science

Flowers that Attract Hummingbirds Confuse Bees

Debbie Blicher is Senior Producer of Ray Brown's Talkin' Birds.

Flowers pollinated mostly by hummingbirds seem to have evolved to confuse bees rather than to attract hummingbirds. So says a recent paper in the journal Ecology, "'Hummingbird' floral traits interact synergistically to discourage visitation by bumble bee foragers,"

Here are some starter facts. Flower preferred by bees ("bee" floral variants) tend to be upright and have blue or purple coloration, since bees have trouble seeing the color red. "Bird" variants, meanwhile, tend to be horizontal with red or orange coloration. Also, bee flowers yield small amounts of concentrated nectar, while bird flowers give pollinators larger amounts of dilute nectar. 

Robert Gegear, assistant professor of biology and biotechnology at Worcester Polytechnic Institute (WPI), wanted to understand how flower characteristics combine to influence the decisions bumblebees make about which flowers to visit. In other words, What kinds of flowers encourage or confuse bees? 

For the first step of the study, Gegear and his team of students trained bees to forage on arrays of paper flowers that all had the same color, orientation, and type of nectar reward. The bees learned that every color and orientation combination yielded the same reward.

The team then gave the bees arrays in which flowers of one color/orientation combination contained nectar and the other combinations contained distilled water. Gegear and his students recorded how long it took the bees to learn which flowers were worth visiting. 

The bees took longer to learn about certain combinations than about other combinations. That is, fake flowers that would favor birds in real life were more confusing for bees than fake flowers that would be better for bees in real life.

Why? Gegear explains, "These data suggest that the reason bee-to-bird evolutionary transitions are often accompanied by a floral shift to classic 'bird' trait complexes is because bees have a particularly difficult time combining red with other sensory traits, including nectar rewards." In other words, bees have a hard time recognizing red flowers, so any trait associated with red flowers is not worth their time to learn, even if learning would mean a greater nectar reward.  

Then where do hummingbirds come in? Well, if bees tend to ignore flowers that are difficult for them, then other pollinators, such as hummingbirds, make their move. Gegear says, "In the case of the two species of Mimulus, the costs associated with bird combinations are much greater than the costs associated with bee combinations, so bees avoid them to increase their foraging efficiency....When you put all this together, you find that 'bird flowers' are really 'anti-bee flowers' that function by exploiting specific sensory and cognitive limitations." That is, hummingbirds forage where bees don't bother to forage. 

Like most pollinators, bees are not genetically programmed to visit only particular flowers; instead, they seek to gather the most nectar in the least time however they can. In other words, they're generalists. From the plant's perspective, however, the best pollinator is a specialist in that plant. (Think of a building toy, like Lego, that clicks only with itself, which forces shoppers to buy only that one brand of building toy.) By combining particular floral characteristics, plants manipulate pollinators to become specialists because generalizing becomes a waste of time. In Gegear's words, "From an ecological perspective, an ideal pollinator is one that always forages on flowers of the same type so pollen is transferred effectively. In reality, pollinators are generalists and they should simply forage randomly. So the big question has been, how do plants get the pollinators to do what they want?"

Gegear suggests that most hummingbird-pollinated flowers once had bee-pollinated ancestors. He says his study shows that at least two floral characteristics had to change for the bird flower Mimulus cardinalis to evolve from the bee flower Mimulus lewisii, and that those changes served to discourage bees.  

Regardless of the flower, we can be kind to pollinators by avoiding pesticides in our gardens and by providing shelter and water for pollinators.

How Zebra Finches Choose their Valentines

Like many birds, Zebra Finches tend to pair up and stay paired. However, how they choose their mates is a bit of a mystery. 

Whereas many animals choose their mates for certain physical traits, Zebra Finches don't seem to do so. Malika Ihle and her colleagues at the Max Planck Institute for Ornithology, in Seewiesen, Germany recently published their research into what makes Zebra Finches pair up. 

The studied 160 single Zebra Finches, allowing groups of 20 males and 20 females in an aviary to become acquainted with one another. Grooming is a sign of Zebra Finch courtship, so when pairs started grooming each other, the researchers knew that those birds were capable of pairing. They let half the couples stay together. The other half they divided into "arranged marriages." They caged all the pairs for a few months so they could develop relationships, then released them into a group aviary to raise their families. 

