Greenery in avian nests

why do some birds add fresh sprigs?

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.

 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 Mr. 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.

Tracks and king eiders

winter sightings and mysteries

Early March brought us a lot of gray skies, but there was one spectacularly sunny day when there was deep, fresh snow at Eaglecrest. The parking lot was jammed, the slopes were thronged with down-hillers, but the nicely groomed lower Nordic loop was little used by humans, at least the morning I was there.

So a friend and I ambled around the loop on snowshoes, looking for tracks and hoping (in vain) to hear an early junco singing. There were a few deer and hare tracks, and a probable mouse. A raven had landed, punching deep in the snow and bracing a bit with one wing tip; then it walked a few feet and took off again—I wondered why it chose that particular spot. Here’s where an ermine dashed out from under a bush, bounded over the snow, and dove under the deep snow blanket. Oh look! Lots of bipedal three-toed tracks circling one blueberry bush after another… clearly a grouse or ptarmigan had come for lunch, nipping dozens of tiny buds off the blueberry twigs.

Just as we were about to move on from the bud-feeding area, we saw a small shrub at the side of the trail, poking up out of soft snow: a single stem with just a few twigs on top. Suddenly it quivered very noticeably, and then did it again. That movement was not caused by wind or a falling clump of snow or anything else detectable on the surface. So I suspect that some small critter under the snow was jostling the base of the stem. A lot goes on down there that we don’t see.

The next day was gray again. I made a quick trip to Point Louisa with another friend. I wasn’t expecting to see very much, because the tide was way out. But when we got out on the point, a gaggle of bird watchers had gathered, drawn by reports that some king eiders were seen there. The watchers were what I call ‘real birders’, who keep up on recent sightings and often carry telescopes as well as binoculars and don’t need to consult the field guides to know what birds they are seeing.

Just off-shore was a big crowd of scoters that even I could identify, along with a few harlequin ducks and goldeneyes. The birders assured me that mixed in with that lot were two female king eiders and even graciously loaned me the use of a telescope. After lots of finger-pointing and information on just where in that big flock I should look, I think I finally saw them—two brownish ducks with smaller bills than those of the scoters. A first, for me!

Photo by Kerry Howard

King eiders nest on the coastal tundra way up north, with the males then in their colorful breeding dress. There they feed on invertebrates in the freshwater ponds. Most of the birds that nest up there spend the winter in the Bering Sea, where they dive for benthic invertebrates.

However, some of them wander down our way at times. In addition to some records from Gustavus, there are records, over several years, of sightings from Sandy Beach to Eagle Beach—that stretch of coast is, of course, where most of the local birders are active; it seems likely that the eiders occur sporadically up and down the coast. Most of the sightings have been in winter and early spring. I have to wonder what brings them to our area…

The birds in the big flock were bobbing about peaceably (https://vimeo.com/520231569). The scoters were diving and coming up with prey, usually a mussel. When a scoter surfaced with prey in its bill, it shook its head with the prey securely clamped in the bill, perhaps to jettison some indigestible bits and extra water. Meanwhile, the eiders were cruising around among the scoters, occasionally dipping down to grab things not far below the surface. I suspect that all those paddling feet and diving birds stirred up small invertebrates, making them more accessible to the eiders. Perhaps the eiders also occasionally snatched up edible fragments that were accidentally discarded by the scoters.

Thanks to Doug Woodby and Mary Hausler for the loan of their telescope and guidance on where to look, and to Gus van Vliet for helpful consultations. Thanks also to Kerry Howard for the photo and to Bob Armstrong for the video.

Bribery

how to find a dispersal agent

As seed plants evolved, they invented many different ways to disperse their seeds away from the maternal plant to new sites. We observe various adaptations for dispersal by wind (maple seed wings, fireweed and dandelion fluff) or on the outsides of animals (stick-tights and burrs) or by bribing an animal with a food reward. The many varieties of bribery have evolved many times in separate lineages.

