MOLT FOCUS REPORT 2015

MOLT FOCUS REPORT 2015

January 29, 2016

Bird age can be determined by a variety of phenotypic characteristics. Skull ossification and feather molt can help determine age in most birds, but factors such as feather wear, degree of body molt, time of year, and variation in molt strategies can make ageing birds difficult in many cases. Ageing birds requires an advanced level of training, as well as great attention to detail. Below, we provide pictures of how we determine age in birds that we capture at our songbird banding stations at the MPG Ranch.

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Bird age can be determined by a variety of phenotypic characteristics. Skull ossification and feather molt can help determine age in most birds, but factors such as feather wear, degree of body molt, time of year, and variation in molt strategies can make ageing birds difficult in many cases. Ageing birds requires an advanced level of training, as well as great attention to detail. Below, we provide pictures of how we determine age in birds that we capture at our songbird banding stations at the MPG Ranch.

Individual age and sex information are the two most critical pieces of data collected on birds at passive-effort mist-netting stations (DeSante et al 2015). A wealth of demographic information can be generated from these data, including estimates of productivity and survivorship (DeSante et al 2015, Ralph et al 1993). Techniques such as assessing a bird’s degree of skull ossification and other non-plumage related characteristics can be reliable methods for determining age. However, they are not useful in the early part of the breeding season; differences in feather shape, quality, and patterns of replacement during the annual cycle allow for micro-ageing at this time of the year. An understanding of life history strategies and molt strategy, as well as a highly trained eye, all contribute to accurate age classifications.

Important molt terms.

Contrary to our colloquial usage of the word ‘nest’ to describe a safe, homey place, actual bird’s nests are extremely risky places. There is selective pressure on young passerines to leave the nest as quickly as possible. When nestlings undergo pre-juvenal molt, it is the only time in a bird’s life it will grow all of its flight feathers simultaneously. The energetic demands required to grow feathers, coupled with the pressure to reach fledging stage, result in feathers poorer in quality than their adult counterparts. Thus, there are detectable differences between feathers grown during the pre-juvenal molt and feathers grown during subsequent molts. Juvenile feathers are characterized by a lower barb-density, a more tapered shape, the presence of growth bars, and, oftentimes, less vibrant and extensive coloration

Here is an open wing of a Lazuli Bunting, with its feather tracts and individual flight feathers labeled.

The tail of this second-year Black-headed Grosbeak, photographed on 8/21/15, is an excellent example of why feather replacement is necessary. Note the chips and nicks along the tips and sides of the feathers. One of the central rectrices is worn down almost totally to its shaft. These feathers were grown during the pre-juvenal molt, and are over a year old in this photograph. This bird will soon undergo its pre-basic molt.

Occasionally, we can age birds without looking at any feathers. Assessing a bird’s level of skull ossification is the most reliable technique, but some species have other diagnostic characteristics. For example, the lower mandible of young Yellow Warblers is a fleshy color (top), which transitions to the darker color seen in the Yellow Warbler on the bottom.

This hatch-year Gray Catbird, photographed on 8/12/15, is undergoing its pre-formative molt. The undertail coverts in the yellow circle were grown during its pre-juvenal molt. The feathers in the red circle are its incoming formative feathers. This is a great example of the textural differences between juvenile feathers and feathers grown during subsequent molts. The incoming feathers are richer in color, with a much higher barb density.

For some species, we can use the shape of the outer rectrices to determine age. The American Robin on the left has a broad, wide, and square rectrix (green line), indicating an after hatch year bird. The American Robin on the right has a pointed and tapered outer rectrix (yellow line), indicating a hatch year bird.

These two Yellow Warblers, photographed in August 2013, are of different age classes. The bird pictured on top is undergoing its adult pre-basic molt. Because it is systematically and symmetrically replacing each of its flight feathers, we know the bird is at least an afterhatch- year bird (second year or older). The bird pictured on the bottom is a hatch-year. Compare the intensity of the yellow edging of the greater coverts and primary coverts between the two birds. The hatch-year bird’s edging (black bracket) is noticeably duller than the after-hatch-year’s (red bracket).

We can see clear plumage differences in a second-year (top) and after-second-year (bottom) Western Tanager captured on 6/1/15. The second year bird grew its primary coverts (yellow arrows), primaries (green arrows), and the majority of its secondaries (purple arrow) during its pre-juvenal molt. Note the difference between the second-year bird’s primary coverts and the after-second-year bird’s below it. The second-year bird’s coverts are a duller color, show more wear, and are slightly more diminutive and less rounded at the tips. Also note the contrast between the second-year bird’s primary coverts and its greater coverts (red arrow).

Molt limits are often more subtle than those shown in the second-year Western Tanager from the previous slide. This second-year Gray Catbird, photographed on 6/1/15, has a molt limit in the greater coverts. The green line marks the boundary between feathers grown during the pre-formative molt on the left (red arrow), and feathers grown during the pre-juvenal molt on the right (yellow arrow). Note the more tattered and loosely-textured juvenile feathers.

This second-year Lazuli Bunting, photographed on 6/23/15, has an eccentric pattern of flight feather replacement. The molt strategy of the Lazuli Bunting is different from most other species. Lazuli Buntings are ‘molt migrants’, meaning they migrate to an area distinct from their breeding and wintering grounds to undergo molt each fall (Greene et al. 2014). This bird replaced its greater coverts (red arrow), outer two primary coverts (green arrow), primaries 3-9 (red line), and secondaries 5-9 (green line) during its pre-formative molt. There is a stark contrast between the replaced and retained flight feathers in terms of degree of weather, brightness of the blue leading edge, overall color, and the color of the shafts of the feather. From this information, we were able to age this Lazuli Bunting a second year bird.

Woodpeckers have more complex molt patterns than most passerines. In contrast to songbirds, woodpeckers do not reach a definitive plumage in their second fall. As a result, banders are able to micro-age woodpeckers to a more precise degree than passerines. For example, this Red-naped Sapsucker, photographed on 6/15/15, was aged as a third-year bird. The inner five primary coverts have been retained since its pre-juvenal molt (yellow arrow).

In contrast to other survey techniques, catching and banding birds give us the opportunity to learn about population demographics. We can ask questions about factors influencing survival, reproduction, and response to change. Accurate ageing of birds generates more robust data and, thus, provides more precise estimates of vital rates (productivity, survival, and population changes). When combined with natural history information of each species, this information can enhance the effectiveness of conservation activities, as well as our overall understanding of avian population dynamics.

Literature Cited

Previous Research Report

American Kestrel Research Summary

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