Over the following five months, the researchers observed as the pairs went through three breeding cycles. Then they repeated the experiment, this time allowing only one third of the birds to stay with their mates. 

The results? Couples who had chosen each other had 37% more surviving young than those who had not. Forced couples produced more unfertilized eggs, lost more eggs, and had more chicks die after hatching. Females in forced pairs were not as interested in mating as those who had chosen mates. Males in forced pairs were less interested in caring for chicks and more interested in mating with other females. 

Dr Ihle and her colleagues say that, if the finches were choosing mates for genetic reasons, more embryos would have died from defects caused by interbreeding between such a limited selection of partners. However, the difference in the survival of chicks appeared to depend on how well they were cared for by their parents. The researchers argue that the results they saw indicate that Zebra Finches select their mates based on how well they get along. We here at Talkin' Birds don't think that's a bad way to choose. 

 

 

Female Songbirds: Quiet for Good Reason

Female birds are capable of song in 71% of songbird species, So how come we don't hear them as often as we hear the males? New research indicates that sometimes female songbirds have good reasons to stay quiet. 

Sonia Kleindorfer, a behavioral ecologist at Flinders University in Adelaide, Australia, studied the singing patterns of female and male Superb Fairywrens (Malarus cyaneus), a small Australian songbird species. Like the females of many species, female Superb Fairywrens often sing for territorial defense, sometimes even when they're on their nests. They use a melody known as a "chatter song." The male uses it too, but he is not as often near the nest when he sings.  This is a key difference, since singing alerts predators such as rodents, cats, and foxes. Kleindorfer investigated whether male singing or female singing poses a greater risk.

During the nesting seasons of 2013 and 2014, Kleindorfer and her colleagues monitored male and female singing on and near 72 wild Superb Fairywren nests, as well as the eggs and chicks in the nests. The scientists counted a nest as “attacked” if eggs or chicks vanished in under 25 days. In other words, if eggs disappeared unhatched or chicks disappeared before they were capable of fledging, researchers assumed they'd been eaten. 

Kleindorfer's team leaned that both males and females sang the chatter song more often when they were just beginning to nest. They sang it less often when they had eggs and chicks in the nest, and with one major gender difference: The males sang away from the nest and the females near or inside it. In fact, some females never sang at all, and some sang only in response to their noisy mates. To put it differently, the females sang less when doing so would endanger their nests. Proof? The scientists baited artificial nests with quail eggs and broadcast female chatter songs infrequently (six calls per hour) and frequently (20 calls per hour). Predators ate the eggs at 40% of the "frequent" nests, but at only 20% of the "infrequent" nests: the quieter nests were safer for the kids. 

Jordan Price, a behavioral ecologist at St. Mary’s College of Maryland, posits that danger to the nest could be the reason that female songbirds of other species sing far less than their partners—or not at all—when they're the primary on-nest parents. It's even possible that, generally, male songbirds sing not to attract mates but simply because there's no reason for them not to. The females, on the other hand, have to be more wary. 

Too-Dark and Too-Light Birds

We can rely on our field guides, right? All birds of the same species and gender look the same?

Wrong. When it comes to color, an occasional bird may be darker or lighter than the others of its species. That's when you find yourself looking back and forth between the bird and your field guide and saying, "It SOUNDS like that one and it ACTS like that one and it's in a whole FLOCK of them--but it's the wrong color!" 

So what's going on?

If a bird is darker than usual: It's "melanistic" or a "dark morph." These birds possesses an excess of dark pigment, called melanin, in their feathers. They are sometimes so dark that their markings are impossible to see. Therefore, it's best to identify them by sound, behavior, and association with other birds. Some species, such as red-tailed hawks and ferruginous hawks, have regularly occurring instances of such birds. These more common dark birds are known as dark morphs.