The food rewards for vertebrate animals that disperse seeds are typically what we call fruits, which vary in size and composition; they also develop in a variety of ways on the maternal plants. Quite different plant tissues can be turned into rewards for seed dispersers—seed plants have been quite inventive!

If you venture farther into this essay, be warned that our common terminology often differs from that of botanists. The common parlance has little relationship to botanical technicalities and often reflects culinary uses rather than biology (and we are not about to change, of course). Furthermore, botanical terminology itself is confusing; it’s meant to distinguish different anatomical relationships and developmental pathways, but these definitions are not always precise and vary even among botanists. Rather than try to untangle the semantic snarls in a way intelligible to readers, I’ve curtailed the elaborate descriptions.

In the angiosperms or flowering plants, the food reward is commonly a fruit of some type, formed from the seed-containing ovary and sometimes additional tissue from the parent. Gooseberries, currants, peppers, and tomatoes are fruits that are simply mature ovaries containing multiple seeds; cherries, plums, peaches, and olives are similar, but with a single hard seed inside. All of those could be called botanical berries, broadly defined. But sometimes considerable material from the base of the flower and the upper stem gets incorporated into the fruit. Thus we get apples, pears, rowan or mountain ash fruits, and serviceberries in which the ovary is now just the core and the co-opted floral base and upper stem are the edible part. And in pomegranates and rose hips, those upper stem tissues make the covering of the fruit.

What we call ‘berries’ of raspberry and blackberry are really aggregates of numerous small fruits borne on an enlarged base of a flower. Mulberries are multiple fruits from several flowers, all lumped together, and so is an ear of corn. Strawberries are not fruit at all (botanically); the tasty part is an expanded flower base, and the things we call seeds are actually tiny fruits.

All of this got started as I peeled a mandarin orange for lunch one day. I’ve peeled many of them over the years, but I suddenly got curious about its botanical description. The citrus fruits are actually specialized berries! They are, however, unique in having the juicy little sacs inside the segments; even other members of their taxonomic family do not have this feature. Most of the commercial varieties of citrus are said to be hybrids of various sorts, but mandarins are one of the original types, coming initially from the Himalayan region, spreading first to southeast Asia and eventually, via trade routes, to Europe, North Africa, and beyond. This made me wonder just how did that initial range expansion occur–Was it all due to human transport or did fruit-eating animals play a role too? In citrus orchards today, lots of different animals feast on the fruit, including rats, possums, raccoons, squirrels, parrots. Could similar animals have served as dispersal agents in Asia?

In some cases, the food reward for animal dispersers comes, not from the ovary and associated tissues, but from the maturing ovule. Again, definitions vary, but broadly defined, these outgrowths of the ovule are called arils. The seeds of pomegranates are covered by arils, as are those of the invasive Asian bittersweet vine and passion flower. 

Even the gymnosperms get into the aril story: juniper ‘berries’ and yew ‘berries’ are really made from modified cone scales that cover the seed like an aril, although the edible part arises from maternal tissue (the cone) rather than the ovule, as in angiosperms.

Many flowering plants make seeds with appendages called elaiosomes; these too have varied developmental pathways. This type of animal dispersal has evolved quite independently from those involving vertebrates. The appendages look like little lumps on the outside of the seed. They are usually full of oils and are attractive to ants and sometimes other insects that carry away the seeds and later eat the elaiosomes. Examples include at least some species of Trillium, Claytonia, Corydalis, and Dicentra.

Two species

an encounter with a wary ermine… and some thoughts about tree swallow nests

On a murky day toward the end of February, I went with a friend on the Boy Scout camp trail. Rain and warm temperatures had turned the snow to unpleasant deep slush and puddles in some places. As usual, we were just looking to see what we could see—and it wasn’t much. There were some crows picking through the wrack on the beach, a tiny group of bufflehead moving farther offshore, and a few geese on the far side of the big meadow. Not even any curious seals popping up to inspect us, no sea lions cruising by. A bit disappointing!

