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350 P. R. GRANT : FAT CONUITIOl\T OF ISLAND BIRDS IBIS 107
THE FAT CONDITION OF SOME ISLAND BIRDS
P. R. GRANT
Received on 7 July 1964
INTRODUCTION In ornithological literature, fat-lability has been considered to be associated with
requirements for migration since, with few exceptions, migrants have been found in a fat condition at the beginning of migration, while residents have little or no fat throughout the year (Wolfson 1945). I t has been demonstrated recently that the accumulation of fat is not an essential process for the release of migratory behaviour (King & Farner 1963, Lofts et at. 1963). However, it is generally agreed that fat deposits are associated principally with energy requirements during migration.
Grayson (1871), referring to the birds of the Tres Marias Islands, Mexico, wrote " Another fact worthy of note is that they are all very fat, so much so as to render the preparation and preservation of the specimens difficult ". Since these remarks apparently refer to the endemic, as well as the non-endemic, forms, and since the endemic forms would presumably not have a marked migration, information on the occurrence and significance of the fat deposits in the Tres Marias passerine birds was obtained during the course of a systematic and ecological study reported elsewhere (Grant 1565).
METHODS AND MATERIALS The passerine birds of the Tres Marias islands were studied in 1961-1963. Specimens
were collected on the mainland and the islands (Fig. 1) as often as possible from February
2 3'
22-
- - _ ._ ? , .-
I' ,.-... -. , I 1. , ,
6 Son Juanito Las M.Madre Trer Mar;as
M.Maqdalena
,-.- . . . I I. a
. - --** %- . . a. I
21'
Puerto Vallarta
3 0 Kmr. 106O 105"
FIGURE 1. Map showing the Tres Marias Islands off Mexico. X=Iocalities at which specimens were collected.
1965 P. R . GRANT : FAT CONDITION OF ISLAND BIRDS 351
to August inclusive. Of the 19 passerine species breeding in both regions (Grant & Cowan 1964), 11 were collected in sufficient numbers for mainland/island comparisons to be made. A few specimens only of the remaining passerine, and some of the non- passerine, species were also collected. Birds were either dissected soon after being collected, or were frozen and later thawed and dissected. The fat condition of each specimen was recorded using the scale of classes proposed by McCabe (1943), the limits of which are " no fat " (fat 0) and " extremely fat " (fat 5). It was possible to apply this to all species, although certain individuals, particularly of the Dusky-capped Flycatcher Myiarchus tuberculifer, were difficult to classify when they were either " moderately fat " (fat 2) or " fat " (fat 3).
RESULTS The data from the three years of study are grouped and summarized in Table 1.
This shows that many of the island samples have average fat values higher than those of
h
E \p 0
h h h hh -N
0 1 o W " O O S S S " C = . 0 n ; r o ; r r ; l
h h h
2 3 S ? P O
c 2 c . y P 1 1 0 0 0
c e a 0
- 3 -
h h h h
'? N
h h
0 3
h
3 ? N
h h
3 3 - 1 - I
h
3 "1 "
h h
3 3 9 0 m -
352 P. R. GRANT : FAT CONDITION OF ISLAND BIRDS IBIS 107
the mainland samples. Thus, on the islands, each of the eleven species has an average fat value of 2.5 or more at some stage during the 24 weeks of the study period, whereas only four reach this level on the mainland. Table 2 shows that the high levels of accumu- lated fat, categories 4 and 5, are reached more frequently by the island sample of each species than by the mainland sample, with the exception of Myiarchus tuberculifer and the
TABLE 2. The number of individual birds in each f a t category (see text) on Tres Marias islands and the mainland,
SPECIES
Platypsaris aglaiae
Tyrannus melancholicus
Myiarchus tyrannulus
Myiarchus tuberculifer
M.viopagis viridicata
Melanotis caerulescens
Vireo hypochryseus
Vireo flavoviridis
Icterus pustulatus
Piranga bidentata
Spinus psaltria
Note. M=mainland; I =island.
