European starlings are among the birds that add fresh, green-leafy, nonstructural material to theirs nests. In general, the added greenery is from species that have aromatic leaves, rich in volatile compounds; these plants are a highly non-random, carefully selected portion of the plants available in the nesting habitat. (Mick Thompson / Flickr)

On The Trails: Greenery in avian nests

Behavior is less well-known to non-ornithologists.

This article has been updated to feature a new photo and caption. A previous version, due to editor’s error, attributed the nesting behavior of a European starling to a European swallow.

By Mary F. Willson

For the Juneau Empire

Most birds build nests chiefly of plant parts — branches, twigs, grass blades, mosses; in some cases, mud is a major component. Feathers, lichen, spider webs, plant down, hair and other materials may be included by various species.

Less well-known (to non-ornithologists) is that many birds, from songbirds to raptors and herons, also add fresh, green-leafy, nonstructural material to the nest. In general, the added greenery is from species that have aromatic leaves, rich in volatile compounds; these plants are a highly non-random, carefully selected portion of the plants available in the nesting habitat. The persistence of such a habit in so many species suggests that the use of greenery contributes in some way to reproductive success and reproductive fitness. The search for fitness consequences has led to numerous studies, but many questions still remain tantalizingly unanswered.

But before we go into all that, let’s first establish that — contrary to much conventional ‘wisdom’ — birds have a decent-to-excellent sense of smell. Depending on the species, they use it to locate insects in leaf litter or krill in the sea or carrion, to identify individuals, to locate a nest burrow when returning to it at night, and I bet that’s how they found my peanut-butter feeders when I first hung them up.

Although many ideas about the function of nest greenery have been suggested, three ideas have been examined most extensively.

Two species of starling express their interest in nest greenery entirely during the time of courtship and pair formation. Males then carry green material into the nest cavity in the presence of a female before egg-laying; that activity is correlated with testosterone levels. Some studies have shown that an increase of greenery led to larger clutches and more male chicks but experimental removal of greenery reduced the likelihood that a female laid eggs in that nest. Perhaps females use the presentation of greenery to judge the quality of the males?? But more testosterone in the males was associated with less paternal care of chicks and more greenery also led to more aggression among females. So the results of the several studies indicate some positive and some negative effects. In addition, other experiments found that nesting (already paired) females with nests decorated by their mates often left the greenery in place during incubation (although they commonly removed greenery added by an experimenter), and one recent study showed that the presence of greenery somehow induced more steady incubation behavior of the female, a shorter incubation period, and bigger chicks.

[Carrots and their wild relatives]

Perhaps the most popular idea about the function of nest greenery is that the volatile compounds from the leaves help deter parasites and pathogens. Many of the birds that use greenery add the greens after egg-laying (unlike starlings), during incubation and nestling periods. The volatile compounds are known to have negative effects on microbes and bugs in other situations. In line with that idea, birds that re-use old nests or nest in cavities (places where residual debris could house dormant pathogens and parasites) are more likely to put greenery into nests than birds that don’t re-use old nests and don’t nest in cavities. And one study of many species of songbirds in Argentina showed that botfly parasitism was much less in nests of species that added greenery.

However, studies of starlings found that the anti-parasite effects differed in different populations. And other results were conflicting too: if parasites were reduced, the nestlings were not measurably healthier than those in parasitized nests; contrarily, another study found that improved offspring survival. Then again, even if parasites were not reduced by greenery, nestlings in greened nests did better anyhow, possible because their immune systems were some bolstered (in some still-to-be-determined way) or because the females became better incubators.

Clearly, this common natural history phenomenon needs a lot more study—of different bird species, in different habitats, with different parasites and pathogens, with breeding birds of different ages and with different stress levels, and so on. The reproductive fitness consequences are there to be found, and they are likely to differ among species and situations.

Mark Twain once remarked that “There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.” And that’s true! But, contrary to Twain’s implication that conjecture is just hot air, it’s the starting point for science to proceed. T. H. Huxley, famous British biologist and staunch Darwinian, noted that, without conjecture, we rarely get as far as actual facts: Reformulate those conjectures based on observations and limited data into testable hypotheses. Those are necessary steps to discovering real facts — such as those that lie beneath disparate, conflicting results. That’s how science works.

• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.

More in News

This undated electron microscope image made available by the U.S. National Institutes of Health in February 2020 shows the Novel Coronavirus SARS-CoV-2, yellow, emerging from the surface of cells, blue/pink, cultured in the lab. Also known as 2019-nCoV, the virus causes COVID-19. A medical director at the British Columbia Centre for Disease Control says the numbers of active COVID-19 cases that are variants of concern are higher than what has been publicly reported in the province. THE CANADIAN PRESS/AP-NIAID-RML via AP
COVID at a glance for Thursday, April 15

These numbers come from reports from the City and Borough of Juneau… Continue reading

Has it always been a police car? (Michael Penn / Juneau Empire)
Police calls for Thursday, April 15, 2021

This report contains public information from law enforcement and public safety agencies.

COVID at a glance for Wednesday, April 14

The most recent state and local numbers.

It's a police car until you look closely and see the details don't quite match. (Juneau Empire File / Michael Penn)
Police calls for Wednesday, April 14, 2021

This report contains public information from law enforcement and public safety agencies.

This photo shows an envelope containing a 2020 census letter mailed to a U.S. resident. On Wednesday, March 24, 2021, a federal judge dismissed a lawsuit filed by the state of Ohio that tried to get the U.S. Census Bureau to provide data used for drawing congressional and legislative districts ahead of its planned release. (AP Photo / Matt Rourke)
Alaska joins 15 other states in backing Alabama’s challenge to Census privacy tool

The case could go directly to the Supreme Court if appealed.

Has it always been a police car? (Michael Penn / Juneau Empire)
Police calls for Tuesday, April 13, 2021

This report contains public information from law enforcement and public safety agencies.

This photo shows the Johnson & Johnson COVID-19 vaccine sits on a table at a pop up vaccinations site the Albanian Islamic Cultural Center, in the Staten Island borough of New York. The U.S. is recommending a “pause” in administration of the single-dose Johnson & Johnson COVID-19 vaccine to investigate reports of potentially dangerous blood clots. (AP Photo / Mary Altaffer)
CDC freeze on Johnson and Johnson vaccine sets clinics scrambling

The odds of being affected are vanishingly rare, but CDC says better safe than sorry.

After over 30 years at 3100 Channel Drive, the Juneau Empire offices are on the move. (Ben Hohenstatt /Juneau Empire File)
The Juneau Empire is on the move

Advertising and editorial staff are moving to Jordan Creek Center.

Most Read