Molt is the process in which birds shed feathers and replace them by growing new ones. It is a costly effort that typically follows breeding in the warmest months of the year and precedes migration. But a bird strategically adjusts the timing and sometimes the pace of its molt. The complete molt is a major undertaking. The bird sheds and then regenerates thousands of feathers, roughly from 25 to 40 percent of its lean dry mass (that is, excluding fat and water content). Molt draws significantly on protein and energy reserves to synthesize feather structure and to offset the costs of poorer insulation and flight efficiency. It has been estimated that adult Blue Jays and Florida Scrub Jays must increase daily metabolism from 15 to 16 percent during peak periods of feather production. Reduced insulation while molting requires increased heat production, and this doubles the cost of molt in Brown-headed Cowbirds at low temperatures. Molting during the warm summer months can thus be advantageous.
Molt is also a period of intense physiological change. Accompanying the replacement of worn feathers is the production of keratin by the skin, increased amino acid metabolism, and increased cardiovascular activity to supply blood to the growing feathers. The long list of changes also includes the moving of water to the developing feathers, changes in bone metabolism and calcium distribution, and an increased need for iron for red blood cell production. Together, these and other metabolic changes impose substantial hidden costs beyond the conversion of amino acids into feather proteins.
Few species breed and molt at the same time. They mostly are species that live in productive, tropical environments with minimal seasonal variation. There, prolonged molts apparently minimize daily costs in the absence of strong seasonal constraints. From 3 to 4 percent of the African birds examined in one study were molting while breeding. From 8 to 10 percent of the Costa Rican birds examined in another study bore signs of both molt and reproductive activity.
Exceptions to the rule are instructive. Some female hornbills molt while imprisoned in sealed nest cavities to incubate eggs and brood young. Their energy requirements for self-maintenance are minimal; as a result, the added costs of molt can be accommodated. Additionally, the flight feathers are not essential during this sedentary period. The high temperatures that build up inside the nest cavity may favor loss of feathers and reduced insulation. In contrast, male hornbills, which feed the incubating females, wait to molt until their families leave the nest.
Tropical birds molt more predictably than they breed. Reproduction may be tied to irregular periods of rain or may require several renesting attempts due to high rates of nest loss to predators. To give breeding priority, some birds interrupt their molts. Desert birds such as Darwin’s finches of the Galapagos and the Zebra Finch of Australia stop the regular seasonal molt to nest whenever the unpredictable rains begin. They resume the interrupted molt after nesting is completed. Tropical terns such as the Angel Tern on Christmas Island turn the molt on and off to breed whenever possible. This delicate seabird has no pigment in its flight feathers, which consequently wear easily and must be replaced more often than those of other terns. Wave after wave of molt is initiated in the flight feathers. The innermost primary feathers often begin to molt again before the outermost primary feathers are replaced in the preceding molt. As many as three successive molts may be in progress simultaneously. When an Angel Tern starts to nest (it simply lays an egg precariously on a bare branch), the molt stops suddenly, no matter which feathers may be missing–the molting equivalent of musical chairs-and after the tern has finished nesting, molt resumes as if there had been no interruption in the complicated pattern of feather replacement. Elsewhere birds adjust the pace of molt in relation to the time available. Gulls and sandpipers that breed in the high Arctic, where the reproductive season is short, start molting before they finish breeding to be ready for migration. The Dunlin, for example, begins to molt its primary feathers just before incubation and then finishes from four to five weeks later.
1
Molt is the process in which birds shed feathers and replace them by growing new ones. It is a costly effort that typically follows breeding in the warmest months of the year and precedes migration. But a bird strategically adjusts the timing and sometimes the pace of its molt. The complete molt is a major undertaking. The bird sheds and then regenerates thousands of feathers, roughly from 25 to 40 percent of its lean dry mass (that is, excluding fat and water content). Molt draws significantly on protein and energy reserves to synthesize feather structure and to offset the costs of poorer insulation and flight efficiency. It has been estimated that adult Blue Jays and Florida Scrub Jays must increase daily metabolism from 15 to 16 percent during peak periods of feather production. Reduced insulation while molting requires increased heat production, and this doubles the cost of molt in Brown-headed Cowbirds at low temperatures. Molting during the warm summer months can thus be advantageous.