0 M 9 I 4
M 15 I 2
M 5 I 3
M 4 I 4
M 7 I 1
M 5 I
M 2 I
M 1 I 1
M 10 I
M 6 I
M 4 I 6
-
-
-
-
FAT CATEGORIES
1 2 11 6 2 6 3 2 5 5 1 6 7 - 5 2 1 2 6 8 1 1 4 4 1 2
11 9 6 3 1 1 2 5
33 13 4 7
18 1 2 1 3 3 _ -
3 2 1 6 3 2 2
1
4
3
3 3 8 4 1
2 7 2
-
-
-
-
-
4
9
14
4 1 1
4
4 1 4 3
12
10 2 3 4 5
-
-
-
-
-
-
Dark-backed Goldfinch Spinus psaltria. An average fat value for the whole study period can be obtained for each sample (Fig. 2), and again this is higher for each of the island samples except in the case of Spinus psaltria. These results apply to both sexes of each species.
The data in Table 1 give an indication that at the beginning of the year most of the island resident birds are at a low level of fat, which then rises and later falls, i.e. they are fat-labile (see Fig. 3). On the other hand all but three mainland residents remain at about the same low level of fat deposits (classes 0-2) throughout this,period (fat-stable). Of the three exceptions, the Golden Vireo Vireo hypochryseus and the Flame-coloured Tanager Piranga bidentata are probably not as fat-labile as their single values of 3.0 in Table 1 suggest, since these values are derived from samples of only two and three individuals respectively. The third species, Spinus psaltria, may be a local migrant rather than a resident on the mainland, because it forms large flocks in the non-breeding part of the year, often removed from the breeding habitat.
Most mainland birds start building nests in the last week of May, and island birds about four weeks later (Grant 1964). If the fat values of mainland samples in the 21 May period are compared with those of island samples in the 18 June period (Table l), it is seen that seven of the island samples are fatter than their mainland counterparts, and only one, Myiarchus tuberculifer, is the same or less fat. Thus, island birds enter the breeding season in a fatter condition than do the mainland birds (see Fig. 3). There is further evidence of this in the fat condition of specimens collected with an egg in the oviduct ('Table 3).
1965
m .- 0 a 0
P. R. GRANT : FAT CONDITION OF ISLAND BIRDS 353
M I
M I
M--I
M-I
M I
M- I
M I
M-I
M I
M I
I-M
P. ugluiae
T . melancholicus
M . tyrunnulus
M . tuberculijer
M . viridicuta
M. caerulescens
V. hypochryseus
V. flavoviridis
I. pustulatus
P. bidentutu
S. psaltria
0 1 2 3 4 5 Mean Fat Category
FIGURE 2.
Notes.
Mean fat value of mainland and Tres Marias island specimens of eleven species:(see text).
Mean fat values are calculated from the data in Table 2. Abbreviations: M=mainland fat value; I =island fat value.
It is worth noting that, at equivalent periods of the year, the fat values of each species were approximately consistent from year to year, which justifies grouping them together. There is one exception to this, however. Between 23 June and 30 June 1962, eight specimens of the Rose-throated Becird Platypsaris aglaiae were collected on Maria Magdalena, and all had low fat values (0-2); between 19 June and 26 June 1963, six more specimens were collected at the same locality, and all had high fat values (4-5). Thus, in successive years, and at essentially equivalent periods of the calendar year,
5 1
Time In 2 - week periods, 26 Fob -12Aug
FIGURE 3. Notes.
Fat values of mainland and Tres Marias island specimens of Icteruspustulutus (see text). Numbers in brackets give sample size. Triangular symbol = start of nest-building.
3 54 P. R. GRANT : FAT CONDITION OF ISLAND BIRDS IBIS 107
samples of the same population were in a completely different fat condition. According to observations, the time of breeding of this and other species was not appreciably different in the two years.
TABLE 3. The fa t condition of birds possessing an egg in the oviduct when collected (see text). SPEC I E S FAT VALUE OF EACH INDIVIDUAL
mainland Platypsaris aglaiae 0 Tyrannus melancholicus 1 Myiopagis viridicata 1 ; 2 Thryothorus fe l ix 1 Melanotis caerulescens - Turdus rufo-palliatus 2 Vireo ffavoziridis - Icterus pustulatus - Piranga bidentnfa -
island 4 5 - -
4
4 ; 2 4 ; s
5
-
Mean value 1.2 4.1
CONCLUSIONS There seems little doubt that in the months just prior to and at the beginning of the
breeding season island birds are in a fatter condition than their mainland counterparts. This is essentially a difference between the residents; migrants are fat before the breeding season in both regions.
