Post by Art Goldsmith
(unless otherwise credited, photos by Art Goldsmith, tree swallow photos by P-G Bentz )
Thanks to the following people for assisting with advice, content and research: Prof. Fran Bonier, Ph.D. candidate Catherine Dale, and Prof. Raleigh Robertson.
The Bird, The Research, The Sordid Truth: Do Tree Swallows Murder Nestlings and Cheat on Their Partners?
Yes, the daily tabloid newspapers are rife with sleazy debauchery, and so is this Blog, at least when it comes to Tree Swallows (Tachycineta bicolor) referred to by the abbreviation TRES. The story of the Tree Swallows at QUBS is integral to the development of the station by two remarkable leaders, Prof. Raleigh Robertson, who was Director of QUBS for 33 years, and Frank Phelan, who has managed the Station for 40 years, and who will be retiring at the end of 2015. Their story, which is also the saga of a small research station on the shores of Lake Opinicon morphing into a primary and pre-eminent 8500-acre regional research, education and conservation centre, deserves a separate post.
Back in 1974, Prof. Robertson had been continuing his studies of Red-winged Blackbirds (RWBL ) when, and this story is worthy of a movie script, the potential acquisition of 1000 acres (Hughson Farm) at the south end of Lake Opinicon prompted some thought about other studies. Coincidentally, Geoff Holroyd came to Queen’s from Long Point Bird Observatory (LPBO) with a number of nest boxes. As with so many human advances, the interplay of events prompted invention and the TRES studies commenced. Having already conducted RWBL habitat selection studies, Prof. Robertson decided to use the newly acquired nest boxes to study habitat selection of TRES on the Hughson property (in the illustration below by Dr. Raleigh Robertson, the Hughson tract is shown in green). The original pre-1974 QUBS is shown in light blue. The additional tracts, up to the year 2007, are shown in various other colours. The illustration shows several Tree Swallow photos, including a natural tree cavity nest and a nest box.
The following four paragraphs are paraphrased from personal communications with Prof. Bonier:
North American Breeding Bird Survey (BBS) data since the mid 1980s shows declining populations of aerial insectivorous birds, particularly in northeastern North America (Shutler et al).
Update: Prof. Bonier has informed me (November 9, 2015) that a new study (science is always progressing!) indicates this geographic pattern for aerial insectivores is not as strong as the Shutler team thought. The exception is Tree Swallows, which are, indeed, increasing or stable outside of northeastern North America.
Tree Swallows are aerial insectivores, and therefore, the network of TRES nest boxes afforded an opportunity to study the population at QUBS. Several hypotheses have been put forward regarding the drops in populations. I have spoken to people (at QUBS and elsewhere) who believe greater survival of parasites may be responsible. This is one of the disease parameters that Prof. Bonier mentioned in her communication with me. Prof. Bonier has determined that malaria parasites have not increased in Tree Swallows over time. Other parasites may have an effect on populations. Other climate-linked hypotheses include phenological changes, especially earlier life cycling of important insect prey.
Prof. Bonier inherited the QUBS TRES boxes from Prof. Robertson, and she ran studies on them since 2007. She compiled all of the demographic data on the population (from 1975 to present) and has used the data to test a couple of hypotheses about proximate causes of the population’s decline. The story seems to be this: the population has been declining fairly rapidly and steadily since ~1990, when it was at its peak (with every single box occupied by a breeding pair). The all time low of 23% box occupancy was last year (2014). That’s an even lower occupancy than 1975 when Prof. Robertson first put up about 70 boxes to start the population (and occupancy was 26%). This year, 2015, occupancy rebounded a bit to 35%. From this amazing data set, we can show that on average, per nest fledging success has not changed over time, nor have adult return rates (a proxy of adult survival). However, first-time breeders are coming into the population at a lower rate. This is true for immigrants into the population as well as resident nestlings returning. At its peak, 10% of all nestlings that hatched in boxes at QUBS in a given year returned later as breeders. Now there are rarely any coming back. So this would suggest that something is happening to dispersal and/or survival during the first year of life, between hatching and first breeding. Prof. Bonier and her colleagues are pursuing analyses on climate and disease to explore possible causes of this change, which might help us differentiate the two.
The population at QUBS started its decline in 1990’s. Significant climate change has been documented in our region from 1980 onward. That causes me to lean to that predominant factor as a major contributor to the drop in our local populations. The research also tells us that TRES are also expanding their range in the southeast, at the same time as return rates drop here. This raises even more questions. Climate change tends to be more pronounced from south to north. An expansion of the range to the south is interesting since it causes me to wonder why these birds have not occupied the southeast in the past, and what has changed to cause them to expand in the south now? More nesting opportunities? Less competition?