According to paragraph 1, molting typically occurs during the warm summer months because
Awarmer temperatures reduce the effort needed for molting
Bthe warmer air assists birds in increasing their metabolic rates
Cfeather structure can be altered more quickly at higher temperatures
Dlow temperatures can prevent feathers from being shed effectively
2
Molt is also a period of intense physiological change. Accompanying the replacement of worn feathers is the production of keratin by the skin, increased amino acid metabolism, and increased cardiovascular activity to supply blood to the growing feathers. The long list of changes also includes the moving of water to the developing feathers, changes in bone metabolism and calcium distribution, and an increased need for iron for red blood cell production. Together, these and other metabolic changes impose substantial hidden costs beyond the conversion of amino acids into feather proteins.
According to paragraph 2, all of the following physiological changes occur during molting EXCEPT
Athe loss of keratin by the skin
Bincreased cardiovascular activity
Cchanges in the distribution of calcium
Dthe conversion of amino acids into proteins
3
Few species breed and molt at the same time. They mostly are species that live in productive, tropical environments with minimal seasonal variation. There, prolonged molts apparently minimize daily costs in the absence of strong seasonal constraints. From 3 to 4 percent of the African birds examined in one study were molting while breeding. From 8 to 10 percent of the Costa Rican birds examined in another study bore signs of both molt and reproductive activity.
The word “constraints” in the passage is closest in meaning to
Aadjustments
Blimitations
Cneeds
Dvariations
4
Few species breed and molt at the same time. They mostly are species that live in productive, tropical environments with minimal seasonal variation. There, prolonged molts apparently minimize daily costs in the absence of strong seasonal constraints. From 3 to 4 percent of the African birds examined in one study were molting while breeding. From 8 to 10 percent of the Costa Rican birds examined in another study bore signs of both molt and reproductive activity.
Paragraph 3 suggests that which of the following is true about bird species that breed and molt at the same time?
AThey tend to molt for a relatively brief period.
BThey live in areas where food is not readily available.
CThey are mostly found in Africa.
DThey usually have longer molting periods than other birds.
5
Exceptions to the rule are instructive. Some female hornbills molt while imprisoned in sealed nest cavities to incubate eggs and brood young. Their energy requirements for self-maintenance are minimal; as a result, the added costs of molt can be accommodated. Additionally, the flight feathers are not essential during this sedentary period. The high temperatures that build up inside the nest cavity may favor loss of feathers and reduced insulation. In contrast, male hornbills, which feed the incubating females, wait to molt until their families leave the nest.
The word “minimal” in the passage is closest in meaning to
Apredictable
Bfavorable
Cinsignificant
Dsimple
6
Tropical birds molt more predictably than they breed. Reproduction may be tied to irregular periods of rain or may require several renesting attempts due to high rates of nest loss to predators. To give breeding priority, some birds interrupt their molts. Desert birds such as Darwin’s finches of the Galapagos and the Zebra Finch of Australia stop the regular seasonal molt to nest whenever the unpredictable rains begin. They resume the interrupted molt after nesting is completed. Tropical terns such as the Angel Tern on Christmas Island turn the molt on and off to breed whenever possible. This delicate seabird has no pigment in its flight feathers, which consequently wear easily and must be replaced more often than those of other terns. Wave after wave of molt is initiated in the flight feathers. The innermost primary feathers often begin to molt again before the outermost primary feathers are replaced in the preceding molt. As many as three successive molts may be in progress simultaneously. When an Angel Tern starts to nest (it simply lays an egg precariously on a bare branch), the molt stops suddenly, no matter which feathers may be missing–the molting equivalent of musical chairs-and after the tern has finished nesting, molt resumes as if there had been no interruption in the complicated pattern of feather replacement. Elsewhere birds adjust the pace of molt in relation to the time available. Gulls and sandpipers that breed in the high Arctic, where the reproductive season is short, start molting before they finish breeding to be ready for migration. The Dunlin, for example, begins to molt its primary feathers just before incubation and then finishes from four to five weeks later.
In paragraph 5, why does the author discuss molting in the zebra finch of Australia?