Melanistic and dark morphed birds have a couple of advantages over normally colored birds. First, their dark feathers absorb sunlight more easily in cold climates, helping them stay warmer than normal birds. Second, darker birds generally have an easier time with camouflage. The disadvantage of excess melanin? It makes feathers brittle, so they break easily.

if a bird is lighter than usual: It's albino or "leucistic." These birds lack dark pigmentation.

Albino birds—very rare—are unable to produce melanin at all. They may have red markings only (red pigment is unaffected) or they may look completely pale, and their eyes appear pink or red. Since their eyes have no protection from UV light, they don't see very well. Their poor eyesight and light coloring make it unlikely that they'll survive until adulthood. 

Leucistic birds might lack either of two kinds of melanin, or their bodies simply might not be able to deposit melanin in their feathers. They appear lighter than others of their species, making them easier for predators to see. Their feathers tend to wear out quickly. 

So next time you see a bird that looks darker or lighter than the picture in your field guide, remember that it might be one of these variants. And let us know!

 

Few Answers (So Far) Concerning West Nile Virus in Bird Populations

 A new study in Proceedings of the National Academy of Sciences indicates that West Nile virus (WNV) is killing millions of birds every year in North America, but few clear patterns are yet visible. 

West Nile Virus (WNW), which arrived on this continent about 16 years ago from Africa, is carried by mosquitoes. It can infect and kill people, but birds are its preferred host. Because North American bird populations had not previously encountered WNV, they had not developed any resistance to it. Therefore, the virus spread across the entire continent in only five years, leaving millions of birds dead. Earlier studies had shown that other factors, such as climate and habitat, influenced the virus's effect on various bird species. For instance, it was found that birds in urban environments seemed more likely to contract it, for reasons that are still unknown.

A team of scientists analyzed 16 years of data collected from 1992 to 2007 at more than 500 Monitoring Avian Productivity and Survivorship (MAPS) stations across the United States. (MAPS data after 2007 weren't included because they have not yet been processed.) Using this information, the scientists were able to determine whether and how the virus first affected various bird populations and whether their numbers have recovered or are still declining. They studied a quarter-million birds from 49 species, focusing on adults.

Twenty-three of the species studied were negatively affected. Some initially suffered huge declines. For example, Red-Eyed Vireo (Vireo olivaceus) populations dropped about 29% in the year they first encountered the disease. As expected by disease ecologists, their numbers subsequently recovered. Ryan Harrigan, an infectious disease biologist at the University of California, Los Angeles (one of the study's authors) explains, “Everyone builds up immunity, and the impact tends to wane.” Eleven of the 23 affected species in the study experienced this type of recovery. 

 

But the other twelve species were not so lucky: their populations are still declining for unknown reasons. For example, populations of Warbling Vireos (Vireo gilvus) dropped only 8.7% when they contracted WNV. But instead of recovering, they've continued to drop every year by roughly the same percentage.

To try to understand why some species overcome the disease better than others, the researchers compared their habitats. Here, too, results were mixed. Ten species, including the Spotted Towhee (Pipilo maculatus) and Song Sparrow (Melospiza melodia), did not show significant losses among adults, but eleven other species in these same urban areas did. The scientists also studied whether closely related species were affected similarly by WNV. Again, the pattern was mixed. Researchers are now looking more closely at regions where certain species are still dying to see whether they can spot the reason. And they have no idea why WNV did not affect at least three species, the black-capped chickadee (Parus atricapillus), American robin (Turdus migratorius), and house wren (Troglodytes aedon). Meanwhile, Staffan Bensch, an animal ecologist at Lund University in Sweden, wonders whether the virus might even be benefitting some songbird species by killing off jays and crows, which prey on them.

One further area of concern is that the virus may be causing particular harm to birds with smaller populations and ranges--that is, endangered birds. No answers are presenting themselves at this time. The researchers say that finding them will require more long-term data of the type collected at the MAPS stations. Ultimately, they hope that understanding the behavior of WNV in bird populations will help us understand its behavior in human populations.

For further reading, check out the study: http://www.pnas.org/content/early/2015/10/27/1507747112

 

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This week, we see how birds are adapting to climate change—or not; why vehicles and birds collide so often (and so tragically); and how you can get involved in a little bit of citizen science, right from your laptop!