We cut through some of the groves on the big berm behind the beach, where the mosses were happily showing off their many shades of green. One spreading tree sheltered several duck decoys. Then, as I was stepping over a few roots, a movement near the toe of my boot made me stop. A small white head with bright black eyes was peering up out of squirrel-size hole in the ground. I signaled to my friend (who walks faster than I do) to come back. Meanwhile the white head disappeared, but briefly, only to re-emerge once more for a quick look-see. The owner of the head did not like two monsters looking at it, so even though we backed well away and waited, it did not reappear. With its wintry white coat, the ermine (a.k.a. short-tailed weasel; called a stoat in the U.K.) would have been very conspicuous on the snowless ground under the trees. We don’t see ermine very often, and this was the highlight of the walk that day.

The same day, in the afternoon, three female mallards arrived on my icy home pond. One of them had scouted the place two days earlier, and now brought along a couple of friends. They were out of luck, though; no open water and no seeds on the ice. The ducks weren’t the only critters that were anticipating spring, however. The previous week, a bear had crossed the ice into my yard, no doubt allured by the aroma of the peanut butter feeders, and left dirty footprints on my downstairs windows. That was not the only bear report for the Valley—ADFG tells me that there have been other early risers (or poor sleepers) this winter.

Some recent reading included a book called White Feathers, by famous naturalist Bernd Heinrich. It’s about tree swallows, those beautiful aerial acrobats that also sing sweetly—some birds seem to have it all! They are cavity nesters, using natural tree holes and readily using nest boxes.

Among many other observations, Heinrich noted that the tree swallows using his nest boxes had a strong interest in white or light-colored feathers, sometimes collecting them from some distance away. Male swallows were especially interested, although females sometimes showed interest too. Small feathers might make a cozy nest, but they had a special use for long, whitish feathers, chiefly during the later stages of egg-laying and the incubation period.

Of course, I wanted to know if our local tree swallows collected white and light-colored feathers too. And they do: inspection of nest boxes here and in Gustavus found white and whitish feathers around the clutches of eggs.

Those long, white feathers are arranged around the edge of the cup that holds the eggs, placed with the quills poked into the bottom of the nest around the eggs, so that the plumes stand up and arch over the eggs. The feathers clearly are not a cuddly cushion for the eggs, and not a snuggly blanket around them; smaller feathers might do that. They might conceal the eggs, but feathers of any color could do that. So why white ones? Are tree swallows the only species that adorns its nests in this particular way?

Photo by Jessica Millsap. This image was taken as part of the Audubon Tree Swallow Project, under permits from the Alaska Department of Fish and Game and the US Fish and Wildlife Service.

Tree swallows are fiercely territorial, aggressively defending an area and sky-space near the nest, sometimes engaging in knock-down-drag-out fights that end in injuries. They defend a chosen nest cavity against other tree swallows and other cavity-nesting species, including wrens, woodpeckers, bluebirds, starlings, chickadees, and others. The supply of suitable cavities is generally limited and competition for them can be ferocious. In some cases, tree swallows even oust chickadees that have already laid eggs and appropriate the cavity.

The long, whitish feathers, arranged to arch over the eggs, would show up well in dark cavities, easily visible from the nest opening. Heinrich suggests that they might possibly be a visible signal that tree swallows occupy that cavity. When the adult swallows are out foraging, such a signal could be useful in turning away other cavity-seekers and thus avoiding injurious battles. More observation and research needed!

February scrapbook

warm and bright observations in an icy world

Winter finally arrived sometime in early February, with good snow on the ground and very cool temperatures. I’ve lived here for three decades, so I’m quite well acquainted with Juneau’s local microclimates—it’s often warmer, wetter, and windier downtown than it is in the upper Valley where I live. But I recently saw what seemed to be an extreme case: as I drove Out the Road one morning, I left my house at a temperature of minus six degrees (F), then the car thermometer registered plus thirteen, dropped quickly to minus two, and rose again to plus fourteen degrees. That’s a twenty-degree span in fewer than twenty minutes. Extraordinary.