ATo emphasize the speed with which some birds molt in order to start breeding
BTo explain why sudden climate change may shorten the molting process
CTo illustrate the claim that environmental conditions may cause some tropical birds to give priority to breeding over molting
DTo show that tropical birds have regular seasonal molts
7
Tropical birds molt more predictably than they breed. Reproduction may be tied to irregular periods of rain or may require several renesting attempts due to high rates of nest loss to predators. To give breeding priority, some birds interrupt their molts. Desert birds such as Darwin’s finches of the Galapagos and the Zebra Finch of Australia stop the regular seasonal molt to nest whenever the unpredictable rains begin. They resume the interrupted molt after nesting is completed. Tropical terns such as the Angel Tern on Christmas Island turn the molt on and off to breed whenever possible. This delicate seabird has no pigment in its flight feathers, which consequently wear easily and must be replaced more often than those of other terns. Wave after wave of molt is initiated in the flight feathers. The innermost primary feathers often begin to molt again before the outermost primary feathers are replaced in the preceding molt. As many as three successive molts may be in progress simultaneously. When an Angel Tern starts to nest (it simply lays an egg precariously on a bare branch), the molt stops suddenly, no matter which feathers may be missing–the molting equivalent of musical chairs-and after the tern has finished nesting, molt resumes as if there had been no interruption in the complicated pattern of feather replacement. Elsewhere birds adjust the pace of molt in relation to the time available. Gulls and sandpipers that breed in the high Arctic, where the reproductive season is short, start molting before they finish breeding to be ready for migration. The Dunlin, for example, begins to molt its primary feathers just before incubation and then finishes from four to five weeks later.
According to paragraph 5, the angel tern’s flight feathers have to be replaced more often than those of other terns because
Aits flight feathers are shed with each molting
Bits flight feathers are easily harmed
Cit carries out more than one molt of those feathers at the same time
Dits outermost feathers are worn down during the process of nesting
8
Tropical birds molt more predictably than they breed. Reproduction may be tied to irregular periods of rain or may require several renesting attempts due to high rates of nest loss to predators. To give breeding priority, some birds interrupt their molts. Desert birds such as Darwin’s finches of the Galapagos and the Zebra Finch of Australia stop the regular seasonal molt to nest whenever the unpredictable rains begin. They resume the interrupted molt after nesting is completed. Tropical terns such as the Angel Tern on Christmas Island turn the molt on and off to breed whenever possible. This delicate seabird has no pigment in its flight feathers, which consequently wear easily and must be replaced more often than those of other terns. Wave after wave of molt is initiated in the flight feathers. The innermost primary feathers often begin to molt again before the outermost primary feathers are replaced in the preceding molt. As many as three successive molts may be in progress simultaneously. When an Angel Tern starts to nest (it simply lays an egg precariously on a bare branch), the molt stops suddenly, no matter which feathers may be missing–the molting equivalent of musical chairs-and after the tern has finished nesting, molt resumes as if there had been no interruption in the complicated pattern of feather replacement. Elsewhere birds adjust the pace of molt in relation to the time available. Gulls and sandpipers that breed in the high Arctic, where the reproductive season is short, start molting before they finish breeding to be ready for migration. The Dunlin, for example, begins to molt its primary feathers just before incubation and then finishes from four to five weeks later.
According to paragraph 5, why do birds such as the Dunlin begin molting before breeding?
AThey need to avoid completing two energy intensive tasks at the same time.
BThey need to replace missing primary feathers for successful incubation.
CThey need to use the limited time available to complete both molting and breeding.
DThey take an unusually long time to complete the process of replacing feathers.
9
Molt is also a period of intense physiological change. [■] Accompanying the replacement of worn feathers is the production of keratin by the skin, increased amino acid metabolism, and increased cardiovascular activity to supply blood to the growing feathers. [■] The long list of changes also includes the moving of water to the developing feathers, changes in bone metabolism and calcium distribution, and an increased need for iron for red blood cell production. [■] Together, these and other metabolic changes impose substantial hidden costs beyond the conversion of amino acids into feather proteins.[■]
Look at the four squaresthat indicate where the following sentence could be added to the passage
While the production of new feathers is the result of molting, additional chemical and structural processes may take place at the same time.
Where would the sentence best fit？Click on a square sentence to the passage.
10
For a bird, molting–the process in which worn feathers are replaced by new ones–is a costly effort.
AMolting draws heavily on birds’ energy reserves to synthesize feather structures, increase heat production, and support the various physiological and metabolic changes that occur.
BRoughly 8 to 10 percent of all bird species molt while breeding, but in most of these cases either the male or the female delays its molt until after the young have left the nest and no longer require feeding.
CMost birds limit their complete molts to warm periods when they can remain largely inactive, thus making flight feathers less essential and greatly reducing the energy requirements for self-maintenance.
DBecause of the high energy requirements of breeding and molting, species that breed and molt at the same time must have low self-maintenance costs or live in stable, warm, productive environments.
EWhen there are multiple molts occurring, each molt starts with the outermost primary feathers and involves the regeneration of 25 to 40 percent of all the bird’s outer feathers.
FSome birds can adjust the start and speed of molt to balance the demands of breeding and migration, and tropical birds can stop their molts to breed when conditions become favorable.