For detailed life history information read the Tree Swallow species account at the Cornell Lab of Ornithology.
The conclusion of the Shutler (citation follows at the end of the TRES portion of this blog post) paper to which Prof. Bonier and Prof. Robertson contributed:
“The broad geographic patterns are consistent with a hypothesis of widespread changes in climate on wintering, migratory, or breeding areas that in turn may differentially affect populations of aerial insects, but other explanations are possible. It is also unclear whether these changes in occupancy rates reflect an increase or decrease in overall populations of Tree Swallows. Regardless, important conservation steps will be to unravel causes of changing populations of aerial insectivores in North America.”
Prof. Bonier also has links to the Virginia Tech people doing research in that part of the U.S. (probable increasing population southward). They have found that TRES displace Bluebirds at nest boxes. Prof. Bonier wrote:
“TRES in the northeast of North America are declining, whereas other locations are stable or increasing. We are studying TRES with collaborators Ignacio Moore (Virginia Tech) and Mark Stanback at Davidson College in North Carolina, where they have been displacing eastern bluebirds from his study site. TRES never used to breed there, and now are abundant and occupy almost all of his nest boxes. Ignacio put up nest boxes near the Virginia Tech campus last fall, and already has higher box occupancy than we have at QUBS. So in the southeast, it does seem that TRES are increasing and also expanding southward.”
For more about Prof. Bonier’s current research, see the news article posted on the Virginia Tech web site.
Thanks to Prof. Robertson’s long term research, we have learned a lot about the mating and nesting behaviour of Tree Swallows, and their selection of nesting sites.
Left, a male Tree Swallow stands guard while a first-year breeding female brings nesting material to one of the nest boxes at QUBS. Professor Robertson studied the unusual “late onset of adult plumage” in Tree Swallow females. Among the many interesting traits of these birds, the delayed adult female plumage characteristic is very unusual in the bird world. Delayed adult plumage is common among adult males of many bird species. Male TRES are fully iridescent blue by their second year. Females only attain such “blueness” later in their 2nd year. Why would this be?
It is this kind of observation and hypothesis development that characterizes good science.
So using a variety of methods, Prof. Robertson and colleagues went about testing their hypotheses regarding the late plumage onset of TRES females. And this is what they found.
When a one-year-old female TRES arrives at a nest box inhabited by a nesting pair, the resident male is less aggressive towards a one-year-old female than toward older intruders. The female is equally aggressive toward all females. Isn’t THAT special? It turns out these young, less brightly feathered females are, indeed, LOOKING for breeding opportunities, as they visit many nests to seek out a tryst with a resident male. And many males respond quite positively. Note that these visits last only a few seconds. Birds are quicker than people! Another interesting result: older females are much more successful at raising young early in the season. Later in the season, the younger females, which were experimenting at a lot of different nest boxes earlier are, later in the season, breeding much more than their older counterparts AND they are being just as successful.
This is a later vintage QUBS nest box, with the effective anti-rodent device pictured below the box. Its location beside the road into the QUBS buildings is not particularly suitable for TRES, which prefer the nest boxes in open hay fields, but are occasionally occupied by House Wrens and Black-capped Chickadees. Raccoon, snakes, and squirrels are among the most prevalent and successful predators of songbird nests. Next time you are thinking about how cute squirrels are, recall this fact! Larger snakes are the only predator capable of circumventing these collars.
A vigilant adult Tree Swallow is pictured above. After reading through this blog, I expect people will wonder more about what goes on in birds’ brains as they look at us. Perhaps some of these surprising revelations will prompt some to wonder all the more.
Sex and Infanticide: Tree Swallows, the Sordid Side
People look on small songbirds as benign, benevolent and harmless. Evidence abounds for these traits. Although, in my opinion, the following evidence doesn’t change an overall positive image, it does give one pause. Your blogger has observed the sweetest of birds, the Black-capped Chickadee joyfully feeding at a deer carcass in mid winter. During a winter snowshoe sojourn, a Red-breasted Nuthatch flew onto my shoulder demanding FOOD, and gave me a most malevolent look when the lack of bird seed was apparent. And, given an opportunity, male Tree Swallows will massacre a previous male’s offspring in order to start its own brood.
Prof. Robertson studied this behaviour by removing males from nest boxes. Sexually selected infanticide is quite rare in birds. It is more common in mammals. So seeing this behaviour in TRES is of great interest to ornithologists. During incubation, with eggs in the nest, Prof. Robertson removed 17 males. In three cases, the males were not replaced. However, a large proportion of the males, 11, killed the nestlings when they hatched. Three others adopted the newly hatched nestlings.
