Studies showing such alterations often do so at the population level (i.e., using all members of a particular species occupying an area). This assumes that environmental conditions, and as a consequence, the rate at which behaviour is altered, to be consistent across a population. There is, however, great variation in the environment a population inhabits. Neighbours can experience drastically different conditions resulting in drastically different rates of change.
In 2021, using data going back as far as 1961, researchers at the University of Oxford showed population level change in the time Great tits lay their eggs to be linked to increased spring temperatures, with laying shifting 2 weeks earlier over the last 60 years. Such advancement of lay date may act to better align the hatching of chicks with food availability, as insect availability is likely to also be advanced by warmer temperatures.
Variation between individuals would normally be hidden
Having 6 decades worth of data also allowed for the analysis of repeated observations made of the same nest boxes. This analysis showed variation between individual nest boxes in the degree to which spring laying was advanced. In some areas of the woodland laying was 20.4 days earlier than in 1961, whilst in other areas it advanced by only 15 days. Such variation between individuals would normally be hidden in population level analyses.
This 5.4 days discrepancy cannot be accounted for by changes in temperature, as it is unlikely the average change in temperature since 1961 has greatly varied across the area being studied. It is more likely that variability in habitat results in variation in lay timing.
Oaks are the foraging ground of Great tits, as their leaves are covered in caterpillars. Great tits foraging in areas plentiful in oak trees have been previously shown to breed earlier and to have greater reproductive success. Further analysis by the team from Oxford showed variation in the health of the oak trees to be linked to the variation in rate of laying advancement. With nest boxes near to healthy oaks having early lay dates than those surrounded by unhealthy oaks. These findings suggest that birds laying in proximity to unhealthy trees struggle algin their breeding as warm spring temperatures get earlier each year.
2 theories are proposed as to why tree health can have such affects:
Findings demonstrate the need to study climate change on the individual level
Whilst work is yet to be done to discerned exactly why tree health impacts the lay date of individual birds, current findings demonstrate the need to study climate change on the individual level. Small differences between individuals living in close proximity reveal what may prevent some individuals from successfully adapting to future changes in their environment.
https://doi.org/10.1038/s41558-021-01140-4