This conclusion is supported by the meagre data provided by the other passerines. For instance the Brown-backed Solitaire Myadestes obscurus, collected on the mainland only, is typically fat-stable (15 specimens examined; maximum fat value 2); the Red- breasted Chat Granatellus venustus, collected on the islands only, is typically fat-labile (19 specimens examined; maximum fat value 5). The only data for non-passerines pertain to the Red-tailed Hawk Buteo jamaicensis. An adult female, collected on the islands on 10 March 1962 was classified as fat 1; another collected there on 26 April 1963 had a fat value of 4. Although there are no data from mainland specimens with which to compare them, these two specimens suggest that the fat-lability of island birds may not be restricted to passerines. In his remarks about the fat condition of the island birds, Grayson (1871) made no distinction between passerine and non-passerine species, but did refer to all his specimens of the Parauque Nyctidromus albicollis, as excessively fat."
DISCUSSION It is possible that the fat-lability of the Tres Marias birds is in some way associated
with limited migratory movements over each island or within the group. However, such movement was not detected in April, June and July, when the longest visits to the islands were made. Furthermore Heilfurth (1938), having spent some time on all four islands in December, January, May and June, considered that most species were widely distributed over the islands, and stationary, throughout- the year.
Even though local migrations are not apparently performed, it is arguable that the accumulation of fat at one season may be of advantage later when food is in short supply. Activities involving courtship and nest-building reduce the avkilable time for feeding, and if food is relatively scarce at this time the stored energy in fat will compensate for diminished intake of food. A critical investigation of this possibility would require at
* J. R. Northern of the Los Angeles County Museum, California, has kindly supplied me with information of the fat condition of birds he collected on the Tres Marias islands in the latter half of March 1964. Whilst most specimens he collected had small or moderate fat deposits, which is to he expected, it is noteworthy that two non-passerines collected on 30 March, one Forpus cyanopygius and one Trogon elegans, were very fat (Fat Category 4).
1965 P. R. GRANT : FAT CONDITION OF ISLAND BIRDS 355
least an estimation of available food and daily intake, neither of which was made. I t can only be said that there was no indication given by the feeding behaviour of the birds (studied quantitatively in four species-Grant 1964), that the supply of several types of food on the islands varied appreciably from one month to the next, or differed noticeably from the supply on the mainland. This was also the view of Heilfurth (1938), but it needs more precise study.
Heilfurth (1938) suggested that lack of water is a more serious problem to the birds than food shortage. He observed that on Maria Cleofas a few brush-living species were found most frequently near water in the dry season (January-June), and that dew- drinking was common at this time of the year. He believed that, in the absence of accumulated fresh water on San Juanito, the drinking of dew was the only means by which the birds there obtained free water. Furthermore, all the transients he saw were near the artificial supply of water on Maria Madre. Dew-drinking, and the attraction to water of both transients and residents, were observed during the present study on Maria Magdalena. Perhaps the storage of fat is related to a shortage of free water (as opposed to preformed and metabolic water-Bartholomew & Cade 1963) at the end of the dry season, particularly in view of the well known fact that the oxidation of a given weight of fat produces an approximately equal weight of water (Peters 1935). There are two reasons for believing this. First, the dry season is effectively longer on the islands than on the mainland. Although the wet season begins in both regions in June, more rain falls on the mainland in the first month, and more frequently, than on the islands. For example, by the end of July only 130 mm. of rain have fallen on the islands compared with more than 500 mm. at Puerto Vallarta and Tepic on the mainland; and Puerto Vallarta and Tepic receive rain on 37 and 35 days respectively in June and July, whereas Maria Madre receives rain on only 12 days (Contreras 1942). Secondly, although the data in Table 1 do not allow a precise description of seasonal fat change, they do indicate that levels of fat drop at the climatically most unfavourable time. In April the level of fat deposits is high in all the resident island species shown in Table 1, and in June is lower in four of them. This suggests that the fat is mainly used in May, June and July, when free water is scarce and diminishing, and when ambient temperatures approach a yearly maximum (Contreras 1942).
