A systematic review of long-term avian studies on phenological changes of birds and their food

Many organisms reproduce in seasonal environments, where selection on timing of reproduction is particularly strong as consumers need to synchronize reproduction with the peaked occurrence of their food. When a consumer species changes its phenology at a slower rate than its resources, this may induce a trophic mismatch, i.e. offspring growing up after the peak in food availability, potentially leading to reductions in growth and survival. However, there is large variation in the degree of trophic mismatches as well as in its effects on reproductive output.

Here, we explore the potential causes for variation in the strength of trophic mismatches in published studies of birds. Specifically, we ask whether the changes in the degree of mismatch that have occurred over time can be explained by a bird's (1) breeding latitude, (2) migration distance, and/or (3) life-history traits.

In particular, the script used contains the following analyses: (I) using linear mixed effect models we test whether the rate of change in food phenology, bird phenology and their difference (i.e. the rate of change in asynchrony) differed significantly from zero, (II) using linear mixed effect models we test whether the rate of change in asynchrony between birds and their food could be explained by breeding latitude, migratory behaviour and/or species life-history traits (body mass, clutch size and incubation duration).

We found that none of these three factors explains changes in the degree of mismatch over time. Nevertheless, food phenology did advance faster at more northerly latitudes, while shifts in bird phenology did not show a trend with latitude. We argue that the lack of support in our results is attributable to the large variation in the metrics used to describe timing of food availability.