Ageing Northern Shrikes in the Field

Distinguishing between adult and immature Northern Shrikes is more difficult than most birders think. Indeed, those individuals with brownish backs, indistinct grayish-black masks, reduced white wing patches, and heavily-barred, dingy underparts are immatures (i.e., hatched the previous summer), as traditional views suggest (Figures 1 and 2). The problem, however, is that many immatures (Figure 3) more closely resemble adults (Figure 4) than most field guides indicate.


Figure 1. Immature, Wisconsin, January. Photo by Ryan Brady.



Figure 2. Immature (with Fox Sparrow), Michigan, November. Photo by Nova Mackentley.



Figure 3. Immature, Wisconsin, January. Photo by Ryan Brady.



Figure 4. Adult, Wisconsin, January. Photo by Ryan Brady.


The reason for this similarity is that many immature Northern Shrikes have a more advanced first prebasic molt than these individuals classically thought of as immatures. The extent of this molt, which is completed before the birds’ first migration south and may involve body feathers, upperwing secondary coverts, and occasionally 1-3 tertials, is highly variable. Thus some immatures more closely resemble brownish, heavily-barred juveniles and others sport body plumage suggestive of adults. In northern Wisconsin at least, where I have trapped dozens and observed hundreds of Northern Shrikes over the past three years, the latter case is more frequent and hence the dilemma associated with ageing this species.

As a result, birders should not rely on the body plumage of Northern Shrikes in order to correctly age them. Rather, the best way to age this species in the field is to examine (1) the flight feathers of the wing and (2) the upperwing secondary coverts.

Use the flight feathers by focusing on the secondaries and tertials of a perched bird. In immature Northern Shrikes, these feathers are brownish and worn and usually have a weak, indistinct whitish trailing edge, especially as winter progresses (Figures 5 and 6). In adults, these feathers are black with a broad and bold white trailing edge (Figures 5 and 7). The only exception is the uncommon case where an immature bird has molted one or more tertials during its first prebasic molt, but here the contrast between newly-replaced, black adult tertials and retained, brownish juvenal secondaries is visible in the field under most conditions. Adults have flight feathers of similar age and wear and thus show no such contrast.


Figure 5. Immature (left) and adult (right). The brownish flight feathers and indistinct white trailing edge of the immature secondaries can be observed under many field conditions. Photo composite by Ryan Brady.



Figure 6. Immature, Wisconsin, April. Even when the secondaries and tertials are not well-lit, the lack of a white trailing edge remains visible. Photo by Ryan Brady.



Figure 7. Adult, Wisconsin, February. Photo composite by Ryan Brady.


For the upperwing secondary coverts, check the median and especially the greater coverts for color, wear, and edging when field conditions allow. In Northern Shrikes, juvenal coverts are brownish, more worn, and have a broad buff edge (Figure 8). Adult coverts are glossy black and usually lack a pale tip (Figure 8). If any or all coverts are juvenal, then the bird must be an immature. However, because some immatures molt all of these coverts during the first prebasic molt, the presence of entirely adult coverts does not necessarily indicate an adult bird. A mix of old and new greater coverts is most common among immatures but up to a third of individuals may molt all of these feathers. Those that molt few or no greater coverts often show a weak wing bar owing to their buff-tipped juvenal coverts (Figure 9).


Figure 8. In-hand comparison of greater secondary coverts in immature (left) and adult (right) Northern Shrikes. Note that some immatures will show fewer or even no juvenal coverts. Photo composite by Ryan Brady.



Figure 9. Immature, Illinois, November. Photo by Carolyn Fields (pbase.com/cfields).


Although popular perception likely leads to more immatures being incorrectly aged as adults than vice versa, some adult female Northern Shrikes can approach the traditional view of immature plumage by showing distinct brown tones throughout the head, back, and scapulars, along with heavy barring and patches of brown on the breast (Figure 10). However, a good feature to look for here, in addition to those mentioned above, is the presence of brown concentrated on the forehead, which is shown by some immatures (e.g., Figures 3, 6, and 9) but not adults. Regardless, getting good looks at the forehead, flight feathers, and/or upperwing coverts can be difficult and many birds should be left unaged until these appropriate field marks are adequately observed.


Figure 10. Adult, Wisconsin, January. This bird would be impossible to confidently age in the field based only on this view. Photo by Ryan Brady.


In summary, do not rely on head/back color, boldness of the mask, or degree of barring below to separate adult and immature Northern Shrikes. Because the extent of the first prebasic molt varies highly among immature birds, examining the flight feathers of the wing and the upperwing secondary coverts is critical for correct age assessment in the field. This becomes especially paramount during late winter and early spring when immatures undergo a limited prealternate molt and may become astoundingly similar to adults with regard to body plumage (Figures 11 and 12).


Figure 11. Immature, Wisconsin, March. Photo by Ryan Brady.



Figure 12. Immature (same bird as Figure 11), Wisconsin, March. Note the brownish, juvenal flight feathers that contrast the black adult tertials and secondary coverts. Photo by Ryan Brady.


All material, unless otherwise noted, copyrighted to Ryan Brady 2008 and cannot be reproduced in any form without permission. Comments and questions are welcome.