Try being a bird. Want more pain? Try being a migrant bird. On their highly evolved natural calendars, spring, as measured by the First Leaf and First Bloom Indices is leaping forward not by hours but by days or even weeks.
Our spring insect emergence, a natural event that migrant birds rely on, started in late February, not early March, after our fourth warmest winter on record. Winter resident birds devoured the insects. As I explain below, a stroke of good luck for them but not so good for the neotropical migrants on their way from central and south America.
Phenology is the study of the timing of recurring biological events. For an important example, for many species of birds a cornerstone life history event is the timing and duration of migration, including stopovers to refuel. At the highest level, annual migration timing is set to seasonality – spring migration to northern breeding grounds and fall migration back to the southern wintering grounds.
Alongside this seasonal avian migration timeframe there are several non-avian flora and fauna phenology that can make – or break – the success of a bird species’ migration. A critical one is the post-winter dormancy emergence of many insect species as the birds move northward. Over the millennia the phenological calendars of northward migrating birds and the northward movement of ‘spring’ insect emergence have synchronized. This exquisite timing means that there is an abundance of insects for protein and fat nutrition when the birds arrive at their progressively northern stopovers. They use these buffets to replenish body mass and health ahead of their arrival on their breeding grounds and the rigors of breeding.
Weather events can impact any or all these phenological calendars. For example, a cold snap returns the temperatures to winter lows and insects return to dormant. Or, the opposite, an abnormal number of days that raise the air and ground temperatures brings out flora blooming and insect emergence. Only to be set back by a frost.
Birds have experienced these non-normal events over the years and have included adaptations for them in their evolutionary survival tactics. For example, the time spent to hunker down waiting for the return of the normal start-of-spring temperatures for stopover latitude is factored into their estimated time of arrival at the breeding ground. So they may arrive a little early or a little late on the breeding ground.
But the problem over the last few decades of climate change is the average start-of-spring temperatures – that is, the arrival of the natural spring – is arriving earlier on the astronomical calendar. And it is romping forward. For some bird species this is not an issue, especially for resident birds. Their insect smorgasbord also comes early. This can benefit them in at least two ways. Their mating and brooding can also start earlier, and they may even get in an extra clutch before autumn. They can also expand their range northward into areas with less competition for their specific habitat and food needs.
This phenological mismatch is not as advantageous for some other species. This is particularly true for the long-distance, neotropical migrants. Many of their calendars are more closely aligned with the astronomical calendar and their trigger events reflect this. For example, hours of daylight and\or changes in celestial bodies in the night sky.
The outcome is that many species arrive on, or close to on, the same astronomical calendar date as they have for millennia only to not find the variety and abundance of food sources they need. The flush of spring insect emergence is over. This scarcity – what we might call food insecurity – has been compounded over these same decades by the collapse of insect populations across large swaths of their migratory routes.
How these birds are adapting or not adapting and what impact(s) on specific species populations are occurring are open and complex research questions.
As these questions are being studied and specific species conservation management practices are being formulated, it seems safe to assume that changing landscaping and agriculture practices that reverse the trend of insect population declines and encourages more insects – remove turf, plant natives, leave the leaves, stop indiscriminate use of pesticides – buys a welcome buffer (and provides some great photo ops).
As a species that struggles to cope with a one hour change on our calendars twice a year it seems this is the least we can do for birds struggling with ‘leaps forward’ measured in days or weeks.
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