Bird molt is strangely divisive. Akin to secularists and people of faith fiercely debating creationism (Fig 1), molt nomenclature – and associated theory – has split the ornithological community. The fault line is the Atlantic Ocean. Generally, banders in North, Central, and South America find the Howell-augmented Humphrey-Parkes (HHP) system intuitively convenient. Across the Atlantic in Europe, experienced ringers find the HHP system “incomprehensible”. The WRP age classification system is based on HHP nomenclature as well as associated assumptions of evolutionary homology. As such, the WRP age classification system has grown in popularity among the next generation of banders throughout the Americas while facing stiff opposition among ringers in Europe. For example, this was the response I received from a British ringer when I posted a link to a WRP tutorial: “The motives were good. Execution of explaining it is beyond comprehension."
Figure 1. Examples of (A) secularists who believe in molt homology and ageing birds using WRP and (B) religious zealots who reject molt homology and the WRP system.
This blog post is not for those that reject HHP (see Fig 1), this post is for WRP aficionados who want to increase their capacity to categorize age of captured birds. As such, if you need an introduction to the WRP system, I suggest you read my tutorial here, followed by photographic examples here. In this post, I want to introduce WRP practitioners to two advanced age codes that I have found particularly useful in the field.
First advanced WRP code: after a given molt
Birds undergoing their second prebasic molt are technically undergoing a definitive prebasic molt. Thus, a bird classified as undergoing its definitive prebasic molt (DPB) is at least undergoing its second prebasic molt (i.e. 2nd or 3rd or 4th or etc. prebasic molt). DPB is an umbrella code. The WRP system offers more specific codes to classify birds undergoing the second prebasic molt (SPB) but does not offer codes for birds “after” their second prebasic molt. For non-molting birds, this is no problem, we simply use the “A” code for “after a given plumage”. For example, after second basic plumage (SAB) indicates a bird is at least in its third cycle. Clearly, an “after a given molt” code is useful and would compliment the SPB and DPB codes. Here, I suggest using “M” for “after a given molt.”
For example, scale-backed antbird (Willisornis poecilinotus) have partial preformative molts whereby worn-juvenal flight feathers are replaced by fresh-definitive flight feathers during the second prebasic molt (SPB; see Fig 2). Subsequent definitive prebasic molts are characterized by individuals replacing worn-definitive flight feathers with fresh-definitive flight feathers (Fig 3). In such cases we would recognize these individuals as after their second prebasic molt and would use SMB as their corresponding age code. If we were unable to differentiate SPB and SMB individuals, we would revert to the more general DPB code.
When using the "M" code, I suggest that the first (cycle) and third (plumage) codes reflect the last known homologous molt. This helps expand the use of the "M" code to other molts such as prealternate and presupplemental molts. For example, a captured bird that is at least undergoing its second prealternate molt can be classified as SMA (instead of SMB).
Figure 2. Scale-backed antbird (Willisornis poecilinotus) from Brazil, central Amazon, undergoing its second prebasic molt where formative body-plumage and juvenal flight feathers are being replaced by definitive basic plumage. Using the WRP system, this individual scale-backed antbird can be aged as undergoing its second prebasic molt (SPB), or more generally and less specifically, as undergoing its definitive prebasic molt (DPB).
Figure 3. Scale-backed antbird (Willisornis poecilinotus) from Brazil, central Amazon, undergoing at least its third prebasic molt where definitive basic plumage (at least its second basic plumage) is being replaced by definitive basic plumage (at least third basic plumage). Using the WRP system, this individual scale-backed antbird can be aged as after its second prebasic molt (SMB), or more generally and less specifically, as undergoing its definitive prebasic molt (DPB).
Second code: suspended molts
Suspended molts appear rare among North American temperate birds and Neotropical birds found in humid rainforests of Central and South America. However, birds found in the Sahel, and eastern and central Africa commonly exhibit suspended definitive prebasic molts. Suspended molts appear common, particularly among bulbuls (Pycnonotidae) and sunbirds (Nectariniidae), across the dry savannas of east Africa as well as the humid Congolese rainforest in central Africa. The capacity to regularly suspend molts is likely a phylogenetically conserved trait within families, and presumably provides an adaptive response to seasonal fluctuations in food resources.
To date, the WRP age classification system has no code to encompass suspended molts. Some practitioners, such as Dr. Erik Johnson, suggest simply using the “P” code to denote suspended molts, accompanied by a note describing the suspended molt. This strategy works fine in the Americas where suspended molts are relatively rare, but is less useful where suspended molts are more common. Given the sheer volume of birds exhibiting suspended molts in Africa, I propose using “S” to denote a suspended molt. For example, a captured yellow-whiskered bulbul (Eurillas latirostris) with a suspended definitive prebasic molt would be classified as suspended definitive prebasic molt (DSB; see Fig 4 and 5). Because Yellow-whiskered Bulbul undergo complete preformative molts, if we encountered an individual with a suspended preformative molt, we would classify its age as suspended first-cycle formative (FSF). Make sense? Good.
Figure 4. Yellow-whiskered greenbul (Eurillas latirostris) from Equatorial Guinea, central Africa, exhibiting "suspension limits" indicative of a suspended prebasic molt. Suspended prebasic molts appear common within the bulbul family (Pycnonotidae).
Figure 5. Western olive sunbird (Cyanomitra olivacea) from Equatorial Guinea, central Africa, exhibiting "suspension limits" indicative of a suspended prebasic molt. Suspended prebasic molts appear common within the sunbird family (Nectariniidae).
Take home message
Flexibility of the WRP age classification system allows practitioners to modify – and in rare cases even create – codes to fit their specific research and monitoring needs. I hope the addition of the “M” and “S” codes help researchers accurately classify bird age in understudied regions of the Earth, such as eastern and central Africa.