Determined Bluebird Pair Lays Third Nest Despite Aggressive Neighbours
Bring Back the Bluebirds Project Update
This has been a challenging and educational season for the Bring Back the Bluebirds Project, as nestbox competition and depredation have become serious and ongoing threats. So far this season, five nests have failed—with a sixth coming close to failing—as a result of these threats, while only two nests have fledged without incident. Although these are known issues in bluebird trails, and our partners on San Juan Island also encountered these problems, we’ve been lucky to escape them in previous years of our work in the Cowichan Valley.
Depredation is a common cause of nest failure for all birds but this year we have discovered that predators are not the biggest threat facing nesting bluebirds in our area: rather, the most danger comes from invasive English House Sparrows. Known to be highly territorial and aggressive, House Sparrows will attack and kill any birds nesting within their territory. As detailed in our last project update, at this year’s first breeding territory, occupied by our first translocated pair of 2014, this is exactly what has happened—twice. We are choosing to share with you a detailed account of one bluebird pair’s struggle with House Sparrows throughout this season. These problems highlight not only a major impact of exotic species but also why it is so important to monitor nestboxes, and why it is better to have no nestbox than to have one that raises House Sparrows.
We have done our best to reduce House Sparrow competition through nestbox management, being careful to always mount nestboxes away from houses and buildings, and to remove boxes that are repeatedly used by them. Until this year, we have avoided any significant issues. Unfortunately, throughout this season we’ve been battling nestbox competition from House Sparrows. Our technician Jemma Green and summer student Reanna Schilling have tested several methods of dealing with them based on our experiences, those of our project experts, and extensive information available from the North American Bluebird Society and other long-time nestbox trail monitors. Depending on the nesting stage and level of aggression displayed by the House Sparrows, we have removed nesting attempts and eggs, trapped and euthanized adults, or tried either to prevent nesting or to eliminate aggression and reduce competition (by either mounting or removing additional nestboxes from the area). A major complication is that these nestboxes are also used by other native cavity-nesters such as Tree Swallows and Violet-Green Swallows: while it is legal to humanely remove adults or nests of invasive House Sparrows, we cannot (and don’t want to!) disturb other native birds. Knowing that any nesting swallows are also threatened by House Sparrows, we have struggled to balance providing nesting opportunities with reducing risk of competitive attack. We also want to ensure that no House Sparrows are raising successful nests, and adding to the population of this non-native competitor.
One day following their release from an aviary a couple of kilometers away, a bluebird pair selected a nestbox along a fencline on a property that we considered prime habitat, between two large, open hayfields and near a lone Garry Oak. As they laid and incubated their eggs, nearby nestboxes were gradually occupied by breeding House Sparrows, even though it was not their typical habitat. Based on what we have learned so far this year, Jemma and Reanna quickly put up additional empty nestboxes to reduce competition, leaving the existing House Sparrow nests alone to avoid sparking aggression in the defensive males. This seemed to work: the bluebird nest was left alone and the nestlings’ growth progressed normally. At 14 days old, 4 or 5 days before they would fledge the nest, we banded these 5 nestlings.
Just 3 days later, on a cool and wet afternoon in late May, our Project Technician arrived to provide an extra supplemental feeding only to discover a House Sparrow looking out of the bluebirds’ nestbox. Three nestlings were severely injured and were rushed to Island Wildlife, a Salt Spring Island facility that provides urgent medical care to injured wildlife. But what to do with the 2 uninjured nestlings? They could not be placed back in the box to face another attack, yet they desperately needed to be kept warm and well-fed by their parents in the safety of a nestbox. Jemma put the nestlings in a hand-made nest in a nestbox mounted nearby but the parents seemed obsessed with a box on the other side of the field that already contained an active House Sparrow nest. Several hours later, with the nestlings still being ignored by their parents, we were left with no other option but to remove the House Sparrow nest from the box and replace it with the bluebird nest with the two nestlings inside, taking the risk that the House Sparrow adults would retaliate against the bluebirds. Minutes later, the bluebird pair and their nestlings were reunited.
Over the coming days, Jemma and Reanna were able to prevent further attacks with help from vigilant project volunteers, and the nestlings fledged to safety. The three nestlings in rehab were assessed and the injuries were indeed serious. One nestling had to be euthanized but the other two, after almost a month of constant care, eventually learned to fly and feed themselves, and reached a stable weight. In late June they were released, together, into their parents’ territory.
After fledging their two surviving nestlings in early June, this pair built their second nest in a different nestbox along the same fence line as their first, again amongst active House Sparrow nests. We made many attempts to actively manage the House Sparrow population, from capturing them using mist nets and “Sparrow Spanker” trap doors over the entrance, to shooting with pellet guns. In spite of all these efforts, the 6 bluebird nestlings in the pair’s second clutch reached 14 days old only to be attacked by yet another male House Sparrow. This time, the attacked nest was discovered after the damage had been done: 3 nestlings killed, 3 very seriously wounded. Again, the injured nestlings were rushed to Island Wildlife where one succumbed to its injuries two days later. Against all odds, the two other nestlings survived, and rehab staff is hopeful of a full recovery.
Having lost their entire second clutch, the pair, with two juveniles from their first clutch still in tow, have built a third, late-season nest. Although still within the same territory, this time the pair have chosen a nestbox on an adjacent property that is hidden from view of the House Sparrow population. We hope that this pair’s determination is rewarded with a successful nest, free from attack by House Sparrows and from depredation, and, as always, we are doing everything in our power to ensure that this nest is successful.
In spite of these challenges, the bluebird reintroduction project continues with successes this season. Three nests have fledged successfully so far, and the last of five nests that are currently active is expected to fledge by mid-August. After 9 translocations this season, in addition to the 6 naturally returned (male) birds, we have: 5 breeding pairs, 2 “widowed” adult males, 27 juveniles, as many as 17 nestlings, and 4 eggs.
In species recovery, as in wildlife management, it is important to always take the long view: after all of our successes in 2012 and 2013 (who can forget the excitement of hatching the first Western Bluebirds on Vancouver Island since 1995?!) we are confident of our overall project success. It took many years of decline before Western Bluebirds disappeared from our Salish Sea area—it will likewise take many years of dedicated, thoughtful stewardship and habitat restoration to re-build a sustainable population.
Learn more about House Sparrows and their impacts: