomical signal, warming pulls them apart rather than moving them together.
The food peak is narrow and does not wait
Caterpillar abundance in temperate woodland rises and falls over a short window, driven by leaf chemistry that becomes less digestible as the season progresses.
Nestlings need the largest food supply during a few days of rapid growth, so hitting that window is what determines how many chicks fledge.
A mismatch of even a week or two reduces the food available at the critical moment, and the effect shows up as smaller, lighter fledglings rather than as immediate failure.
Some species adjust and others cannot
Resident birds that experience local conditions all year can advance their laying dates directly, because their cue is the same warmth that drives the food peak.
Long-distance migrants have less information, and the scope for adjustment is limited by how early they can arrive and still find conditions survivable.
Populations do shift over time through selection for earlier breeders, but that process operates across generations while the climate signal changes within them.
Consequences extend beyond a single season
Lighter fledglings survive their first winter at lower rates, so a mismatch year affects recruitment into the breeding population one or two years later.
Repeated poor years reduce population size gradually, which is difficult to attribute because the visible cause is a season that looked unremarkable.
Predators and parasites tied to those birds experience the change second-hand, which is how a shift in leaf timing propagates through several levels of a food web.
Detecting mismatch requires long records
Establishing that timing has shifted needs decades of consistent observation of the same sites, since year-to-year variation is large compared with the trend.
Much of the available evidence comes from long-running volunteer recording schemes, which have tracked first leaf, first flight and first egg dates for many years.
Those datasets are why the pattern is documented at all, and they are the reason mismatch can be distinguished from ordinary weather variation.