Along the way, I passed a place where a thin blanket of white mist lay over an estuary and shallow inlet. We often see this phenomenon in cold weather and sometimes call it ‘sea smoke’ or ‘steam’. But it’s not steam…steam is hot water vapor, and it’s not really smoke, either…not full of organic particles and carbon dioxide. Whatever the right name is, the cause is well-known. Liquid fresh water cannot be colder than thirty-two degrees (or it would become ice). So the surface of the estuary was warmer than the frigid air and water was evaporating. When that rising water vapor encountered the cold air, which holds less water than warm air does, it condensed into small droplets that hung over the water surface in thin mist.

I met a friend at the Point Bridget trailhead and we set off to see what we could find. The best find was the trail of an otter, bounding and sliding over flat ground and out onto the frozen beaver pond. Even on the flat, this otter was sliding as much as eight feet before gathering itself for another bound and slide. Wouldn’t that be fun to do! Blowing snow had drifted into some other tracks, but we found those of porcupine, moose, and a deer or small moose; red squirrels had made new highways under some of the trees.

A stiff breeze was churning Lynn Canal into a froth and big waves were roaring onto the beach where we look out at Lion’s Head. By the time we got there, it was afternoon and the wind was increasing, as it often does then. So the beach log where we often perch for lunch was not very hospitable. Even behind the beach berm, the wind was making the emergent tall grasses lie almost flat on the snow. So we found a windbreak in a sunny spot for a comfortable lunch.

Home again, with temperatures a relatively balmy plus sixteen degrees. The birds were active on the feeders, among them ‘my’ pair of red-breasted nuthatches. They brought two youngsters to the feeders one day last summer but they have apparently stayed on their territory for the winter. Each pair is socially monogamous; there apparently have been no studies of extra-pair matings (which are common in many other birds). Nuthatches defend their territories from other nuthatches; the male is especially vigorous in defense when the pair is excavating a nest in a dead tree. They also defend the nest cavity from red squirrels, which are potential predators of eggs and chicks.

Nuthatches have the odd habit of putting sticky conifer resin around the opening of the nest cavity. It is thought that this helps deter predators. One study found that more resin was placed around the nest entrance right after a face-off with a squirrel. Rarely, however, this tactic backfires, and one parent gets inextricably stuck in the resin and dies.

After the nest is built, females incubate five to eight eggs and the male brings her food. Incubation takes about twelve days and chicks stay in the nest for almost three weeks. Sometimes the male joins the female in the nest during incubation and brooding very small chicks. After the chicks fledge, the parents feed them for another two weeks and then the youngsters sometimes stay with the parents for many weeks, or they may become independent and disperse. Most nuthatches probably live only a few years; the maximum known lifespan is just over seven years.

Nuthatches forage by walking up and down and around tree trunks and big branches, especially in winter, presumably because dormant arthropods lurk in the crevices. They can walk head-first down a tree trunk and even walk upside down underneath a branch. They have a very short tail, not usable for bracing again the wood as woodpeckers and creepers do. Having a relatively long hind toe helps them scamper down and sideways. Outside of the winter season, they also forage on twigs and leaves, even on the ground sometimes, and occasionally catch insects out of the air.

Photo by Gwen Baluss

Captured food is often cached in holes and crevices, sometimes covered with bits of lichen or bark. A big item is wedged into a crack and then hacked into smaller bits (unlike chickadees, which hold such items in their feet). In fact, their English name may have originally been nut-hacker. At a seed feeder, nuthatches can be very choosy, carefully selecting the largest and heaviest items.

Little appears to be known about how they manage in extremely cold weather. They do join mixed-species foraging flocks in winter, along with chickadees, kinglets, and other small birds. Presumably their insulation is quite good, but they don’t seem to roost communally or have elevated metabolic rates then (as some other birds do). More questions to be answered!

From spores to seeds

evolution of a powerful reproductive strategy

A seed consists of an embryo with a packet of nutrition (usually), all housed in a protective coat. They range in size from the dust-like seeds of orchids to coconuts. And they all trace their origin to spores. 

