BLACKBIRDS and Identification of Damage

 

Richard A. Dolbeer
Project Leader
Denver Wildlife Research Center
USDA-APHIS-Wildlife Services
Sandusky, Ohio 44870

For additional information on blackbird control click Blackbird Control

Fig. 1. The red-winged blackbird Fig. 1. The red-winged blackbird (Agelaius phoeniceus) is the most abundant bird in North America. The black male, with red and yellow shoulder patches, is about 40% larger than the female. The female resembles a large sparrow more than a blackbird.

redwinged blackbirds

Introduction

The term blackbird loosely refers to a diverse group of about 10 species of North American birds that belong to the subfamily Icterinae. In addition to blackbirds, this subfamily includes orioles, meadowlarks, and bobolinks. The various species of blackbirds have several traits in common. The males are predominantly black or iridescent in color. All blackbirds have an omnivorous diet consisting primarily of grains, weed seeds, fruits, and insects. The relative proportions of these food groups, however, vary considerably among species. Outside of the nesting season, blackbirds generally feed in flocks and roost at night in congregations varying from a few birds to over one million birds. These flocks and roosting congregations are sometimes comprised of a single species, but often several species mix together. Sometimes they are joined by non-blackbird species, notably European starlings (Sturnus vulgaris) and American robins (Turdus migratorius).

The species also have many important differences in their nesting biology, preferred foods, migration patterns, and their damage and benefits to agriculture. Summarized below for each of seven species of blackbirds is information on identification, geographic range, preferred habitats, feeding habits, general biology, and damage.

Damage Identification and Assessment Corn damage caused by blackbirds

Blackbird damage to agricultural crops is often readily discernable because of the conspicuousness of the flocks of birds and the visible signs of the damage. However, correct identification of the species of birds in the agricultural field is important, along with evidence that the birds are actually feeding on the crop. For example, starlings superficially resemble blackbirds and sometimes feed in cornfields, yet they usually feed on concentrations of insects such as armyworms, doing little damage to corn. Also, red-winged blackbirds will often be attracted to agricultural fields, such as corn, initially to feed on rootworm beetles and other insect pests. They will not damage the crop itself until the grain has reached the milk stage. Blackbirds often forage in newly planted grain fields such as winter wheat, feeding on previous crop residue, weed seeds, and insects without bothering the sprouting grain.

Blackbird damage is also sometimes confused with other forms of loss. Raccoon and squirrel damage to corn can be mistaken for blackbird damage (Fig. 4). Also, seed shatter in sunflower caused by wind may resemble bird damage; however, the difference can usually be detected by examining heads for the presence or absence of bird droppings and by looking on the ground for hulls or whole seeds. Careful observation of the birds in the field and a little detective work will usually result in the correct identification of damage.

To estimate accurately the amount of blackbird damage in an agricultural field, examine at least 10 locations widely spaced throughout the field. For example, if a field has 100 rows and is 1,000 feet (300 m) long, walk staggered distances of 100 feet (30 m) along every 10th row (for example, 0 to 100 feet [0 to 30 m] in row 10, 101 to 200 feet [31 to 60 m] in row 20, and so on). In each of the 100-foot (30-m) lengths, randomly select 10 plants and visually estimate the damage on the head or ear of each plant to the nearest 1% (for instance, 2% destroyed, 20% destroyed). For corn, six kernels usually represent about 1% of the corn on an ear; for sunflower, it may be easiest to visually divide the head into four quarters and then estimate the percentage of seeds missing. When finished, simply determine the average damage for the 100 plants examined. This will give an approximation of the percent loss to the field. Multiplying the percent loss by expected yield can give a rough estimate of yield loss. In small grains, such as rice, estimates of loss are more difficult to obtain. One possibility is to simply compare the yields from plots in damaged and undamaged sections of a field.

Legal Status

Blackbirds are native migratory birds, and thus come under the jurisdiction of the Federal Migratory Bird Treaty Act, a formal treaty with Canada and Mexico. Blackbirds are given federal protection in the United States. They may be killed only when found “committing or about to commit depredations upon ornamental or shade trees, agricultural crops, livestock, or wildlife, or when concentrated in such numbers and manner as to constitute a health hazard or other nuisance,” as stated in federal laws regarding migratory birds (50 CFR 21). Some states have additional restrictions on the killing of blackbirds.

Editors

Scott E. Hygnstrom; Robert M. Timm; Gary E. Larson

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