The Western Bluebird ( Sialia mexicana) is a familiar sight in the western United States, southwestern Canada, and Mexico. These small thrushes, often found in partly open terrain ranging from valley farms and orchards to clearings in mountain pine forests, have a diet that shifts with the seasons, adapting to what's available. Recognizing the importance of a varied diet and understanding what these birds eat can help support their populations, especially as they face habitat loss and competition for resources.
Seasonal Shifts in Diet
The Western Bluebird's diet primarily consists of insects and berries, with insects making up the majority, especially during the summer months. These birds are often seen perching on fence wires by open meadows, fluttering down to pluck insects from the grass. In winter, they form small flocks, feeding on berries in trees, sometimes gathering by the hundreds where juniper woods have heavy berry crops.
Summer: An Insect-Rich Feast
During the breeding season, insects become an even more crucial component of the bluebird's diet, particularly for feeding their young. Studies analyzing the stomach contents of bluebirds revealed a preference for grasshoppers, caterpillars, beetles, and spiders. Other insects consumed include ants, wasps, bees, flies, myriapods, angleworms, snails, sow bugs, black olive scales, moths, weevils, and termites.
One study in south-central Washington found that nestling diets consisted of 37.5% beetles (Coleoptera), 29.2% wasps and bees (Hymenoptera), 17.5% true bugs (Hemiptera), 9.4% grasshoppers (Orthoptera), 2.5% caterpillars (Lepidoptera), and 1.0% spiders (Arachnida). The adult diet also heavily featured beetles (68.2%) and caterpillars (12.2%). After nestlings hatched, the diet shifted to 37.5% Coleoptera, 29.2% Hymenoptera, and 15.6% Hemiptera.
Winter: Relying on Berries and Fruits
As insect populations decline in the colder months, berries and small fruits become vital for the Western Bluebird's survival. Fruits of mistletoe, juniper, and elderberry are among their favorites. Planting native fruit-bearing shrubs and trees can significantly aid bluebirds during this time, providing a consistent food source when insects are scarce.
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Foraging Behavior
Western Bluebirds are adept foragers, employing various strategies to obtain their meals. They often perch fairly low, flying down to the ground to capture insects. Sometimes, they hover briefly before pouncing on their prey. They may also catch insects in mid-air or seek them among foliage. When it comes to berries, they perch or flutter among branches to pluck them.
The Impact of Diet on Nestlings
During nesting season, the proportion of insects in the food collected by bluebird parents is significantly higher. Caterpillars, grasshoppers, and spiders are primary food sources for nestlings. It's crucial to avoid using pesticides, as they not only kill insects but can also harm or kill the birds themselves.
Human Intervention: Feeding Bluebirds
Many people enjoy feeding bluebirds in their backyards, supplementing their natural diet with various treats.
Mealworms: A Bluebird Favorite
Mealworms are perhaps the most popular offering for bluebirds. These darkling beetle larvae can be purchased live or dried at most pet stores. If offering live mealworms, ensure the container has smooth sides to prevent escape. Dried mealworms can be soaked in warm water for 10 to 15 minutes before serving to provide moisture. However, overfeeding mealworms can cause calcium deficiency, so they should be offered as an occasional treat. Some advise limiting quantities to no more than 15 mealworms per day, per bird.
Berries and Fruits: A Healthy Supplement
Bluebirds naturally eat various berries, including dogwood, juniper, hackberries, and mistletoe berries. Safe options for feeding include blueberries, blackberries, raspberries, and cranberries. Chopped fruits like apples and pears, as well as dried fruits such as raisins and cranberries, can also be offered.
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Suet: Fat and Protein for Winter
Suet, with its abundant fat and protein, can be beneficial, especially during winter when insects are scarce. It's best to offer suet in feeders that don't require the birds to hang and feed.
Other Treats
Some birdwatchers offer peanut butter, smearing a small amount on a tree trunk at ground level. Additionally, there are commercially available treats specifically formulated for bluebirds, including nuggets and pellets with ingredients like peanut butter and insect parts. Shelled sunflower, safflower and peanut chips/nut meats can also be offered.
What to Avoid
It's important to avoid offering dead insects that have been poisoned. While bluebirds may occasionally eat vertebrates like small snakes, salamanders, tree frogs, and lizards, it's best to stick to insects and berries.
Earthworms should not be fed to nestlings, as they can make them very sick and even cause death.
The Importance of Native Plants
Landscaping with native plants that produce berries throughout the seasons is one of the best ways to support a healthy, varied diet for bluebirds. This ensures a natural and sustainable food source, reducing the need for human intervention.
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Conservation Efforts and Habitat Management
Western Bluebird populations have declined in recent decades due to habitat loss and competition for nest sites. Providing birdhouses can help offset the loss of natural nest sites, but it's essential to consider the surrounding habitat. Western Bluebirds prefer semi-open grassland habitat with scattered trees, fence lines, and short ground cover.
Supporting Bluebirds on Farms
Western Bluebirds can play a beneficial role in agricultural ecosystems, acting as natural enemies of pests like the beet armyworm. Adding nest boxes to vineyards and farms can increase the abundance of insectivorous birds, leading to a reduction in pest populations and increased crop yields.