The first land plants evolved from green algae, perhaps five hundred million years ago or more, and dispersed their offspring as spores; modern mosses and ferns still do so. A long time passed before seeds evolved (and of course they have not stopped evolving). Botanists have deduced that spores evolved to become seeds by several major steps and conjectured about what innovative features made such steps successful, such that the next generations maintained and continued those traits. I thought it might be interesting to visualize those steps, to begin to understand what was involved with the process.

Start with spores: Spores germinate to form tiny organisms, called gametophytes because they produce gametes (sperm and eggs) that have one set of genetic chromosomes (one set is termed 1N). Sperm swim around, looking for eggs to fertilize, sometimes joining with an egg from the same gametophyte. The fertilized egg and then an embryo with two sets of chromosomes (termed 2N) is held by the gametophyte as it grows into a recognizable moss or fern that will mature and produce spores (therefore called a sporophyte). So the life cycle is complete—alternating a sporophyte generation with a gametophyte generation (see diagram).

Life cycles of spore-producing plant (left) and seed-producing plant (right). Illustration by K. Hocker

In most spore-bearing species, the spores are all alike, making gametophytes that produce both sperm and eggs. However, in a number of lineages, the sexes became separated such that distinct male and female spores were produced, and these became much smaller. Botanists have suggested that, in one of these lineages, the separation of sexes was one of the important early steps on the evolutionary way that led to seed plants. What might have been the advantages to this arrangement? The two sexes would produce two kinds of gametophytes that could now evolve different characteristics and exploit their habitats in different ways. Females could invest all their energies in producing eggs and rearing embryos, no longer investing in producing sperm, and males could invest all of their energies in sperm. Separation of the sexes also helped reduce the rate of self-fertilization, thus reducing the risks of inbreeding.

The next major innovation was the retention of female gametophytes with their embryos on the sporophyte. These gametophytes became even smaller; fossils show step-by-step examples of how maternal tissues eventually formed integuments surrounding a gametophyte and embryo, thus providing a protective covering that we call a seed coat. The whole package is called an ovule, which matures into a seed. That happened about three hundred and fifty million years ago or so.

Presumably those primitive seeds were simply shed, as spores were, to be dispersed on breezes. Having a protective coat may have allowed seeds to remain dormant until conditions were right for germination—something that most spores cannot do.

Then, about a hundred and sixty million years ago, a major division occurred. Some plants, called gymnosperms (naked seed), kept their ovules and seeds exposed on the surface of the spore-bearing structure. Another set of plants, called angiosperms (enclosed seed), began to put additional layers of maternal tissue around the seed, probably by folding a leaf-like ovule-bearing structure to enclose and protect the maturing seeds from desiccation and consumers such as beetles. (The gymnosperms would have had to solve such problems differently). This structure evolved in many different directions in various lineages of angiosperms, forming the pistils in our familiar flowers. The lower part of the pistil became the ovary, housing the seeds, while the upper part became the receptive surface (the stigma) for pollen.

The evolution of pollen is complex, but it broke the sperm cells’ dependence on water. Instead of swimming, wind-or animal-carried pollen became the sperm delivery system. Somehow the many sperm produced by the male gametophytes of seed-plant ancestors were reduced in number, such that each pollen grain contained one very tiny male gametophyte that produced two sperm.

In gymnosperms, one sperm fertilizes the egg, and the other one just degenerates. Nutrition for the embryo and seedling are provided by the female gametophyte, as was true for spores but now the gametophyte and embryo are inside the seed.

A new invention arose, somehow, in the angiosperms: one sperm fertilizes the ovule and the non-fertilizing sperm unites with certain other nuclei in the ovule to create essential food material (commonly starches or oils) for the developing embryo and eventually the early seedling. (The angiosperm ancestors of orchids did this too, but orchid seeds have lost that stored nutrition and depend on mycorrhizal fungi for food.)

After those major steps, natural selection in angiosperms led to the evolution of flowers and a great variety of pollination techniques and fruit types. Much of that diversification was related to interactions with animals.