Montmorency Forest: birds are nesting later and later

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The white-throated sparrow is one of a group of short-distance migrants whose ne
The white-throated sparrow is one of a group of short-distance migrants whose nesting date has been pushed back by two weeks in the last quarter century at Forêt Montmorency. - lwolfartist
Despite earlier springs in North America, migratory birds are nesting 2 to 4 weeks later than 25 years ago at the Université Laval teaching and research forest.

Thanks to earlier springs, bird spring migrations have been occurring earlier and earlier in North America over the past few decades. Do these disruptions to the normal course of migration mean that birds are nesting earlier? Not necessarily, at least not for the 36 migratory species studied at Forêt Montmorency by a research team from Université Laval and Environment and Climate Change Canada. Not only are these species not nesting earlier, they are also nesting 2 to 4 weeks later than 25 years ago, reports the team in a study published in the journal Avian Conservation & Ecology.

Sara Boukherroub, André Desrochers and Junior Tremblay came to this astonishing conclusion after analyzing data collected at the Université Laval teaching and research forest over 16 breeding seasons between 1996 and 2020. These data are based on 7106 bird inventories carried out by experienced ornithologists.

"To carry out the inventories, observers stationed themselves at sampling sites and, for an average of 15 minutes, identified all the birds they saw, as well as noting whether any parental behavior was present. On average, 1,387 bird observations were made each year," says André Desrochers , professor in the Department of Wood and Forest Sciences at Université Laval.

Determining the exact date on which a forest bird begins nesting is no simple matter. The researchers therefore used an indirect method to establish the date on which birds at Forêt Montmorency nested in a given year.

"When birds are seen carrying food in their beaks, they have nested and at least one egg has hatched. Another clue to nesting is when birds are seen carrying fecal sacs produced by nestlings. These are droppings wrapped in a membrane that the parents carry away from the nest to keep it clean and avoid attracting predators," explains Professor Desrochers.

These observations enabled the researchers to establish the date at which half of the sites were home to birds raising chicks, enabling inter-annual comparisons. Analyses revealed that between 1996 and 2020, birds nested later and later at Forêt Montmorency. For species that migrate over short distances - a few hundred kilometers - such as the White-throated Sparrow, this time lag is around two weeks. For species that migrate thousands of kilometers, such as the yellow-rumped warbler, the time lag is up to four weeks.

The researchers found a link between nesting date and accumulated degree-days as of May 20. This variable is a climatic indicator linked to the emergence of buds and leaves, as well as to insect abundance," points out Professor Desrochers. However, its influence on the nesting date is not very high. The "year" variable has an effect 19.5 times greater. However, we don’t know exactly what lies behind this variable. Above all, it reveals our ignorance of the factors that influence the birds’ nesting date."

The researcher admits to having been surprised by the extent of the delay in bird nesting observed at Forêt Montmorency: "The nesting period cannot be pushed back much further because the arrival of autumn imposes constraints on the birds of the boreal forest. After nesting, the adults must complete their annual moult, and the young must be self-sufficient in time for the autumn migrations. At Forêt Montmorency, three quarters of the warblers had already left by the end of August."

"It’s reductionist to try to make rules that apply to every bird species on every planet."

-- André Desrochers Professor Desrochers doesn’t deny that certain regions of the planet are going through a period of global warming, and that this could lead to earlier migrations and nesting for some bird populations. "However, this is not what we have observed at Forêt Montmorency. This suggests that it’s reductionist to try to make rules that apply to all bird species across the planet."