If the new male is introduced during egg-laying, (Robertson removed 11 in this instance), 4 were not replaced and all of the other 7 replacement males did adopt the eggs and nestlings. Finally, the same experiment was completed during the nestling stage (15 males removed), and five of the fifteen killed the nestlings. Of these 5, only 1 re-nested with the widowed female; two re-nested with a new female and two did not re-nest. So why would so many kill the newly hatched nestlings? Researchers studying with Prof. Robertson found that the behaviour is adaptive. That is, there is an advantage to those males who kill the previous male’s offspring, as they disproportionately pass on their genes. This raises more questions, which future Queen’s students may answer using the unequaled resources at QUBS.
What about the secret mating among TRES, that is, one of a mated pair sneaking off to pair with another TRES, and, as already discussed, the young females with their juvenile plumage, visiting males in nest boxes? The facts are surprising. The euphemistic ornithological expression is “extra-pair” mating and paternity. Based on observations, it appeared that TRES are models of social monogamy. Hold on! That is another fun feature of good science. What appears to be happening based on observation may conflict with the facts. A good detective follows the evidence. And the evidence is:
80% of nests contain some extra-pair offspring!
50% of all nestlings are sired by extra-pair males!
So seeing ISN’T believing. There is a lot of “visiting” going on, and it happens quickly and surreptitiously…so much so that trained observers don’t see it.
Seeing this result from a MALE perspective, the benefits are obvious: the male has more offspring spread over more nests, making survival more likely (literally male Tree Swallows have their eggs in more than one basket!).
What does the female get? She still has her nestlings all in one basket, and she still gets the same contribution to care by one male. Once again, the facts are both interesting and unexpected. It is the FEMALES that control extra-pair copulation. They are the ones who stray.
After some very intensive study and some very difficult analysis, research has concluded that the female, as well as the overall population, does receive an important benefit. These pairings result in better genetic compatibility, not in overall better genes. Genetic compatibility (i.e. simply the genes from each partner vary in how well they work together. Some genes pair up for greater benefits. The more pairings, the greater likelihood of compatible pairings) is an adaptive advantage.
How Tree Swallows Select Nest Location
In the photo series below, Tree Swallows are photographed at various natural tree cavity nest locations. As you would expect, forestry, farming and other land uses have limited and reduced the numbers of tree swallow nests. Natural cavities produced by wood peckers, disease or physical damage must be, ideally, close to water or open fields, so Tree Swallows have a ready source of food, such as insects emerging from water and fields. Artificial boxes do help alleviate this limiting factor.
Tree Swallows, outside of nesting season, are very social. They form large flocks for migration and over-wintering. The author has seen flocks of thousands over bays in Florida. That changes when they nest. They like some space between nests.
On average, artificial nest boxes are larger, roomier and safer than natural cavities. However, whether a natural cavity or nest box is selected, TRES like their nests to be spaced an average of 28 meters apart. Researchers at QUBS determined this optimal distance through a spiral alignment of boxes in a field.. The second birds to arrive to nest usually choose the most distant nest box form the first pair. After a few weeks, nest selection becomes more random, but males do defend their nests aggressively from other TRES males. QUBS nest boxes are now set up in a grid pattern in optimum habitats. This territorial behaviour differentiates Tree Swallow from other swallow species which are more colonial.
Current research on TRES is focused on applied ecology, especially ecotoxicology. Professor Fran Bonier, who contributed greatly to this blog, is studying how Tree Swallows adapt to environmental stresses, for example through endocrinology field studies. See her current Queen’s web page for more information.
Spatiotemporal Patterns in Nest Box Occupancy by Tree Swallows Across North America, Avian Conservation and Ecology 7(1): 3., D. Shutler et al, 2012.
- Field Ornithology-Agent for Science, Education and Conservation, presentation to the Society of Canadian Ornithologists Annual Meeting, Dr. Raleigh Robertson, September, 2007.
- Personal Communication, Dr. Frances Bonier
- Personal Communication, Dr. Raleigh Robertson
- Personal Communication, Catherine Dale
I want to leave you with a few images of the Queen’s University Biological Station:
Your blogger, Art Goldsmith, with Professor Raleigh Robertson, left. The new Jack Hambleton Library (officially opened in June) is at the upper right. The building also now houses the Fowler Herbarium in state-of-the-art facilities. We believe this is the only field station with on-site access to such a large herbarium collection. Below, the large central QUBS building, with a cafeteria, labs, offices and a large seminar room, is named for Professor Robertson.
Next time, we will start our series of field course blogs. This includes the courses on Insects, Aquatic Ecology (a different course from the China-Canada Course), and Fabulous Fall Fungi. See you then.