However, this seemingly plausible explanation encounters a serious difficulty when the temperature balance of a bird is considered. The oxidation of a given weight of fat produces more heat than can be dissipated by the evaporation of the water liberated in this process, as has been emphasised by D. S. Farner (pers. comm.). Heat production will not be as disadvantageous at the lower temperatures of night (when fat metabolism may principally occur) as it certainly is in day-time, and some heat may be eliminated by means other than evaporative cooling (see Eliassen 1963) ; but, nevertheless the storage of water as fat is of doubtful economy. Furthermore there is recent evidence that fat is not metabolised by birds during considerable heat stress (Zimmerman 1965).
Thus the significance of the fat reserves remains to be demonstrated. It is not known whether seasonal fat accumulation of resident species is a specifically insular characteristic (on these or other islands), or whether it also occurs in birds living in mainland areas which are climatically similar to the Tres Marias islands e.g. near Mazatlin (see Fig. 1). Similarly, the factors governing fat deposition under these circumstances are completely unknown.
ACKNOWLEDGMENTS
This work was financed by a grant in aid of research from the National Research Council of Canada. The co-operation and assistance of the Direccidn General de Cam, Departamento de Prevencidn Social, Mtxico, D. F., and Prof. B. Villa R., and A. R. Phillips of the Universidad Nacional Autdnoma de Mkxico is gratefully acknowledged. G. G. E. Scudder and D. S. Farner read the manuscript and offered valuable suggestions, which are also acknowledged with gratitude.
356 P. R. GRANT : FAT CONDITION OF ISLAND BIRDS IBIS 107
SUMMARY Several resident passerine species of birds on the Tres Marias islands, Mexico, are in a fatter
condition than their mainland counterparts just prior to, and at the beginning of, the breeding season. At least two non-passerine species on the islands are also fat at this time of the year. The significance of the fat reserves of the island birds is discussed, and it is concluded that the fat may serve the functions of energy-storage and possibly water-storage, to be used later at the climatically most unfavourable time of the year.
REFERENCES BARTIIOLOMEW, C. S. & CADE, T . J. 1963. CONTREHAS, A. A. 1942.
Secretaria de Agricultura y Fomento, Instituto Geogrifico: 1-54. ELIASSEN, E. 1963.
during flight. GRANT, P. R. 1964.
thesis, University of British Columbia, Vancouver 8, B.C. Canada. GRANT, P. R. In press. GRANT, P. R. & COWAN, I. McT. 1964.
Nayarit, MCxico. GRAYSON, A. J. 1871.
Marias. HEILFURTH, F. 1938.
Tres-Marias Inseln (Mexico). KING, J. R. & FARNER, D. S. 1963.
Condor 65 : 200-223. LOFTS, B., MARSHALL, A. J. & WOLFSON, A. 1963.
migration activity in the absence of fat deposition in birds MCCABE, T . T . 1943. PETERS, J. P. 1935.
Thomas. WOLFSON, A. 1945.
Condor 47 : 95-127. ZIMMERMAN, J. L. 1965.
The water economy of birds. Auk 80 : 504-539. Mapa de las provincias climatol6gicas de la Repitblica Mexicana, MCxico.
Preliminary results from new methods of investigating the physiology of birds
Unpubl. Ph.D.
Evolution. A review of the avifauna of the Tres Marias islands,
The physical geography and the natural history of the islands of the Tres
Beitrag zur faunistik, okologie und beseidelungsgeschichte der vogelwelt der
The relationship of fat deposition to zugunruhe and migration.
The experimental demonstration of pre- Ibis 105 : 99-105.
Ibis 105 : 234-237. The significance of some insular characteristics in birds.
The adaptive significance of some size trends in island birds.
Candor 66 : 221-228.
Proc. Bost. Nat. Hist. SOC. 14 : 261-302.
Proc. VIII Intern. Ornith. Congr. : 450475.
An aspect of the collector’s technique. Auk 60 : 550-558. Body Water-The Exchange of Fluids in Man. 1 4 0 5 , Springfield, Ill:
The role of the pituitary, fat deposition and body weight in bird migration.
Wilson Carcass analysis of wild and thermal-stressed Dichcissels. Bull. 77 : 55-70.
Dr. P. R. Grant, Department of Zoology, University of British Columbia, Vancouver, B.C. Present address : Department of Biology, Yale University, New Haven, Connecticut, U.S.A.
